BASIC COURSE FOR MARINE
SURVEYORS ON STABILITY
Monfalcone, 30 Giugno -1 Luglio 2010
2007
COURAGE ACE
BALLAST WATER EXCHANGE AT SEA
2
LOADING OPERATION IN PORT
3
4
SHIP HYDROSTATIC – ARCHIMEDES’ PRINCIPLE
When a body is partially or completely immersed in a liquid, then it experiences
an
upward buoyant force which is equal to the weight of the fluid displaced by
the immersed part of the body:
S=
where:
■
 = volume of the fluid displaced (m3)
 = density of the liquid (1,026 t/m3 for seawater at 15° and standard
conditions of salinity).
In this way intensity, direction and vector arrow of the upward force can be
determined but not its point of application, known as the centre of thrust.
■
Since this point is hard to determine and poor in terms of practical application,
another one is preferred in naval architecture, namely the centre of buoyancy
(centroid of volume of the underwater portion of the hull)[1] intersected by the
thrust line of action and whose co-ordinates are more easily identifiable.
5
[1] Since the thrust is vertical, the horizontal co-ordinates of the centre of buoyancy and the centre of thrust are the same. It can be shown, on the other hand, that the height of the centre of thrust is always greater than
that of the centre of buoyancy.
SHIP HYDROSTATIC – GENERAL CONDITIONS
OF EQUILIBRIUM
From the study of rigid bodies we know that the following constitutes a necessary
and sufficient condition for the static equilibrium for a solid object:
■
the sum of all the forces acting on it is zero;
■
the sum of all the moments of these forces at any reference point is zero.
For a solid object that is partially or completely submerged, only two forces act:
■
the weight force of the object, called displacement, equal to the sum of all the
weights constituting the ship, acting vertically downwards and through the
centre of gravity of the vessel;
■
the positive buoyancy acting vertically upwards and through the centre of
buoyancy.
6
SHIP HYDROSTATIC – GENERAL CONDITIONS
OF EQUILIBRIUM
For the above-mentioned conditions of equilibrium to be satisfied, it is therefore
necessary that displacement and positive buoyancy :
■
have the same modulus and opposite direction ( F = 0);
■
lie on the same line of action ( M = 0).
7
SHIP HYDROSTATIC – CONDITIONS OF EQUILIBRIUM
In addition to the general conditions of equilibrium, the stability conditions also
need to be satisfied. There are three conditions of equilibrium:
■
STABLE (a) : the system returns to the initial position of equilibrium after slight
disturbances from outside forces;
■
UNSTABLE (b): the system moves away from the initial position of equilibrium
equilibrium after disturbances from outside forces;
■
NEUTRAL (c): the system is unaffected by the outside force and is always in
equilibrium in any position.
8
SHIP HYDROSTATIC – EQUILIBRIUM FOR TOTALLY
IMMERSED BODIES
9
SHIP HYDROSTATIC – EQUILIBRIUM FOR TOTALLY
IMMERSED BODIES
If the displacement is equal to the buoyancy and both forces lie on the same line
of action, the body is in equilibrium.
However, there are three different conditions of equilibrium depending on the
reciprocal positions of the centre of gravity and the centre of buoyancy.
It is clear that if a body is disturbed from its original condition of equilibrium and is
inclined, the sole condition of stable stability is for the centre of gravity to be
under the centre of buoyancy (c).
The other conditions are of unstable (a) and neutral (b) equilibrium.
10
SHIP HYDROSTATIC – EQUILIBRIUM FOR TOTALLY
IMMERSED BODIES
The value of the righting moment is:
Mr= a sin
where a, the height of B above G, remains constant for all inclinations.
11
SHIP HYDROSTATIC – EQUILIBRIUM FOR PARTIALLY
SUBMERGED BODIESSURFACE OF
ISOVOLUME CENTRES OF BUOYANCY
The basic difference, with regard to equilibrium, between totally and partially
immersed bodies is that for the latter the position of the centre of buoyancy
changes together with the waterplane.
A ship floating freely in an upright position with waterline WL, hull volume  and
centre of buoyancy B, is subjected to inclination “” and moves to waterline WL1,
enclosing an isovolume (having the same buoyancy) hull in respect of the
original one.
The centre of buoyancy B moves right and upwards and out of the transverse
vertical plane, passing through B and perpendicular to the axis of inclination y
(intersection of the two waterplanes WL - WL1), since the floating object is
generally not symmetrical in respect of this plane.
12
SHIP HYDROSTATIC – EQUILIBRIUM FOR PARTIALLY
SUBMERGED BODIES SURFACE OF
ISOVOLUME CENTRES OF BUOYANCY
The movement from the initial position WL to the inclined position WL1 with angle  occurs by
isovolume hulls, having all the same buoyancy, so that the centre of buoyancy moves from B
to B1 with continuity along the trajectory of isovolume centres of buoyancy in respect of the
volume  and the direction of inclination Y.
If we consider the inclinations along the X axis, again bisecting B and lying on the horizontal
plane XY, we will find another curve of isovolume centres of buoyancy lying, in this case, on
the diametrical plane of the hull in respect of the plane YZ.
Due to the infinite lines lying on the plane XY and bisecting B, we will have infinite curves of
isovolume centres of buoyancy combining to form a surface called the surface of isovolume
centres of buoyancy in respect of the hull volume .
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SHIP HYDROSTATIC – EQUILIBRIUM FOR PARTIALLY
SUBMERGED BODIES SURFACE OF
ISOVOLUME CENTRES OF BUOYANCY
14
SHIP HYDROSTATIC – EQUILIBRIUM FOR PARTIALLY
SUBMERGED BODIES SURFACE OF
ISOVOLUME CENTRES OF BUOYANCY
The properties of the curve and therefore of the surface of the isovolume centres
of buoyancy in respect of a given hull volume  are:
at the various inclinations, for a given hull volume  and a given direction of
inclination, the same curve of the isovolume centres of buoyancy is always
obtained;
■ a tangent to the curve of the isovolume centres of buoyancy and parallel to a
generic waterline defines, at the tangent point, the centre of buoyancy
corresponding to that waterline;
■
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SHIP HYDROSTATIC – EQUILIBRIUM FOR PARTIALLY
SUBMERGED BODIES
SURFACE OF CENTRES OF BUOYANCY
■
the line perpendicular to the curve at points B, B1, B2… defines the direction of
the positive buoyancy passing through that centre of buoyancy;
■
the curve is convex, i.e. situated entirely above the tangent plane parallel to
the original waterline, in respect of any tangent plane parallel to a generic
waterline.
The above may also be applied to the surface of the isovolume centres of
buoyancy where tangent planes are considered instead of tangents.
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SHIP HYDROSTATIC – EQUILIBRIUM FOR PARTIALLY
SUBMERGED BODIES METACENTRIC EVOLUTE
Let’s consider a free floating ship of displacement  and hull volume  = / and
apply small isovolume transverse inclinations along the Y axis, assuming that the
isovolume centres of buoyancy obtained lie on the plane of inclination XZ.
17
SHIP HYDROSTATIC – EQUILIBRIUM FOR PARTIALLY
SUBMERGED BODIES METACENTRIC EVOLUTE
On this plane XZ lie the corresponding lines of action of the buoyancies and,
while the centres of buoyancy describe the curve of the isovolume centres of
buoyancy relative to the transverse inclinations along the Y axis, the generic
intersections M1 of two buoyancies corresponding to infinitesimal variations of
inclination describe another line. The latter curve, termed E, is the geometrical
locus of the centres of curvature of the curve of isovolume centres of buoyancy
and is known as the metacentric evolute[1].
18
SHIP HYDROSTATIC – EQUILIBRIUM FOR PARTIALLY
SUBMERGED BODIES METACENTRIC EVOLUTE
[1] We know from geometry that, given a plane curve s, taking a point A and a
point B very near to A, the intersections of the perpendicular to the curve at points
A and B result in the centre O of an osculating circumference of radius OA; the
circumference overlaps the curve s at points near A.
It is shown that the centre O and the radius OA, as well as being the centre and
radius of curvature of the osculating circumference, are also the centre and
radius of curvature of the curve s in A.
19
SHIP HYDROSTATIC – EQUILIBRIUM FOR PARTIALLY
SUBMERGED BODIES METACENTRIC EVOLUTE
The infinitesimal lengths B - B1, B2 - B3, etc. of the curve corresponding to
infinitesimal variations of inclination can be considered arcs of a circumference
whose centre is M, M1, …..Mn.
20
SHIP HYDROSTATIC – EQUILIBRIUM FOR PARTIALLY
SUBMERGED BODIES METACENTRIC EVOLUTE
The points of the evolute are called metacentric points and correspond to the
centres of buoyancy defined earlier.If, starting from the upright position of the
vessel, we consider infinitesimal inclinations, the centre of buoyancy B will pass
through B1, describing an infinitesimal trajectory.
The two lines of action of the buoyancy passing through B and B1 will meet at a
point M lying on the plane of symmetry.
21
SHIP HYDROSTATIC – EQUILIBRIUM FOR PARTIALLY
SUBMERGED BODIES METACENTRIC EVOLUTE
This point is called the initial transverse
metacentre of the hull of volume  and
centre B; as the inclination increases the
metacentric points leave the diametrical
plane and move on the metacentric
evolute.
The lines of action of the buoyancy
intersect the diametrical plane (line of
action of the buoyancy with upright ship)
at points p called false metacentres
relative to the direction of inclination Y of
the hull of volume .
Therefore the initial transverse metacentre
of the hull can also be defined as the limit
which the false metacentres tend to
approach
as
the
inclination

approaches zero.
The segment BM labelled r is known as
the transverse metacentric radius of the
hull of volume  and centre B.
