Chapter 7 : Trials
Ch7. Sea trials / Manoeuvring characteristics of ships
IMO Recommendations MSC 137(76)
• The manoeuvrability of ships can be evaluated from
the characteristics of conventional trial manoeuvres.
• Two methods can be used:
– Scale model tests or computer predictions using
mathematical models at the design stage / full scale trials
must be conducted to validate these results
– Full scale trials
• Test speed = at least 90% of full speed = 85% of full
engine power
Ch7. Sea trials / Manoeuvring characteristics of ships
Imo Manoeuvring Standards
• By resolution A.751(18) in 1993 IMO adopted
Manoeuvring Standards
• The standards apply to:
– All ships of 100m in lenght and over
– All chemical tankers and gas carriers
• They consist of:
– Turning circles to Port and starboard
– Stopping Test
– Zig-Zag Test
Ch7. Sea trials / Manoeuvring characteristics of ships
Conditions at which the standards apply
In order to evaluate the performance of a ship,
manoeuvring trials should be conducted to both port
and starboard and at conditions specified below:
.1
deep, unrestricted water (> 4xmean draft)
.2 calm environment (Wind< 5Bft / Sea< 4)
.3 full load (summer load line draught), even keel
condition
.4 steady approach at the test speed(min90% full).
Ch7. Sea trials / Manoeuvring characteristics of ships
• Manoeuvring performance has traditionally received
little attention during the design stages of a commercial
ship.
• Consequently some ships have been built with very poor
manoeuvring qualities, resulting in marine casualties /
pollution.
• Designers have relied on shiphandling abilities of human
operators to compensate for deficiencies in inherent
manoeuvring qualities of the hull.
• The implementation of manoeuvring standards will
ensure that ships are designed to a uniform standard, so
that an undue burden is not imposed on shiphandlers in
trying to compensate for deficiencies in inherent ship
manoeuvrability.
(Extract of IMO MSC/Circ1053)
Ch7. Sea trials / Preliminary
Forces and motions in manoeuvrability
Definition of the Pivot Point:
• the point around which the ship rotates
• The centre of the hydrodynamic forces acting on the
ship’s hull
Position of the Pivot Point:
• Depends on the shape of the hull
• With no forward speed: pivot point at midship
• At speed: pivot point shifts forward
Ch7. Sea trials /Preliminary
The Pivot Point at forward speed
Ch7. Sea trials / Manoeuvring characteristics of ships
1. Course keeping ability and dynamic stability
• Dynamically stable ship moves along a new straight
course without using rudder after a small disturbance
• Dynamically unstable ship performs turning circle
with rudder amidship
•
More difficult to handle dynamically unstable ships
• Infos on course keeping and dynamic stability:
obtained from « Initial turning test »
Ch7. Sea trials / Manoeuvring characteristics of ships
Dynamic stability: dynamically stable ships maintain
A straight course with zero rudder
Dynamically unstable ships can only
maintain a straight course by repeated
use of rudder control
Ch7. Sea trials / Manoeuvring characteristics of ships
Factors determining the Directional stability of vessels
• Increase with the depth of the water
• Increase with the lenght of the ship
• Increase with Trim by the stern
• Decrease with big blockage factor
• Decrease for large vessel (ratio L/B)
• Decrease when cross sectional area fwd larger than
cross sectional area after (pivot point moves
forward)
Ro-Ro ships are directionally unstable
They need more rudder to stop a swing than to start a swing
Ch7. Sea trials / Manoeuvring characteristics of ships
Change of trim
• Ship by the stern has a better course keeping ability
• Ship by the head:
– Slow to start a swing
– Difficult to stop a swing
– In shallow water, a ship gets trim by the head and
looses directional stability
3 STANDARD MANOEUVRES
TURNING CIRCLE
Turning circle: measure of turning ability of vessel
TURNING CIRCLE
To determine the turning ability
- The measure of the ability of a ship using hard-over
rudder
- The result is a minimum « advance at 90° change of
heading » and « tactical diameter » defined by the
« transfer at 180° change of heading »
- Tactical diameter is usually given as multiplacity of
ship lenght
• The advance should not exceed 4.5 ship lengths (L)
• the tactical diameter should not exceed 5 lengths
• Turning circle to be performed with 35°Rudder angle
Statendam
Lenght:196m / beam:25m / 24300DWT / Steamship/ 2 propellers/ 19Knots
Advance: 426m
Transfer: 99m
Diameter: 263m
Tact.Dia: 290m
Advance: 426m
Transfer: 94m
Diameter: 258m
Tact.Dia: 292m
Kick
Advance
Transfer
Advance: the distance traveled in the direction
of the original course by the midship point of a
ship from the position at which the rudder order
is given to the position at which the heading
has changed 900 from the original course.