22
SHIP HYDROSTATIC – EQUILIBRIUM FOR PARTIALLY
SUBMERGED BODIES METACENTRIC RADII
It is shown that the metacentric radius BM of a hull of volume  relative to the direction of
inclination Y[1] is:
r
Iy

where Iy is the moment of inertia of the initial waterplane surface in respect of the centroid
axis parallel to the direction of inclination Y.
It follows that for isovolume inclinations, the volume and buoyancy being constant, the
metacentric radius will vary as a function of the moment of inertia which, in turn, depends on
the axis of inclination and the inclination itself.
The metacentric radius BML of the same hull in respect of the direction X will be:
R
Ix

Considering the L/B ratio of ships it is evident that, Ix and Iy being calculated in respect of
the main axes of inertia of the vessel, they will be the maximum and minimum values and
the moment of inertia in respect of any other axis will be somewhere between the two:
Ix  I  Iy
Since:
Ix  Iy
therefore:
R  r
23
SHIP HYDROSTATIC – EQUILIBRIUM FOR PARTIALLY
SUBMERGED BODIES METACENTRIC RADII
[1] The moment of inertia of a plane surface in respect of any axis of the plane is
the product of the elementary area and the square of the distance from that axis.
Therefore:
Iy= x2 da
Iy=  y2 da
The waterplane surface varies with the various transverse inclinations; therefore,
once the centre of the new area and the associated main axes have been
established, the moments of inertia will be calculated in respect of these new
axes.
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SHIP HYDROSTATIC – EQUILIBRIUM FOR PARTIALLY
SUBMERGED BODIES METACENTRIC RADII
25
SHIP HYDROSTATIC - EQUILIBRIUM STABILITY
FOR FREE FLOATING OBJECTS
Having defined the geometric characteristics and the equilibrium conditions for a
free floating body subject to inclinations in various directions, we will now analyse
the stability conditions for such equilibrium.
First of all an examination is made of all the transverse isovolume inclinations,
which, for conventional vessels, are intuitively the most critical in terms of stability.
From the qualitative standpoint, it is noted that for isovolume inclinations the
magnitude of the forces acting, displacement and buoyancy, will not vary in the
modulus; however, as the direction of application of the buoyancy varies with the
shift of the centre of buoyancy , a torque is created generating a moment.
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SHIP HYDROSTATIC - EQUILIBRIUM STABILITY
FOR FREE FLOATING OBJECTS
The moment due to the torque (displacement  and buoyancy S) may be righting (a), zero
(b) or heeling (c); the moment depends on the trajectory of the isovolume centres of
buoyancy in relation to the angle of heel.
For infinitesimal inclinations, the action line of the buoyancy will bisect the initial transverse
metacentre, the centre of curvature of the trajectory at the origin.
The nature of the torque generated depends on the reciprocal positions of the transverse
metacentre of the hull and the centroid of the ship; specifically, it will be[1]:
■
■
■
RIGHTING if the centroid is below the metacentre;
ZERO if the centroid coincides with the metacentre;
HEELING if the centroid is above the metacentre.
[1] If G is below B (special cases, sailing boats) the equilibrium is always stable.
27
SHIP HYDROSTATIC - EQUILIBRIUM STABILITY
FOR FREE FLOATING OBJECTS
Using a to indicate the height of the centroid G above the initial centre of
buoyancy B, the equilibrium stability conditions can be expressed as follows:
(r – a)  0 : STABLE EQUILIBRIUM
■ (r – a) = 0 : STABLE, UNSTABLE OR NEUTRAL EQUILIBRIUM[1]
■ (r – a)  0 : UNSTABLE EQUILIBRIUM
■
[1] For further information it is necessary to esamine the pattern of the metacentric
evolute about the upright position.
28
SHIP HYDROSTATIC – TRANSVERSE STABILITY TORQUE
We will now examine the general expression of the stability torque moment with
variation of heel and fixed displacement (isovolume inclinations).
For the sake of simplicity it is assumed that the position of the ship’s centroid does
not vary with variation of heel (absence of mobile or shifting loads); it is also
assumed that the heel does not cause variations in trim.
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SHIP HYDROSTATIC – TRANSVERSE STABILITY TORQUE
The torque generated by the weight  and the buoyancy S, called transverse stability torque, is:
M=(GE)
With:
P : transverse false metacentre in respect of a generic angle ;
h : false metacentric height above the centre of buoyancy B;
a : height of the centroid above the initial centre of buoyancy
the following formula is obtained:
M = (GP) sin = (h-a) sin
where the variables, with fixed displacement, are:
 : angle of heel;
h = h () : false metacentric height as a function of the angle of heel.
30
SHIP HYDROSTATIC – STABILITY DIAGRAM
In the most general case, the stability torque moment is:
M=(h-a) sin
where h and  are the variables. It is therefore necessary to determine the value of
M each time as a function of the angle .
This problem is tackled with the aid of a mathematical graph; this is called the
transverse static stability diagram (or curve).
The x-axis shows the angles of heel  and the y-axis plots the moments M or the
transverse righting levers or arms (h – a) sen, which differ from the moments on
account of the displacement.
31
SHIP HYDROSTATIC – STABILITY DIAGRAM
The stability diagram depends on the shape of the hull and the load conditions and varies from ship to ship. For any given vessel, it
depends on the weight distribution and displacement (and therefore draught).
The diagram normally shows only the positive angles  up to the angle of vanishing stability c; given the symmetry of the ship about
the longitudinal plane, the part of the diagram for negative angles would be the same as the positive part.
The points in the diagram where the y co-ordinate is zero are interesting, particularly the points  = 0 and  = c.
Here, the stability torque moment is zero so the ship is in equilibrium, though this equilibrium differs:
=0
The vessel is initially upright; if it is given a slight positive heel and left, it will be subjected to positive torque that will
tend to restore it to its initial position; hence this is a position of stable equilibrium;


= c
The vessel heels initially at the angle c, the angle of vanishing stability (between 60° and 90°).
If the ship is given a slight positive heel and left,
it will be subjected to negative torque that will
tend to upset it.
If it is given a negative heel, it
will be subjected to positive torque that will
tend to restore it to its initial position.
Therefore, the position  = c is one of unstable
equilibrium.
32
SHIP HYDROSTATIC – STABILITY DIAGRAM
These conditions appear on the stability diagrams taking the tangents to the curve
at the points of zero moment; if the semi-tangent above the axis of the X coordinates is positive, i.e. inclined in the direction of increasing , the equilibrium is
stable (a), if it is negative the equilibrium is unstable (b).
Using  to indicate the angle formed by such tangent with the axis of the X coordinates, the following situations may occur:
tg   0
tg  0
tg  = 0
STABLE EQUILIBRIUM
UNSTABLE EQUILIBRIUM
STABLE, UNSTABLE OR NEUTRAL EQUILIBRIUM
33
SHIP HYDROSTATIC –HEEL RESISTANCE COEFFICIENT
Let’s consider the diagram stability tangent traced from the origin: the value of the Y co-ordinate of this line for
 = 1 rad is:
 (r – a)
This expression is called the heel resistance coefficient and it is an index of the vessel’s ability to withstand
forces upsetting its initial position of stable equilibrium  = 0.
The difference (r – a) is called the transverse metacentric height and represents the height of the transverse
metacentre of the hull above the centre of gravity with the ship upright.
The value measured in a position of equilibrium other than the one previously considered ( = 0) also
measures the vessel’s
ability to withstand forces upsetting its initial position and is a
coefficient of resistance to heel starting from that position.
dM
d
34
SHIP HYDROSTATIC – RESIDUAL STABILITY
If we want to subject a vessel already heeled at an angle  to a further
infinitesimal heel d, we need to work on the resistance forces, particularly on the
transverse stability torque.
This work is roughly expressed as the product of the stability torque moment in the
condition  considered and the infinitesimal rotation d;
dL=Md=(h-a) sin d
represented by the area under the stability curve about the angle considered.
35
SHIP HYDROSTATIC – RESIDUAL STABILITY
The work necessary to move the ship from the upright position in stable
equilibrium to the heel angle  with the vessel stationary (no acceleration) in still
water without friction will be:


0
0
L   Md   (h  a) sin d
whose measurement, represented by the area of the stability diagram between
= 0 and  = , is known as the residual stability of the ship in respect of the heel
.
The total area under the stability curve up to the angle of vanishing stability
therefore measures the work needed to capsize the vessel, i.e. the total work that
the ship with its stability torque moment is capable of performing to withstand the
external forces acting to overturn it.
The measurement of this work is known as the total residual stability.
36
SHIP HYDROSTATIC - METACENTRIC METHOD
We have seen that the value of transverse stability torque for any angle of heel
can be calculated exactly using the formula:
M = (h-a) sin
For the solution to hydrostatic problems regarding small angles of heel and to
obtain rough approximations, the foregoing expression can be simplified.
It is noted, in fact, that for heel angles up to 10° - 12° the trajectory of isovolume
centers of buoyancy may be mistaken, with negligible error, for an arc of a
circumference with centre M and radius r.
37
SHIP HYDROSTATIC - METACENTRIC METHOD
This is akin to assuming that the lines of action of the buoyancy for such heels
(10°-12°) all pass through the initial transverse metacentre M, instead of through
their false metacentres, therefore:
M0=(r-a) sin
where, for a given displacement, only the variable  appears, while r is constant.
38
STABILITY OF FORM AND WEIGHT
By developing the product that appears at the 2nd member of the expression:
M=(h-a) sin
we obtain:
M=h sin-a sin
39
STABILITY OF FORM AND WEIGHT
Both terms represent the torque moment; more specifically:
 r sin  :
form stability torque containing the term r as a function only of the
geometric elements of the hull (obtainable from the lines plan);
 a sin  :
weight stability torque containing the term “a”, which depends on the
position of the centroid G of the ship and therefore of the weight
distribution on board (obtainable from the theorem of static
moments).
40
SHIP HYDROSTATIC
- REMARKS ON THE STABILITY DIAGRAM
For the same displacement the stability diagram is influenced by various
geometric and mechanical factors, such as the following:
HEIGHT OF SIDES
FORM OF SIDES
POSITION OF CENTROID
BREADTH OF SHIP
41
SHIP HYDROSTATIC - REMARKS ON THE STABILITY
DIAGRAM HEIGHT OF THE SIDES
The height of the sides and therefore the amount of freeboard affect the stability
diagram.
The diagram is the same until the heel is such as to submerge the ship’s stringer,
thereby decreasing the form stability.
42
SHIP HYDROSTATIC - REMARKS ON THE STABILITY
DIAGRAM FORM OF THE SIDES
The form of the sides above the waterline is important in that it affects the form
stability; if the sides are such that the waterplane area grows with heel, the
stability increases and vice versa.
It is noted how, for the three examples, the tangent at the origin is the same
insofar as the initial metacentric radius is the same.
43
SHIP HYDROSTATIC - REMARKS ON THE STABILITY
DIAGRAM POSITION OF THE CENTROID
The position of the centroid G influences weight stability and therefore
metacentric height; it is clear that it is important to have the centre of gravity as
low as possible.
44
SHIP HYDROSTATIC - REMARKS ON THE STABILITY
DIAGRAM BREADTH OF THE SHIP
The breadth of the waterplane is vital as far as form stability is concerned; it is an
important design parameter, whose value is determined on the basis of the stability
requirements to be obtained.
Ship 1 in particular lacks stable equilibrium, to reach it at an angle of 15°.
45
SHIP HYDROSTATIC - REMARKS ON THE STABILITY
DIAGRAM DISPLACEMENT
Another important parameter – held constant up till now – is the ship’s displacement; clearly,
for any given vessel as its load conditions vary, so will the stability diagram. We will now look
at the effect of displacement and analyze in particular the pattern of righting levers for both
military vessels and cargo ships.
Navy ships are designed so that the most favourable diagram in terms of stability is the full load
diagram in that the tanks are full so the centre of gravity is very low; also the hull forms are such
as to have the greatest breadth of the waterplane.
For cargo ships the situation is the opposite in that there is little variation in the breadth of the
waterplane (vertical sides) between the full load draught and the ballast draught; in addition at
full load the weight is up very high (full holds, containers on deck etc) while in ballast the
double bottoms, fuel tanks and ballast tanks are full.
46
SHIP HYDROSTATIC - VALUE OF THE METACENTRIC
HEIGHTSTIFF OT TENDER VESSEL
The heel resistance coefficient  (r – a) measures the resistance of the vessel to forces upsetting it at small
angles of heel in still water.
Depending on the degree of resistance, great or little, the ship is said to be stiff or tender.
It would seem advisable to build ships with high (r – a) values so as to reduce heeling to a minimum.
However, it should be stressed that a vessel with excessive stability in still waters tends greatly in rough seas to
maintain its deck parallel to the surface of the water and to follow the continuous wave motion.
A stiff ship in calm water is therefore “upset” under heavy weather conditions.
On the other hand, the property called platform stability, which is the tendency of the vessel to maintain its
decks parallel to rough seas, is of paramount importance.
This property is indispensable for ferries, ro-ro and cruise ships for passenger comfort and for navy ships for
weapons sighting systems, deck landing of aircraft and the proper operation of equipment in general.
What is to be avoided, in fact, is not so much shifting or speed, but rather the accelerations inevitably
associated with forces of inertia causing physical discomfort and equipment malfunction.
The stiffer the vessel, the more it tends to roll with a short period and high amplitude; the ship’s rolling period is
obtained from the following formula:
T  2n
Ig
(r  a)
(sec.)
where Ig is the mechanical moment of inertia, i.e. of mass, of the ship about its longitudinal axis passing
through its centre of gravity.
47 of
It is therefore readily apparent that T depends on the initial metacentric height and not on the magnitude
heel (isochronous inclinations).
DETERMINATION OF THE SHIP’S CENTRE OF GRAVITY
– STABILITY TEST
We have seen that the value of (r – a) depends on both geometric (r) and
mechanical (a) factors. While the determination of the former, using the lines plan,
does not pose any problem, there is considerable uncertainty concerning the
exact position of the centroid G.
During the design stage, the centre of gravity is estimated for the various load
conditions by analysing the weights constituting the ship and their position;
however, it is also necessary to carry out a practical test to determine the centroid
with greater precision and to verify the accuracy of the preliminary calculations as
well as the exact metacentric height in the various load conditions.
The stability test is normally performed:
after the ship has been launched;
■ after the ship has been fitted out and is in the desired load condition (for
contractual trials);
■ after the ship has been subjected to major conversions or alterations (so as to
update or amend the stability information);
■ periodically while the ship is in service (passenger vessels).
■
48
DETERMINATION OF THE SHIP’S CENTRE OF GRAVITY
– STABILITY TEST
The test is carried out in a protected area in still water and windless conditions, with
the vessel perfectly upright in the absence of significative trim and ensuring that
the mooring lines do not interfere with the inclination.
Before starting it must be checked that there are no shifting loads or free surface
effects (which must be taken into account) and that only personnel conducting the
test are on board.
A crane is to be arranged at the broadest part of the ship so as to transversely shift
the inclining weights. One or more pendulums are to be used to calculate the
heeling angles and heeling moment after each weight movement.
Once shifted, the weight p will create a torque moment balanced by the
transverse stability torque and the vessel will reach a position  obtainable from the
equality of the moments:
(h-a) sin = p x cos
49
ra 
px
 * tg
DETERMINATION OF THE SHIP’S CENTRE OF GRAVITY
– STABILITY TEST
(h-a) sin = px cos
where x is the transverse shift of the weight p.
The weight p is chosen so that the final inclination  is relatively small and,
consequently, the simplified metacentric method can be applied:
(h-a) sin = px cos
from which, by isolating “a”: a  r 
px
 * tg
ra 
px
 * tg
50
DETERMINATION OF THE SHIP’S CENTRE OF GRAVITY
– STABILITY TEST
The only unknown variable is “a”.
The draught marks are used to determine the isovolume upright draught; this
figure is used in the table of hydrostatic properties to find the displacement D, the
initial transverse metacentric radius r, and the vertical position of the centre of
buoyancy ZB.
Since p and x are given and the pendulums are used to measure the angle of
heel , it is simple to obtain the height of the centre of gravity G above the centre
S
of buoyancy
tg and therefore the height of G above the base line.
l
The pendulums terminate in a pool of water to reduce their oscillations; tg  is
obtained from the expression:
ar
px
 * tg
51
DETERMINATION OF THE SHIP’S CENTRE OF GRAVITY
– STABILITY TEST
Generally the angle found at the end of the test is in the range 1°3°, both so as to
be able to use the metacentric method and so that the centre of buoyancy does
not leave the plane of inclination and the inclination is purely transverse.
Naturally the test is repeated, shifting the weights to the opposite side and
comparing the results.
The amount of inclining weights needed can be easily predicted beforehand
since:
■
■
■
■
the displacement D of the ship is known approximately;
the metacentric height (r – a) is estimated based on the hull forms and the
preliminary index of the weights;
the quantity X is slightly less than the breadth of the ship;
the desired angle is roughly between 1°3°.
So, for example, if D = 7900 t, the breadth B = 18 m from which x  8 m, (r – a)  0,5
m and the angle   2°, then:
P
(r  a)
7900 * 0,5
tg 
tg (2)  15,56t
x
8
52
DETERMINATION OF THE SHIP’S CENTRE OF GRAVITY
– STABILITY TEST
Inclining test and lightweight check are to be carried out in compliance with the following
class and statutory requirements:
■
RINA Rules for the Classification of Ships part B ch.3 appendix 1 “Inclining Test and
Lightweight Check”;
■
SOLAS chapter II-1 part B reg.22;
■
Code on Intact Stability for All Types of Ships covered by IMO Instruments (IMO Resolution
A.749 (18) chapter 7 “Determination of lightship displacement and centres of gravity”;
Instructions to Surveyors II– B–2).
RINA Rules Pt B Ch 3 Appendix 1 takes partially into account also the content of the Code of
Intact Stability Chapter 7 - "Determination of Lightship Displacement and Centres of Gravity".
The Code on Intact Stability for All Types of Ships covered by IMO Instruments (IMO Resolution
A.749 (18)) is a not mandatory instrument and it is to be used as guidance only.
53
DETERMINATION OF THE SHIP’S CENTRE OF GRAVITY
LIGHTSHIP – LIGHTWEIGHT CHECK
Lightship is a ship complete in all respect, but without consumables, stores, cargo,
and crew and effects, and without any liquids on board except for machinery and
piping fluids, such as lubricants and hydraulics, which are at operating levels.
RINA may allow a lightweight check to be carried out in lieu of an inclining test in
case of:
A) an individual ship, provided basic stability data are available from the
inclining test of a sister ship and a lightweight check is performed in order to
prove that the sistership corresponds to the prototype ship (the result of the
lightweight check show a deviation from the displacement of the prototype
ship not greater than than 1%);
B) special types of ship, such as pontoons (centre of gravity to be considered at
the level of the deck);
C) special types of ship, such as catamarans (centre of gravity to be determined
by calculation on the basis of a detailed list of weights and their position).
54
DETERMINATION OF THE SHIP’S CENTRE OF GRAVITY
– STABILITY TEST SOLAS CHAPTER II-1 PART B REG.22
“Stability information for passenger ships and cargo ships*”
SOLAS chapter II-1 part B reg.22 “Stability information for passenger ships and cargo ships*”states the
following:
■
■
■
■
■
Every passenger ship regardless of size and every cargo ship having a length, as defined in the
International Convention on Load Lines in force, of 24 m and upwards, shall be inclined upon its
completion and the elements of its stability determined. The master shall be supplied with such
information satisfactory to the Administration as is necessary to enable him by rapid and simple
processes to obtain accurate guidance as to the stability of the ship under varying conditions of
service. A copy of the stability information shall be furnished to the Administration.