Tactical diameter :
the distance
traveled by the
midship point of a
ship from the
position at which
the rudder order is
given to the
position at which
the heading has
changed 1800 from
the original course.
Kick
It is measured in a
direction
perpendicular to
the original
heading of the
ship.
Final Diameter
Tactical Diameter
TURNING CIRCLE
Comments
• Advance of the ship smaller than the distance ahead
with an emergency stop manœuvre
• Request sufficient searoom on the beam (tactical
diameter)
• Test are carried out at sea and not in shallow waters:
parameters are bigger in shallow water because
rudder effect decreases in shallow water due to the
reduced waterflow
• Parameters of the turning circle do not change for
different speeds of the ship
TURNING CIRCLE
Drift angle and Pivot point
•The pivot point (D) is at the intersection of the longitudinal
axis of the vessel with the radius of the turning circle
•The drift angle at the pivot point is zero
•The drift angle at the centre of gravity (G)
TURNING CIRCLE
In shallow waters, the drift angle is smaller : the water
resistance decreases and the turning circle is larger
Crablike motion of the ship:
Water resistance reduces the speed
and the diameter of turning circle
TURNING CIRCLE
Forces acting on a ship when turning
TURNING CIRCLE
TURNING CIRCLE
The turning circle is affected by the effects of wind and
current
Turning characteristics of full
and slender ships
TURNING CIRCLE
Comparison of turning characteristics of full and
slender ships:
• Two ships of the same lenght have nearly the same
transfer
• Tactical diameters almost the same
• Radius of turning circle smaller for tanker
• Drift angle much larger for tanker
• Pivot point closer to the bow in tanker
TURNING CIRCLE
Water resistance on starboard
Beam during turning circle
ZIG-ZAG TEST
ZIG-ZAG TEST (Kempf)
• Yaw checking ability a measure of :
– the response to counter-rudder (Overshoot angle and
overshoot time)
– Measure of the ability to initiate and check course
changes
Two tests are included: the 10°/10° and 20°/20° tests
10°/10° zig-zag test: rudder is turned alternately by 10° to
either side following a heading deviation of 10° from
original heading
ZIG-ZAG TEST (Kempf)
10°/10° Zig-Zag Test
ZIG-ZAG TEST/ Procedure
•after a steady approach, rudder is put over to 10° to starboard
(port) (first execute)
•when heading has changed to 10° off original heading, rudder
reversed to 10° to port (starboard) (second execute)
• after the rudder has been turned to port/starboard, the ship
continues turning in original direction with decreasing turning rate.
• In response to rudder, ship should then turn to port/starboard.
• When ship has reached a heading of 10° to port/starboard of the
original course the rudder is again reversed to 10° to starboard/port
(third execute).
•The first overshoot angle is the additional heading deviation
experienced in the zig-zag test following second execute
Recommendations of IMO
The value of the first overshoot angle in the 10°/10° zig-zag
test should not exceed:
. 10° if L/V is less than 10 s;
. 20° if L/V is 30 s or more; and
. (5 + 1/2(L/V)) degrees if L/V is 10 s or more, but less than 30s
where L and V are expressed in m and m/s, respectively.
The value of the second overshoot angle in the 10°/10° zigzag test should not exceed:
. 25°, if L/V is less than 10 s;
. 40°, if L/V is 30 s or more; and
. (17.5 + 0.75(L/V))°, if L/V is 10 s or more, but less than 30 s.
ZIG-ZAG TEST
ZIG-ZAG TEST
• The 20°/20° zig-zag test is performed using the same
procedure using 20° rudder angles and 20° change
of heading, instead of 10° rudder angles and 10°
change of heading, respectively.
• The value of the first overshoot angle in the 20°/20°
Zig-Zag test should not exceed 25°
Recommendation of IMO MSC 137(76)
20°/20° Zig-Zag Test
STOPPING
TEST
Shiphandling: Single
Screw Ships
Ship Ahead
Propeller Astern
Rudder Amidships
STOPPING TEST
• The "crash-stop" or "crash-astern" manoeuvre is
mainly a test of engine functioning and propeller
reversal. The stopping distance is a function of the
ratio of astern power to ship displacement.
Procedure
1. ship brought to a steady course and speed
2. The recording of data starts.
3. The manoeuvre is started by giving a stop order. The
full astern engine order is applied with rudder amidship.
4. Data recording stops and the manoeuvre is
terminated when the ship is stopped dead
STOPPING TEST
Parameters:
• track reach
• head reach
• lateral deviation
• time to dead in water
STOPPING TEST
Measure of the ability to stop while maintaining control
• Full astern stopping test determines the track reach
of a ship from the time an order for full astern is given
until the ship stops in the water.