Where any alterations are made to a ship so as to materially affect the stability information supplied to
the master, amended stability information shall be provided. If necessary the ship shall be reinclined.
At periodical intervals not exceeding five years, a lightweight survey shall be carried out on all passenger
ships to verify any changes in lightship displacement and longitudinal centre of gravity. The ship shall
be re-inclined whenever, in comparison with the approved stability information, a deviation from the
lightship displacement exceeding 2% or a deviation of the longitudinal centre of gravity exceeding
1% of L is found or anticipated.
The Administration may allow the inclining test of an individual ship to be dispensed with provided basic
stability data are available from the inclining test of a sister ship and it is shown to the satisfaction of the
Administration that reliable stability information for the exempted ship can be obtained from such basic
data, as required by paragraph 1.
The Administration may also allow the inclining test of an individual ship or class of ships especially
designed for the carriage of liquids or ore in bulk to be dispensed with when reference to existing data
for similar ships clearly indicates that due to the ship's proportions and arrangements more than
sufficient metacentric height will be available in all probable loading conditions
* Refer to the Code on Intact Stability for All Types of Ships Covered by IMO Instruments adopted by the Organization by Resolution
A.749 (18). Refer also to: MSC/Circ. 456, Guidelines for the preparation of intact stability information; MSC/Circ.706, Guidance on
intact stability of existing tankers during transfer operations; and MSC/Circ.707, Guidance to the master for avoiding dangerous
situations in following and quartering seas.
55
DETERMINATION OF THE SHIP’S CENTRE OF GRAVITY
– STABILITY TEST REVISED SOLAS CHAPTER II-1 PART B-1
REG.5 “Intact stability information”applicable after 1 January 2009
56
DETERMINATION OF THE SHIP’S CENTRE OF GRAVITY
– STABILITY TESTREVISED SOLAS CHAPTER II-1 PART B-1
REG.5 “Intact stability information”applicable after 1 January 2009
57
DETERMINATION OF THE SHIP’S CENTRE OF GRAVITY
– STABILITY TEST PLANNING OF EXECUTION
Prior to the execution of the inclining test, it is submitted to the attention of the
attending surveyor a detailed calculation, showing that the below-mentioned
requirements are complied with during the foreseen test.
GENERAL CONDITIONS OF THE SHIP
INCLINING WEIGHTS
PENDULUMS
MEANS OF COMMUNICATIONS
DOCUMENTATION
58
DETERMINATION OF THE SHIP’S CENTRE OF GRAVITY
– STABILITY TEST PLANNING OF EXECUTION
GENERAL CONDITIONS OF THE SHIP
Prior to the test, the RINA Surveyor is to be satisfied of the following:
■
the weather conditions are to be favourable (still water and windless
conditions);
■
the ship is to be moored in a quiet, sheltered area free from extraneous forces,
such as to allow unrestricted heeling. The ship is to be positioned in order to
minimize the effects of possible wind, stream and tide;
■
the ship is to be transversely upright and the trim is to be taken not more than
1% of the length between perpendiculars. Otherwise, hydrostatic data and
sounding tables are to be available for the actual trim;
■
cranes, derrick, lifeboats and liferafts capable of inducing oscillations are to
be secured (no shifting loads);
■
main and auxiliary boilers, pipes and any other system containing liquids are
to be filled;
■
the bilge and the decks are to be thoroughly dried;
59
DETERMINATION OF THE SHIP’S CENTRE OF GRAVITY
– STABILITY TESTPLANNING OF EXECUTION
GENERAL CONDITIONS OF THE SHIP
■
preferably, all tanks are to be empty and clean, or completely full, in order to
minimize the relevant free surface correction. The number of tanks containing
liquids is to be reduced to a minimum taking into account the abovementioned trim. The shape of the tank is to be such that the free surface effect
can be accurately determined and remain almost constant during the test. All
cross connections are to be closed. In general, the total free surface
correction should not be greater than 100 mm: in particular cases, upon
authorization of this Head Office, greater values may be accepted but in any
case not exceeding 150 mm.
■
the weights necessary for the inclination are to be already on board, located
in the correct place;
■
all work on board is to be suspended and crew or personnel not directly
involved in the inclining test are to leave the ship;
■
the ship is to be as complete as possible at the time of the test. The number of
weights to be removed, added or shifted is to be limited to a minimum.
Temporary material, tool boxes, staging, sand, debris, etc., on board is to be
reduced to an absolute minimum.
60
DETERMINATION OF THE SHIP’S CENTRE OF GRAVITY
– STABILITY TESTPLANNING OF EXECUTION
INCLINING WEIGHTS
The total weight used is preferably to be sufficient to provide a minimum
inclination of 1° and a maximum of 4° of heel to each side.
Test weights are to be compact and of such a configuration that the VCG
(vertical centre of gravity) of the weights can be accurately determined.
Each weight is to be marked with an identification number and its weight.
Recertification of the test weights is to be carried out prior to the incline.
A crane of sufficient capacity and reach, or some other means, is to be available
during the inclining test to shift weights on the deck in an expeditious and safe
manner.
Water ballast is generally not acceptable as inclining weight.
61
DETERMINATION OF THE SHIP’S CENTRE OF GRAVITY
– STABILITY TESTPLANNING OF EXECUTIONPENDULUMS
The use of three pendulums is recommended but a minimum of two are to be
used to allow identification of bad readings at any one pendulum station.
However, for ships of length equal to or less than 30 m, only one pendulum can
be accepted.
They are each to be located in an area protected from the wind.
The pendulums are to be long enough to give measured deflections, to each side
of upright, of at least 10 cm even due to a partial shifting of the inclining weights
to each side of the vessel.
To ensure recordings from individual instruments are kept separate, it is suggested
that the pendulums be physically located as far apart as practical.
The use of an inclinometer or U-tube is to be considered in each separate case. It
is recommended that inclinometers or other measuring devices only be used in
conjunction with at least one pendulum.
62
DETERMINATION OF THE SHIP’S CENTRE OF GRAVITY
– STABILITY TESTPLANNING OF EXECUTION MEANS OF
COMMUNICATIONS - DOCUMENTATION
MEANS OF COMMUNICATIONS
Efficient two-way communications are to be provided between central control and
the weight handlers and between central control and each pendulum station. One
person at a central control station is to have complete control over all personnel
involved in the test.
DOCUMENTATION
The person in charge of the inclining test is to have available a copy of the
following plans at the time of the test:
• hydrostatic curves or hydrostatic data;
• general arrangement plan of decks, holds, inner bottoms,etc.;
• capacity plan showing capacities and vertical and longitudinal centres of
gravity of cargo spaces, tanks, etc. When water ballast is used as inclining
weights, the transverse and vertical centres of gravity for the applicable tanks,
for each angle of inclination, must be available
•
tank sounding tables.
•
draught mark location;
63
• docking drawing with keel profile and draught mark corrections (if available).
DETERMINATION OF THE SHIP’S CENTRE OF GRAVITY
– STABILITY TESTPLANNING OF EXECUTION
DETERMINATION OF THE DISPLACEMENT
The Society’s Surveyor is to carry out all the operations necessary for the accurate
evaluation of the displacement of the ship at the time of the inclining test, as listed below:
■
draught mark readings are to be taken at aft, midship and forward, at starboard and port
sides (additionally freeboard readings may be taken), the vessel is to be transversally
upright (heel not greater than 0.5 °);
■
The mean draught (average of port and starboard reading)is to be calculated for each
of the locations where draught readings are taken and plotted on the ship’s lines drawing
or outboard profile to ensure that all readings are consistent and together define the
correct waterline. The resulting plot is to yield either a straight line or a waterline which is
either hogged or sagged. If inconsistent readings are obtained, the freeboards/draughts
are to be retaken;
■
The specific gravity of the sea water is to be determined.Samples are to be taken from a
sufficient depth of the water to ensure a true representation of the sea water and not
merely surface water, which could contain freshwater from run off of rain. A hydrometer is
to be placed in a water sample and the specific gravity read and recorded. For large
ships, it is recommended that samples of the sea water be taken forward, midship and
aft, and the readings averaged. For small ships, one sample taken from midship is
sufficient. The temperature of the water is to be taken and the measured specific gravity
corrected for deviation from the standard, if necessary. A correction to water
specific gravity is not necessary if the specific gravity is determined at the inclining
experiment site. Correction is necessary if specific gravity is measured when the sample
temperature differs from the temperature at the time of the inclining (e.g., if the check of
specific gravity is performed at the office).Where the value of the average calculated
specific gravity is different from that reported in the hydrostatic curves, adequate
corrections are to be made to the displacement curve.
64
DETERMINATION OF THE SHIP’S CENTRE OF GRAVITY
– STABILITY TESTPLANNING OF EXECUTION
DETERMINATION OF THE DISPLACEMENT
■
all double bottoms, as well as all tanks and compartments which can contain
liquids, are to be checked, paying particular attention to air pockets which
may accumulate due to the ship’s trim and the position of air pipes.
■
it is to be checked that the bilge is dry, and an evaluation of the
liquids which cannot be pumped, remaining in the pipes, boilers,
condenser, etc., is to be carried out.
■
the entire ship is to be surveyed in order to identify all items which
need to be added, removed or relocated to bring the ship to the
lightship condition. Each item is to be clearly identified by weight and
location of the centre of gravity.
■
the possible solid permanent ballast is to be clearly identified and
listed in the report.
65
DETERMINATION OF THE SHIP’S CENTRE OF GRAVITY
– STABILITY TESTPLANNING OF EXECUTIONTHE INCLINE
The standard test generally employs eight distinct weight movements as shown in figure 1.
The weights are to be transversally shifted, so as not to modify the ship’s trim and vertical
position of the centre of gravity. After each weight shifting, the new position of the transverse
centre of gravity of the weights is to be accurately determined.
After each weight movement, the distance the weight was moved (centre to centre) is to be
measured and the heeling moment calculated by multiplying the distance by the amount of
weight moved.
The tangent is calculated for each pendulum by dividing the deflection by the length of the
pendulum.
The resultant tangents are plotted on the graphics shown in figure 2. The pendulum deflection
is to be read when the ship has reached a final position after each weight shifting.
During the reading, no movements of personnel are allowed.
66
DETERMINATION OF THE SHIP’S CENTRE OF GRAVITY
– STABILITY TEST USE OF BALLAST WATER
IN LIEU OF SOLID WEIGHTS
Where the use of solid weights to produce the inclining moment is demonstrated to be
impracticable, the movement of ballast water may be permitted as an alternative method.
This acceptance would be granted for a specific test only, and relevant test procedure is to
be submitted in advance to RINA Plan Approval Centre for approval.
As a minimal prerequisite for acceptability, the following conditions are to be required:
■
inclining tanks are to be wall-sided (regular shape) and free of large stringers or other
internal members that create air pockets;
■
tanks are to be directly opposite to maintain ship’s trim;
■
specific gravity of ballast water is to be measured and recorded;
■
pipelines to inclining tanks are to be full. If the ship’s piping layout is unsuitable for internal
transfer, portable pumps and pipes/hoses may be used;
■
blanks must be inserted in transverse manifolds to prevent the possibility of liquids being
“leaked” during transfer. Continuous valve control must be maintained during the test;
■
all inclining tanks must be manually sounded before and after each shift;
67
DETERMINATION OF THE SHIP’S CENTRE OF GRAVITY
– STABILITY TEST USE OF BALLAST WATER
IN LIEU OF SOLID WEIGHTS
■
vertical, longitudinal and transverse centres are to be calculated for each
movement;
■
accurate sounding/ullage tables are to be provided. The ship’s initial heel
angle is to be established prior to the incline in order to produce accurate
values for volumes and transverse and vertical centres of gravity for the
inclining tanks at every angle of heel. The draught marks amidships (port and
starboard) are to be used when establishing the initial heel angle;
■
Additional verification of the quantity shifted may be achieved by a flow
meter or similar device;
■
the time to conduct the inclining is to be evaluated. If time requirements for
transfer of liquids are considered too long, water may be unacceptable
because of the possibility of wind shifts over long periods of time.
68
DETERMINATION OF THE SHIP’S CENTRE OF GRAVITY
– STABILITY TEST EVALUATION OF RESULTS
Following the execution of the inclining test, it is to be submitted to the attention of
the attending surveyor a detailed calculation, showing that the above-mentioned
requirements have been complied with during the foreseen test.
In particular, the RINA attending surveyor is to verify the following:
■
measured pendulums’ deflections, to each side of upright, are comparable
and of at least 100 mm even due to a partial shifting of the inclining weights to
each side of the vessel;
■
it has been achieved a minimum inclination of 1° and a maximum of 4° of heel
to each side;
■
the total free surface correction is not greater than 100 mm: in particular cases,
upon authorization of the Plan Approval Centre, greater values may be
accepted but in any case not exceeding 150 mm.
69
DETERMINATION OF THE SHIP’S CENTRE OF GRAVITY
– STABILITY TEST FOR PONTOONS
An inclining experiment is not normally required for a pontoon, provided a
conservative value of the lightship vertical centre of gravity (KG) is assumed for
the stability calculations.
The KG can be assumed at the level of the main deck although it is recognized
that a lesser value could be acceptable if fully documented.
The lightship displacement and longitudinal centre of gravity should be
determined by calculation based on draught and density readings (lightweight
check).
70
DETERMINATION OF THE SHIP’S CENTRE OF GRAVITY
– STABILITY TESTOBJECTIONS AGAINST TEST
Inclination < 1°
Pendulums’ deflections < 100 mm
Pendulums’ deflections non regular and comparable
Weights movements less than those requested
Too many additional weights on board
Too many missing weights to be taken into account
Water ballast and/or consumables tanks partially filled
Free surfaces correction greater than 100-150 mm
OBJECTIONS ARE TO BE RAISED IMMEDIATELY
DURING THE TEST
AGAINST ANY MISTAKE OR IN CASE OF DOUBT
71
DETERMINATION OF THE SHIP’S CENTRE OF GRAVITY
– STABILITY TESTOBJECTIONS AGAINST TEST
It is quite difficult for the Plan Approval Centres to demand a second inclining test.
A new inclining test takes time and is expensive.
The ship’s delivery from the shipyard could be delayed.
The vessel might be already in service.
72
DYNAMIC STABILITY
We have seen that the total area under the stability curve up to the angle of
vanishing stability constitutes the total residual stability of the ship. The integral
curve for the static stability diagram is called the dynamic stability curve.
The y co-ordinates of this curve show for each angle, properly scaled, the area
under the static stability curve to the left of the angle considered from the upright
position.
The highest y co-ordinate of the dynamic stability curve measures the area of the
static stability curve (in way of the angle of vanishing stability).
73
STATIC ANALYSIS OF THE BEHAVIOUR OF A SHIP
SUBJECTED TO HEELING FORCES
Permanent transverse heeling forces can be represented on the stability diagram with the
values of their moments; the latter are in first approximation sinusoidal or cosinusoidal.
Let’s examine the stability conditions in which a ship is subjected to such forces.
The following considerations approach the problem statically without taking into account the
inertia forces at work or, consequently, the accelerations and kinetic energy.
The analysis is performed graphically, overlapping the stability curve (positive moments) with
the (negative) heeling moment and subtracting the corresponding y co-ordinates; this yields
the residual stability curve.
First we take cosinusoidal torque (curve II). When it is subjected to the heeling moment, the
ship will behave as if its stability diagram were the residual stability curve (III).
The stability properties change as is readily apparent from the analysis of the residual stability
curve:
■
the angle A corresponding to point A where the two curves meet is the new angle of
stable equilibrium (the two moments counterbalance each other);
■
the heel resistance coefficient changes; for  =A, it is represented by the segment that the
tangent to curve III in A bisects on the y co-ordinate corresponding to A + 1 rad;
■
the total residual stability represented by the positive area under diagram III changes; such
area will always be reduced;
■
the angle of vanishing stability is generally reduced.
74
STATIC ANALYSIS OF THE BEHAVIOUR OF A SHIP
SUBJECTED TO HEELING FORCES
75
STATIC ANALYSIS OF THE BEHAVIOUR OF A SHIP
SUBJECTED TO HEELING FORCES
HEELED SHIP
Similarly, the effect of sinusoidal
heeling forces is also considered in
the special case in which the two
curves intersect at point A.
Note that in this case  = 0 is no
longer a stable angle of equilibrium
but is unstable in that the resistance
coefficient is negative. It will be seen
that this condition describes a heeled
ship.
Thus the residual stability curve is
used to deal with all static inclination
problems.
76
DYNAMIC ANALYSIS OF THE BEHAVIOUR OF A SHIP
SUBJECTED TO HEELING FORCES
In the previous section we did not consider the influence of the accelerations and
consequent forces of inertia and potential energy at work every time a vessel is
subjected to variable forces over a short period.
The analysis is performed using the static stability diagram and overlapping a
cosinusoidal heeling moment (wind gust athwartships) applied as impelling force.
77
DYNAMIC ANALYSIS OF THE BEHAVIOUR OF A SHIP
SUBJECTED TO HEELING FORCES
DYNAMIC STABILITY ANGLE
In the initial instant for  = 0 the ship is no longer in equilibrium; due to the heeling moment it will
accelerate and heel at increasing speed. It will reach the angle s with a certain amount of
kinetic energy due to the excess work exerted by the heeling moment compared to the
stability torque (area OMS).
In s, since the moments are equal in absolute value, the acceleration will be eliminated and
then increase again with an opposite sign as heel grows. The figure shows how the righting
moment prevails and reduces the vessel’s acquired kinetic energy (OMS area), stopping it
altogether (area OMS = area SAB) when the work performed by the heeling moment (area
OMSBD) is equal to that performed by the stability torque (area OSABD).
The angle V, called the dynamic stability angle, is that at which the ship stops insofar as the
dynamic energies at work are in equilibrium. The area SAB represents the energy accumulated
by the ship as potential energy; however, the vessel is not in stable equilibrium and the righting
moment prevails over the heeling moment restoring the ship to the upright position.
Due to the dissipation effect caused by friction
from air and water, the ship will stop in way of
the static equilibrium angle s , after a certain
number of oscillations about such angle.
The behaviour of the vessel can therefore be
compared to that of a pendulum released
from a position other than that of equilibrium.
78
DYNAMIC ANALYSIS OF THE BEHAVIOUR OF A SHIP
SUBJECTED TO HEELING FORCES
From this analysis the importance of the total residual stability, i.e. the area under
the stability curve, is clear; if A1  A2, the ship will not find a position of dynamic
equilibrium and will overturn due to the impelling force.
79
ELEMENTS AFFECTING THE STABILITY OF THE SHIP
– VERTICAL WEIGHT SHIFT
When a weight p is raised vertically by an amount z, the ship's centre of gravity is
pz
raised vertically from G to G1 by an GG
amount:
1 

and by indicating with ZG and ZG1 the positions of the centroid in respect of the
base line, and with ZP and ZP’ the initial and final positions of the weight p, the
theorem of static moments will be as follows:
80
ELEMENTS AFFECTING THE STABILITY OF THE SHIP
– VERTICAL WEIGHT SHIFT
 x ZG1=  x Zg -(p x Zp)+ (pXZp)
from which, since Z=Z p-Zp and
GG1 = ZG1-ZG, the foregoing relation is
obtained.