• Track reach is the distance along the path described
by the midship point of a ship measured from the
position at which an order for full astern is given to the
position at which the ship stops in the water
• Track reach must not exceed 15 ship’s lenghts
excepted for very large vessels: maximum 20 Ship’s L.
Comparison between
different manœuvres
for stopping a ship
ADDITIONAL TESTS FOR UNSTABLE SHIPS
• Where standard manoeuvres indicate dynamic
instability, alternative tests may be conducted to
define the degree of instability : « Initial turning
test »
• Guidelines for alternative tests such as a « spiral
test » or « pull-out manœuvre » are included in the
Explanatory notes to the Standards for ship
manoeuvrability, referred to in paragraph 6.1 above.∗
• ∗ Refer to MSC/Circ.1053 on Explanatory notes to the
Standards for ship manoeuvrability
INITIAL TURNING TEST
INITIAL TURNING TEST
Initial Turning ability
• Measure of change of the heading in response to a
moderate helm
• Expressed in :
distance covered before course change of 10° when 10°
of rudder is applied (also with 20° rudder angle)
• Assessed by the « Initial Turning Test »: Test to be
performed for unstable ships (IMO Recommandations)
Initial Turning Test
• Measure of nonlinear
directional stability
• Ability to control yaw
motion with small rudder
angles
With 10° rudder angle
to port/starboard, the
ship should not have
travelled more than 2.5
lengths by the time the
heading has changed
10° from original
heading
PULL-OUT TEST
Additional test for
ships with
unsatisfactory
manoeuvring
standards
Measure of course
keeping ability and
dynamic stability of
a ship
PULL-OUT TEST
1.
The ship is first made to turn with a certain rate of
turn
2.
The rudder is returned to midship position
3.
With a stable ship: rate of turn decays to zero
4.
Unstable ship: rate of turn reduces but residual
rate of turn will remain
SPIRAL TEST
SPIRAL TEST
• The Standard Manoeuvres are used to evaluate
course-keeping ability based on the overshoot
angles resulting from the 10°/10° zig-zag manoeuvre.
• The zig-zag manoeuvre was chosen for reasons of
simplicity and expediency in conducting trials.
• However, where more detailed analysis of dynamic
stability is required some form of spiral manœuvre
(direct or reverse) should be conducted as an
additional measure.
SPIRAL TEST
DIRECT SPIRAL TEST
• The direct spiral is a turning circle manoeuvre in
which various steady state yaw rate/rudder angle
values are measured by making incremental rudder
changes throughout a circling manoeuvre.
• In the case where dynamic instability is detected
with other trials or is expected, a direct spiral test
can provide more detailed information about the
degree of instability.
• In cases where the ship is dynamically unstable it
will appear that it is still turning steadily in the
original direction although the rudder is now slightly
deflected to the opposite side.
DIRECT SPIRAL TEST
• steady course and speed
• recording of data starts
• rudder turned 15 degrees and held until yaw rate
remains constant for one minute
• rudder angle is then decreased in 5 degree
increments. At each increment the rudder is held
fixed until a steady yaw rate is obtained, measured
and then decreased again
• this is repeated for different rudder angles starting
from large angles to both port and starboard
• when a sufficient number of points is defined, data
recording stops.
REVERSE SPIRAL MANOEUVRE
• In the reverse spiral test the ship is steered to obtain
a constant yaw rate, the mean rudder angle required
to produce this yaw rate is measured.
• the yaw rate versus rudder angle plot is created.
RESULT OF SPIRAL TEST FOR STABLE SHIP
RESULT OF SPIRAL TEST FOR UNSTABLE SHIP
DIEUDONNE SPIRAL MANOEUVRE
the vessel path follows a growing spiral, and then a
contracting spiral in the opposite direction.
Suppose that:
a) the first 15° rudder deflection (Sb) causes the vessel
to turn right
b) At zero rudder, the yaw rate is still to the right: the
vessel has gotten “stuck” here, and will require a
negative rudder action to pull out of the turn.
the rudder in this case has to be used excessively
driving the vessel back and forth.
We say that the vessel is unstable, and clearly a poor
design.
Comments to IMO Standards
• For deep water and service/design speed only
• Give no indication of the handling characteristics in
wind, waves and current
• Do not look at manoeuvres normally carried out by
most merchant ships
• Full astern stopping test results in extreme termal
loads on the engine
• Criteria derived from databases heavily biased
towards (old) tankers and bulk carriers
Comments to IMO Standards
• From operational aspects additional requirements
should be developed:
– Manoeuvrability in shallow water
– Low speed manoeuvring capabilities
– Maximum tolerable wind forces in harbour
manoeuvres
– Limited heel angles
– Steering in waves
– Steering with special devices