It immediately follows that the
metacentric height will vary as follows:
(r-a)’ =
ra
the
pz

Whether this amount increases or decreases
depends on whether the shift is downwards
or upwards. Note in particular that an
upward shift is critical for stability.
As a result of the weight shift, the vessel's
static stability curve will change as
follows:
81
ELEMENTS AFFECTING THE STABILITY OF THE SHIP
– VERTICAL WEIGHT SHIFT
As a result of the weight shift, the vessel's static stability curve will change as
follows:
M a  (h  a' ) sin   (h  (a 
pz
)) sin 

by subtracting from the initial stability curve a sinusoidal moment
ma=p z sin
shown in the figure with the fictitious torque p/–p, obtained assuming the initial
centroid G fixed and applying in the initial position of the weight the forces p and
-p, which counterbalance each other but generate with force p applied to the
final position the inclining moment now introduced.
As is apparent from the residual stability curves, an upward weight shift leads to a
reduction in the heel resistance coefficient, total residual stability and angle of
vanishing stability.
Conversely, a downward weight shift generally brings about an increase in these
82
values.
ELEMENTS AFFECTING THE STABILITY OF THE SHIP
– HEELED SHIP
If the weight is shifted far enough upwards so that the centre of gravity is above the
initial transverse metacentre M, the ship will no longer be in a stable upright
position in that r – a < 0.
Accordingly, it is necessary to analyse the behaviour of the vessel around the
upright position, this being influenced by the form of the metacentric evolute.
83
ELEMENTS AFFECTING THE STABILITY OF THE SHIP
– VERTICAL WEIGHT SHIFT HEELED SHIP
In fact, if the evolute has initially descending branches the ship will be unable to
find any position of stable equilibrium and will capsize.
If, on the other hand, the evolute has initially ascending branches the ship will list
until it finds an angle of stable equilibrium, known as the listing angle, which is
obtained by taking from the centroid G1 the tangent to the evolute itself.
84
ELEMENTS AFFECTING THE STABILITY OF THE SHIP
– VERTICAL WEIGHT SHIFT
Consequently, a heeled ship lists not because of the eccentricity of the load but
because of initial instability. We will see shortly that there are many causes of
listing and that to right the vessel we need to remove these causes and, in any
case, move the loads down so that the centroid G is below the initial transverse
metacentre; if we were merely to move the load transversely to the other side
from that of inclination, the best we could hope for would be for the vessel to heel
suddenly to that side, albeit at a greater inclination than previously.
All of the phenomena discussed here can be conveniently studied by using
stability diagrams.
85
ELEMENTS AFFECTING THE STABILITY OF THE SHIP
– TRANSVERSE WEIGHT SHIFT
When the weight p shifts transversely by an amount x, the vessel is subjected to
heeling torque equal to:
M=p x cos 
causing heel . Such inclination will be easy to determine from the equilibrium of
the moments:
p x cos  =  (h-a) sin 
hence:
tan  
px
(h  a)
86
ELEMENTS AFFECTING THE STABILITY OF THE SHIP
– TRANSVERSE WEIGHT SHIFT
If the angle of heel does not exceed
10°-12°, the simplified metacentric
method may be used, replacing h
with r.
By analysing the residual stability
curve obtained overlapping the intact
stability diagram with the heeling
moment p x cos , it is plain that the
initial position = 0 is no longer one of
equilibrium.
The angle S will be the new angle of
stable equilibrium used to measure
the total residual stability and heel
resistance coefficient for the new
condition.
87
ELEMENTS AFFECTING THE STABILITY OF THE SHIP
– SUSPENDED LOADS
A weight p, suspended from any raised point and left free to oscillate, affects the
stability characteristics of the vessel.
Indeed, for any inclination the line of action of this weight passes through the point
of suspension and, for stability purposes, it is as if the load was applied precisely at
this point.
This will be akin to the case of a vertical weight shift and the transverse static
stability curve will be reduced by a sinusoidal inclining force.
Inclining torque may be assumed of the following type:
Mt = p t sin 
where:
t sen  is the fictitious torque lever p, -p created by suspension of the load;
■ t is the height of the point of suspension.
■
The centre of gravity will be raised by the amount pt/delta (being delta the ship’s
displacement), which also represents the reduction of the metacentric height.
88
ELEMENTS AFFECTING THE STABILITY OF THE SHIP –
SUSPENDED LOADS
89
ELEMENTS AFFECTING THE STABILITY OF THE SHIP
– SUSPENDED LOADS
It is readily apparent from the above that it is vitally important, when carrying
raised weights of a certain magnitude, to secure them with chains, lashing and
other means to reduce swinging.
This is both to limit the reduction in transverse stability when the vessel is in a critical
situation and to prevent the load from damage as well as from causing damage or
injury.
90
ELEMENTS AFFECTING THE STABILITY OF THE SHIP
– SHIFTING LOADS
Where an initially stable and upright ship carries an unlashed load and the vessel
lists for any reason, this may cause considerable problems to stability. Indeed,
when the angle of heel exceeds the breakout friction angle, the body will start to
slide or roll until it hits an obstacle such as a bulkhead. If this angle is not
exceeded, the load will behave as if it were tightly secured to the ship.
This is akin to the case of a transverse weight shift equivalent to a cosinusoidal
heeling action given by:
mx = p x cos theta
where x is the transverse shift of the weight p.
This transverse shift is sudden and uncontrolled and therefore dangerous for the
crew, structures and equipment; in the event of a ship carrying heavy loads such
as plates or coils, the shifting of such cargoes may capsize the vessel, particularly
in critical sea states.
Consequently, it is vitally important prior to departure to carefully lash material
and cargo that would otherwise be left to move freely in response to the
movements of the ship.
91
ELEMENTS AFFECTING THE STABILITY OF THE SHIP
– BULK LOADS
The presence on board of bulk loads (cereals, grain, coal, minerals) causes
problems similar to those discussed for shifting and liquid loads.
A small angle of heel, less than the angle of repose, will not cause any change in
stability insofar as the cargo will behave as if it were tightly secured to the ship.
As soon as such angle is exceeded, the material will begin to shift and will not
return to its original position even if the vessel is restored to its upright position. This
may be dangerous, even for angles less than the angle of slide, due to the
dynamic forces generated by accelerations arising in very heavy seas.
92
ELEMENTS AFFECTING THE STABILITY OF THE SHIP
– BULK LOADS
The angle of repose is the angle of the pile formed when the bulk material is
poured onto a horizontal surface: it corresponds to the angle of internal friction of
the material.
This angle varies for different materials and is generally around 30°, though it may
decrease markedly in response to the dynamic forces mentioned above.
93
ELEMENTS AFFECTING THE STABILITY OF THE SHIP
– BULK LOADS
To reduce the danger from the shifting of bulk material, specially designed selftrimming bulk carriers are used to prevent empty spaces from developing in the
wings of the hold.
Small vessels without self-trimming holds need to be filled to the top or, otherwise,
mobile longitudinal divisions made of wood or plates are inserted during loading.
In every compartment partially filled with grain, all free grain surfaces are to be
levelled after loading; in every filled hold, the space below the hatch cover is filled
with bagged cargo or weights.
94
ELEMENTS AFFECTING THE STABILITY OF THE SHIP
– LIQUID LOADS
Among the loads constituting the displacement of a ship there are always liquid
cargoes or consumables that may pose stability problems.
If the liquid were to fill a receptacle or tank completely, it would behave like a
solid load rigidly connected to the hull.
Conversely, if a liquid load of weight P and density G partially fills a compartment,
its fluidity will cause it to deform when the ship lists so that the free surface remains
parallel to the new waterline; thus its centroid, which always coincides with the
centre of volume, will move in respect of the walls of the compartment and the
hull, describing a curve and causing a shift in the centre of gravity of the ship,
varying depending on the inclination.
The effect of the liquidity of the load on the transverse stability of the vessel can be
compared to a weight P which is free to shift along a curve g lying on the plane ZX.
We can also assume the load itself suspended at point O, the centre of curvature of
curve g with the ship upright, similar to the case of a shifting load.
95
ELEMENTS AFFECTING THE STABILITY OF THE SHIP
– LIQUID LOADS
96
ELEMENTS AFFECTING THE STABILITY OF THE SHIP
– LIQUID LOADS
Accordingly, it is assumed that the ship behaves as if its heel resistance coefficient
and the metacentric height, rather than being:
C = (r-a) and (r-a) as for a solidified liquid, were: C’ = (r-a)-pt and where t is
the radius of curvature of curve G at point g, the centroid of the liquid with the ship
upright.
(r  a 
pt
)

97
ELEMENTS AFFECTING THE STABILITY OF THE SHIP
– LIQUID LOADS
Since curve g is simply the trajectory of the centre of volume of the liquid when
the ship heels, it will coincide with the trajectory c of the isovolume centres of
buoyancy of the receptacle R whenever it is likened to a liquid hull whose
waterline coincides with the external liquid level and when in such conditions it is
subjected to isovolume inclinations in direction y.
98
ELEMENTS AFFECTING THE STABILITY OF THE SHIP
– LIQUID LOADS
Therefore t is simply the metacentric radius of the inclinations in direction y of the internal liquid
hull of volume
constituting the receptacle R; this metacentric radius is:
t = i/V
V 
P

where i is the moment of inertia of the free surface level of the liquid in respect of the centroid
axis parallel to the ship’s plane of symmetry.
Consequently, it can be said that the reduction in initial stability due to the liquidity of the load:
does not depend on:
■ the weight p of the liquid cargo;
■ the volume v of the liquid cargo;
■ the height of its free surface in respect of the base line;
■ the distance from the axis of inclination of the ship
does depend on:
■ its density;
■ the moment of inertia i of its free surface level with the ship upright.
The presence of free surface liquid cargo therefore places the ship of metacentric radius r in
the same condition of initial stability as another with the same displacement and the same
value of the term a, but effective metacentric radius :
If Ω = ω then
  r (1 
i
)
Iy
  r (1 
 i
)
 Iy
r
Iy

99
ELEMENTS AFFECTING THE STABILITY OF THE SHIP
– LIQUID LOADS
If there is more than one free surface liquid load, then the expression of the heel
resistance coefficient will include the summation of the products of the individual
densities  for the respective moments of inertia of area i:
C’=(r-a)-i
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INTACT STABILITY - CLASS REQUIREMENTS
IACS Unified Requirement L1 “Intact stability – matter of class” requires the
following:
All new ships with a length of 24 m and above will be assigned class only after it
has been demonstrated that their intact stability is adequate for the service
intended.
Adequate intact stability means compliance with standards laid down by the
relevant Administration or those of the Classification Society taking into account
the ship’s size and type. The level of intact stability for ships of all sizes in any case
should not be less that that provided by IMO Resolution A.749 (18) “Code on
Intact Stability for All Type of Ships Covered by IMO Instruments”, chapters 3.1, 3.2
and 4.1 as amended by MSC Resolution 75(69).
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INTACT STABILITY - CLASS REQUIREMENTS
RINA Rules for the Classification of Ships (Pt.B Ch 3 Sec 1) request that “all ships
equal to or greater than 24 m in length may be assigned class only after it has
been demonstrated that their intact stability is adequate for the service intended”.
Adequate intact stability means compliance with the requirements specified in the
RINA Rules taking into account the ship’s size and service.
For classification purposes, for all type of ships, the intact stability criteria to be
complied with are laid down in Part B Chapter 3 Section 2 “Intact Stability” and
coincide with those included in “Code on Intact Stability for All Type of Ships
Covered by IMO Instruments”.
For each service notation, specific intact stability criteria are laid down in the
relevant chapter of Part E (for example: tugs, supply vessels, fire-fighting vessel,
cement carrier, etc.).
102
INTACT STABILITY - CLASS REQUIREMENTS
Evidence of approval by the Administration may be accepted for the purpose of
classification.
When the Administration of the State whose flag the ship is entitled to fly has issued
specific rules covering stability, RINA may accept such rules for classification
purposes in lieu of those given in the Part B chapter 3.
In case of application of the above requirements, an appropriate entry is made in
the classification files of the ship.
103
INTACT STABILITY - STATUTORY REQUIREMENTS
IMO Resolution MSC.75 (69) “Code on Intact Stability for All Type of Ships Covered
by IMO Instruments” as amended by Resolution MSC 75(69) supersedes the
following IMO recommendations, which application was not compulsory for
statutory certification purposes:
■
Recommendation on intact stability for passenger and cargo ships under 100
metres in length (resolution A.167(ES.IV));
■
Amendments to the recommendation on intact stability for passenger and
cargo ships under 100 metres in length (resolution A.167 (ES.IV)) with respect to
ships carrying deck cargoes (resolution A.206(VII));
■
Recommendation on intact stability of fishing vessel (resolution A.168(ES.IV));
■
Recommendation on a severe wind and rolling criterion (weather criterion) for
the intact stability of passenger and cargo ships of 24 metres in length and
over (resolution A.562 (14)).
The Code recommends stability criteria and other measures for ensuring the
safe operation of all ships to minimize the risk to such ships, to the personnel on
board and to the environment.
104
INTACT STABILITY - STATUTORY REQUIREMENTS
The recommended criteria are relevant to:
■
The minimum area under the righting lever curve (GZ curve) up to 30°(0.055 metre-radian);
■
The minimum area under the righting lever curve (GZ curve) up to 40°(0.09 metre-radian);
■
The minimum area under the righting lever curve (GZ curve) between 30° and 40° (0.03
metre-radian);
■
The minimum righting lever GZ (0.20 m);
■
The minimum angle at which minimum righting lever GZ occurs (30°);
■
The minimum angle at which it occurs the maximum righting arm (preferably exceeding
30° but not less than 25°);
■
The minimum initial metacentric height GM (0.15 m);
■
The severe wind and rolling criterion (weather criterion), which refers to the ability of a ship
to withstand the combined effects of beam wind and rolling.
The “Code on Intact Stability” is recalled as a footnote in SOLAS Part B “Subdivision and
Stability” Chapter II-1 regulation 22 “Stability Information for passenger ships and cargo ships”
and, therefore, is not a compulsory instrument to be applied for statutory certification purposes.
105
INTACT STABILITY - STATUTORY REQUIREMENTS
MAXIMUM PERMISSIBLE KG OR MINIMUM
PERMISSIBLE GM CURVES
The International Convention for the Safety of Live at Sea (SOLAS) chapter II-1 part
B “Subdivision and stability” and chapter II-1 part B-1 “Subdivision and damage
stability of cargo ships” request that the master shall be supplied with such
reliable information as is necessary to enable him by rapid and simple process to
obtain accurate guidance as to stability of the ship under varying condition of
service.
The information shall include a curve of minimum operational metacentric height
(GM) versus draught which assures compliance with the relevant intact and
damage stability requirements, alternatively a corresponding curve of the
maximum allowable vertical centre of gravity (KG) versus draught, or with
equivalents of either of these curves.
106
STATUTORY AND CLASS (ADDITIONAL CLASS NOTATION DMS)
DAMAGE STABILITY REQUIREMENTS
In the past, followings casualties occurred at sea with heavy loss of human lives
and severe pollution, damage stability requirements have been developed by
the International Maritime Organization for statutory certification purposes and
included in international conventions, such as for example the International
Convention for the Safety of Live at Sea and the International Convention for the
Prevention of Pollution from Ships.
Nowadays, both international conventions and non-compulsory statutory
instruments request damage stability calculations to be carried out, depending
on the ship type (passenger or cargo ship), the assigned freeboard (B reduced,
tankers having length greater than 150 m, etc.) and the kind of cargo carried (oil,
chemical, gas cargo).
For classification purposes, RINA Rules for the Classification of Ships foresee
damage stability requirements only for ships to be assigned with the additional
class notation Damage Stability (DMS), in compliance with part F ch.10.
107
STATUTORY AND CLASS (ADDITIONAL CLASS NOTATION DMS)
DAMAGE STABILITY REQUIREMENTS
Damage stability calculations are required in order to achieve a minimum
degree of safety after flooding.
In order to assess the behavior of the ship after damage, two approaches have
been developed:
■the
deterministic approach; and,
■the
probabilistic approach,
which are to be applied depending on the ship type as specified in the
applicable statutory instruments and RINA Rules for the Classification of Ships (Pt.
F Ch 10 Sec 11).
108
STATUTORY AND CLASS (ADDITIONAL CLASS NOTATION DMS)
DAMAGE STABILITY REQUIREMENTS
DETERMINISTIC APPROACH
The deterministic approach is based on standard dimensions of damage extending anywhere along the ship’s
length or between transverse bulkheads depending on the relevant requirements.
The consequence of such standard of damage is the creation of a group of damage cases, the number of
which, as well as the number of compartments involved in each case, depend on the ship’s dimensions and
internal subdivision.
For each loading condition, each damage case is to be considered, and all the applicable criteria are to be
complied with.
In the past, for statutory certification purposes, different deterministic methods in damage stability have been
developed and included in the following statutory instruments:
■
■
■
■
■
■
■
for passenger ships, the International Convention for the Safety of Live at Sea chapter II-1 part B
Subdivision and stability;
for oil tanker, the International Convention for the Prevention of Pollution from Ships, 1973 as amended;
for chemical tankers, the International Code for the Construction and Equipment of Ships Carrying
Dangerous Chemicals in Bulk (IBC Code);
for gas carriers, the International Code for the Construction and Equipment of Ships Carrying Liquefied
Gases in Bulk (IGC Code);
for supply vessels, the Guidelines for the design and construction of offshore supply vessel (IMO Resolution
A.469 (XII));
for special purpose ships, the Code of Safety for Special Purpose Ships (IMO resolution A.534 (13) as
amended;
for ships assigned with a reduced B-type freeboard and tankers having length greater than 150 m, the
International Convention on Load Lines 1966 as amended, regulation 27 (damage stability requirements);
The deterministic methods to be applied for classification purposes coincide with those established by the
above-mentioned statutory instruments and are reported in RINA Rules for the Classification of Ships Pt. F109
Ch 10
Sec 11.
STATUTORY AND CLASS (ADDITIONAL CLASS NOTATION DMS)
REQUIREMENTSPROBABILISTIC APPROACH
The probabilistic concept takes the probability of survival after collision as a
measure of ship safety in the damaged condition, referred to as the attained
subdivision index A.
The damage stability calculations are performed for a limited number of draughts
and relevant GM values in order to draw a minimum GM curve where the attained
subdivision index A achieves the minimum required level of safety R.
For cargo ships, each case of damage is not required to comply with the
applicable criteria, but the attained index A, which is the sum of the contribution
of all damage cases, is to be equal to or greater than R.
The probabilistic method developed on the basis of the above-mentioned
concepts is detailed in RINA Rules for the Classification of Ships Part F Section 11
App 1 and coincide with that included in International Convention for the Safety of
Live at Sea chapter II-1 part B-1 “Subdivision and damage stability of cargo ships”.
As a general rule, the probabilistic method applies to cargo ships of a length not
less than 80 m, and for which no deterministic methods apply.
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PROGRESSIVE FLOODING - OPENINGS
Progressive flooding is the additional flooding of spaces which were not previously assumed to
be damaged. Such additional flooding may occur through openings or pipes as indicated in
the following.
The openings may be listed in the following categories, depending on their means of closure:
UNPROTECTED
Unprotected openings may lead to progressive flooding if they are situated within the range of
the positive righting lever curve or located below the final waterline after damage.
Unprotected openings are openings which are not fitted with at least weather tight means of
closure.
■
WEATERTIGHT
Openings fitted with weather tight means of closure are not able to sustain a constant head of
water, but they can be intermittently immersed within the positive range of stability.
Weather tight openings may lead to progressive flooding if they are located below the final
waterline after damage..
■
WATERTIGHT
Internal openings fitted with watertight means of closure are able to sustain a constant head of
water corresponding to the distance between the lowest edge of this opening and the
bulkhead/freeboard deck.
Air pipe closing devices complying with normal classification requirements may not be
considered watertight, unless additional arrangements are fitted in order to demonstrate that
such closing devices are effectively watertight.
111
■
PROGRESSIVE FLOODING - PIPES
Progressive flooding through pipes may occur when:
■
■
the pipes and connected valves are located within the assumed damage, and no valves
are fitted outside the damage;
the pipes, even if located outside the damage, satisfy all of the following conditions:
a) the pipe connects a damaged space to one or more spaces located outside the
damage;
b) the highest vertical position of the pipe is below the waterline; and,
c) no valves are fitted.
The possibility of progressive flooding through ballast piping passing through the assumed
extent of damage, where positive action valves are not fitted to the ballast system at the
open ends of the pipes in the tanks served, is to be considered.
Where remote control systems are fitted to ballast valves and these controls pass through the
assumed extent of damage, then the effect of damage to the system is to be considered to
ensure that the valves would remain closed (fail safe) in that event.
If pipes, ducts or tunnels are situated within assumed flooded compartments, arrangements
are to be made to ensure that progressive flooding cannot thereby extend to compartments
other than those assumed flooded.
However, RINA may permit minor progressive flooding if it is demonstrated that the additional
flooding of those compartments cannot lead to the capsizing or the sinking of the ship.
112for
Requirements relative to the prevention of progressive flooding are specified in RINA Rules
the Classification of Ships Pt C, Ch 1, Sec 10, [5.5].
INFORMATION TO THE MASTER – STABILITY BOOKLET
CLASS REQUIREMENTS
Each ship is to be provided with a stability booklet, approved by RINA, which
contains sufficient information to enable the Master to operate the ship in
compliance with the applicable requirements contained in this Section.
Where any alterations are made to a ship so as to materially affect the stability
information supplied to the Master, amended stability information is to be
provided.
If necessary the ship is to be re-inclined.
Stability data and associated plans are to be drawn up in the official language or
languages of the issuing country.
If the languages used are neither English nor French the text is to include a
translation into one of these languages.
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INFORMATION TO THE MASTER – STABILITY BOOKLET
STATUTORY REQUIREMENTS
The following statutory instruments include requirements relevant
information to be supplied to the master concerning the ship’s stability.
to the
■
International Convention on Load Lines 1966 as amended, regulation 10
“Information to be supplied to the master”;
■
International Convention for the Safety of Live at Sea (SOLAS) chapter II-1 part
B Subdivision and stability reg. 8 “Stability of passenger ships in damaged
conditions” and 22 “Stability information for passengers ships and cargo ships”;
■
International Convention for the Safety of Live at Sea (SOLAS) chapter II-1 part
B-1 Subdivision and damage stability of cargo ships re. 25-8 “Stability
information”;
■
Requirements of MARPOL Convention (Annex I reg. 25 (5)), IBC Code (chapter
II reg.2.2) and IGC Code (chapter 2 reg. 2.2);
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INFORMATION TO THE MASTER
– STABILITY BOOKLETSTATUTORY REQUIREMENTS
International Convention on Load Lines 1966 as amended, reg. 10
“Information to be supplied to the master”
The master of every new ship shall be supplied with information to
arrange for loading and ballasting of his ship in such a way as to avoid
the creation of any unacceptable stresses in the ship’s structure ,
provided that this requirement need not apply to any particular length,
design or class of ship where the Administration considers it to be
unnecessary;
Information shall be provided to the master in a form that is approved by
the Administration or a recognized organisation.
Stability information and loading information also related to ship strength
when required under paragraph (1), shall be carried on board at all
times together with evidence that the information has been approved by
the Administration.
115
INFORMATION TO THE MASTER
– STABILITY BOOKLETSTATUTORY REQUIREMENTS
International Convention for the Safety of Live at Sea, chapter II-1
part B “Subdivision and stability”
Reg. 8 “Stability of passenger ships in damaged conditions”
■ 7.1 The master of the ship shall be supplied with the data necessary to maintain sufficient
intact stability under service conditions to enable the ship to withstand the critical damage.
■ 7.2 The data referred to in paragraph 7.1 to enable the master to maintain sufficient intact
stability shall include information which indicates the maximum permissible height of the
ship’s centre of gravity above keel (KG) , or alternatively the minimum permissible
metacentric height (GM), for a range of draughts or displacements sufficient to include all
service conditions. The information shall show the influence of various trim taking into
account the operational limits.
Reg. 22 “Stability information for passengers’ ships and cargo ships”
■ The master shall be supplied with such information satisfactory to the Administration as is
necessary to enable him by rapid and simple process to obtain accurate guidance as to
stability of the ship under varying condition of service.
Reg. 25-8 “Stability information”
■ The master shall be supplied with such reliable information as is necessary to enable him
by
rapid and simple process to obtain accurate guidance as to stability of the ship
under
varying condition of service. The information shall include:
■ a curve of minimum operational metacentric height (GM) versus draught which assures
compliance with the relevant intact stability requirements and the requirements of
regulation 25-1 to 25-6, alternatively a corresponding curve of the maximum allowable
vertical centre of gravity (KG) versus draught, or with equivalents of either of these curves;
■ instructions concerning the operation of cross-flooding arrangements; and
116
■ all other data and aids which might be necessary to maintain stability after damage.
INFORMATION TO THE MASTER – STABILITY BOOKLET
STATUTORY REQUIREMENTS
MARPOL – IBC CODE – IGC CODE
MARPOL Convention Annex I as amended reg. 25
The master of every new oil tanker and the person in charge of a new non-self
propelled oil tanker to which this Annex applies shall be supplied in an
approved form with:
■ information relative to loading and distribution of cargo necessary to ensure
compliance with the provisions of this regulation; and
■ data on the ability of the ship to comply with damage stability criteria as
determined by this regulation, including the effect of relaxations that may
have been allowed …
IBC Code (chapter 2 reg.2.2.5) and IGC Code (chapter 2 reg. 2.2.5)
■
The master of the ship should be supplied with a loading and stability
information booklet. This booklet should contain details of typical service and
ballast conditions, provisions for evaluating other conditions of loading and a
summary of the ship’s survival capabilities. In addition, the booklet should
contain sufficient information to enable the master to load and operate the
ship in a safe and seaworthy manner.
117
ENDORSEMENT OF THE STABILITY DOCUMENTATION IN
CASE OF CHANGE OF CLASS AND/OR FLAG
The procedures to be applied for the endorsement of the stability
documentation, in case of change of class and/or flag, are outlined in the
Instructions to Surveyors part II-A-6 Change of Flag, an abstract of which is
included here below.
TRANSFER OF CLASS WITH CHANGE OF FLAG
The surveyor has to verify that all manuals and booklets required on board are
available, written in a language understood by the crew members and that no
modifications have been carried out to the ship so that the validity of such
manuals/booklets have been impaired.
For all those manuals and booklets required to be approved by the flag
Administration which are stamped for approval by the previous flag
Administration or an IACS Society on its behalf , the instructions contained in the
following table shall be followed, taking due care of changing the ship name,
owner, management, etc. where necessary.
118
ENDORSEMENT OF THE STABILITY DOCUMENTATION IN
CASE OF TRANSFER OF CLASS WITH CHANGE OF FLAG
119
ENDORSEMENT OF THE STABILITY DOCUMENTATION IN
CASE OF TRANSFER OF CLASS WITH CHANGE OF FLAG
120
ENDORSEMENT OF THE STABILITY DOCUMENTATION IN
CASE OF CHANGE OF FLAG FOR VESSELS
ALREADY CLASSED BY RINA
The surveyor has to verify that all manuals and booklets required on board are
available, written in a language understood by the crew members and that no
modifications have been carried out to the ship so that the validity of such
manuals/booklets have been altered; if not, the Fleet Services Department has to
be contacted prior to issuing the relevant certificates.
Unless detailed instructions are given by the incoming Administration or
contained in ITS II-C for specific re-approval by RINA Head Office, all those
manuals and booklets required to be approved by the flag Administration which
are stamped for approval by RINA on its behalf, shall be endorsed with the words
“APPROVED ON BEHALF OF THE GOVERNMENT OF …....” taking due care of
changing the ship name, owner, management, etc. where necessary.
A copy of the endorsed front page shall be sent to RINA Head Office by the
surveyor in charge.
In all other cases, the Fleet Services Department is to be contacted prior to issuing
the relevant certificates.
121
ENDORSEMENT OF THE STABILITY DOCUMENTATION IN
CASE OF CHANGE OF CLASS AND/OR FLAG
NATIONAL REQUIREMENTS
(ITALIAN SAFETY RULES ARTICLE 35, UNITED KINGDOM, ETC.)
Various flag Administrations (for example Italy and United Kingdom) specifically
request that the stability documentation is to be approved by the Classification
Society/Recognized Organization on its behalf.
In such a case, in case of a change of class and/or flag, the stability
documentation is to be approved by the RINA Head Office and the
documentation existing on board is to be endorsed as “PROVISIONALLY
APPROVED PENDING FINAL APPROVAL BY RINA HEAD OFFICE/FLAG
ADMINISTRATION”.
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