A New Proposal Method for Estimation of Tug Boat Dimensions
Walid Bahgat* and Mohamed Abdel-Aziz*
College of Maritime Transport & Technology
Arab Academy for Science & Technology and Maritime Transport
ABSTRACT
There are many methods available to find out the principal dimensions of tug boats. However,
due to the remarkable development of such vessels during the last decades, such methods have
become out of date. Therefore, it was necessary to find out new statistics for most new modern
ships that actually exist or under construction, in order to develop design charts for estimating
the principal dimensions of tug boats. Such charts will be of great help in the preliminary design
stage of such vessels.
Key words: Tug boats, Design chart, Ocean-going tug, Coastal tugs, Harbour tugs, Ship
stability.
Nomenclature:
C = 7:25 for freighters with trial speed of
V = 15:5–18:5 kn
Cb = block coefficient
CD = dead weight coefficient
= V g.L = Froude number
L pp = length between perpendiculars (m)
V
= speed (knots)
dwt = dead weight (ton)
= gravitational acceleration (m/sec2)
g
 = sea water density (ton/m3)
 = displacement (ton)
 = volume of displacement (m3)
Fn
Introduction
Modern tugs are capable of meeting the
requirements of the shipping industry in
harbour, deep sea, and towage, salvage
operations. Floating docks, dredgers, have
been towed over vast distances and the
modern oil rigs are taken to their location,
all by tugs. In general, tugs may be
classified according to ocean-going and
salvage, coastal, harbour and river
(R.Munro-Smith, 1975).
The basic requirements for all tugs are
stability under all conditions of operations,
maneuverability, and adequate towing
power. The tug efficiency depends on the
percentage of power the vessel can transmit
through a tow-rope to another vessel. In
order to tow successfully, a tug must be
highly maneuverable. One arrangement that
provides a tug with maneuverability is to
have its propeller encased in a swiveling
ring which acts as a rudder-the kort
propulsion nozzle. Another form of
propeller is the Voith Schneider type where
the propeller blades are vertical. These two
fittings alone have created a high standard of
maneuverability. The controllable pitch
propeller and bridge control of the
propelling machinery are now almost
standard installations on tugs.
*Lecturer, Arab Academy for Science, Technology and Maritime Transport
Ocean-going and salvage tugs must be
capable of spending long periods at sea.
Their main characteristics are high power,
typical sea-going qualities, displacement and
radius of action.
Coastal tugs are generally of short
dimensions and of medium power, i.e. the
length between 30 and 40 m and the power
from about 1100 to 2200 KW. They have a
free speed in the region 12 to 13 knots and a
bollard pull of 25 to 30 tons. It has good
sheer forward and is fitted with forecastle.
Large freeing ports and numerous scuppers
are installed since the tugs are required to
operate in coastal water that is considerably
rougher than the sheltered waters of harbors.
These tugs are also designed for good free
running speed and equipped with fire and
salvage pumps, which are necessary to
provide salvage services.
Harbour and river tugs are restricted to a
length which is generally less than 30 m and
their power is in the region of 1000 KW.
The normal duty of these vessels comprises
assistance to large vessels when they are
docking and undocking, and ship-handling
in rivers.
The tug and towboat are vessels that provide
propulsive power as an auxiliary motive
force for a specific purpose or for a
combination of uses in waterborne
transportation systems. Some uses of tugs
are: to assist other larger, less maneuverable
vessels in docking and escorting to and from
the open sea through crowded hazardous
inland waters; to provide detachable
propulsive power for non-self propelled
vessels; to push massive integrated river
tows; to pull large ocean tows.
Some considerations that affect the design of
a tug are based on the need for the
following: a watertight under water buoyant
body; restrictions and modifications due to
trade, such as operating draft and fuel
capacity;
a
maximum
height
of
superstructure above the water line; shape of
the fore body; superstructure limitations,
e.g., length of deckhouse; any auxiliary deck
power needed; stowage for consumable and
expendable gear; living accommodations
and
any hotel
services
provided.
(EDWARD M.BRADY, 1977)
Tug Boat Design Starting Point
The following basic requisites have to be
considered in evolving a fundamental
design pattern for a tug, whether it is used
for ocean, harbor or inland two purposes:
1. The main propulsion engine and
auxiliary machinery should be capable
of providing maximum power, when
towing or pushing under full load or
light load condition.
2. The engine has to have fast transient
response in maneuvering starting from
zero to full power.
3. The machinery must be completely
reliable under all conditions of
operation with minimum time lost due
to breakdown.
4. The hull must be strongly built and
heavy enough to utilize the contained
horsepower. This is considered as a
great important in ocean tugs. The
vessel must be rugged enough to
withstand heavy usage with a minimum
wear and tear to
hull and
appurtenances.
5. The vessel in towing condition has to
be readily manoeuvrable and capable of
executing sharp turns within a
minimum radius.
6. In conjunction with its manoeuvrability,
the tug should possess favourable
stability characteristics under all
condition of loading. In addition to
being able to ride easily in heavy seas,
the tug must also resist any adverse
stability condition imposed upon it by
towing.
7.
The tug must be economical to operate.
Operating costs must be kept at a
minimum include fuel, crew wages,
maintenance, fitting out and insurance.
The good manoeuvring is one of the most
important considerations taken into account
in tug designed. The design factors, singly
and in combination tend to improve or
impede manoeuvring are hull form,
propulsive power, propeller and rudder. In
addition, the location of two engines, “H”
bitts and tow hooks greatly affect
manoeuvrability.
Generally, it is desirable to locate the centre
of buoyancy at approximately amidships or
slightly aft, which is a condition difficult to
attain in design because of the unusually
fine form of the after body, a form that is
necessary to provide a full flow of water to
the propeller. It is preferred to locate the
towing connection at a point as near as
possible to the centre of buoyancy or
slightly aft. This is to minimize the force
tending to adversely affect steering control.
These forces work in a couple; the propeller
force opposed to towline pull.
The main dimensions of tug boat estimate
many of its characteristics, e.g., stability,
power requirements and even economic
efficiency. Therefore determining the main
dimensions and form ratios is particularly
important phase in the overall design. In
determining the main dimensions for a new
tug, guidance can be taken from similar tug
boats for which basic details are known.
This is known as a "basic tug" and must be
similar in type, size and speed to the new
tug.
When a tug owner makes an initial enquiry,
he usually gives the designer six basic items
of information: type of tug, deadweight,
required service speed, route on which the
new tug will be operate, class and flag.
Estimation of the light weight of the new tug
boat is the first step for the naval architect.
This can be done by knowing the dead
weight coefficient which is the ratio between
deadweight and displacement. (BARRAS,
2004)
The second step in preliminary design stage
is to determine the length of the new tug
boat. The resulting length provides the basis
for finding the other main dimensions. The
following formulae are the traditional
methods to determine the length of a new
ship,
Schneekluth’s:
Lpp  0.3  V 0.3  3.2
Ayre:
Cb  0.5
,
(0.145 )  0.5
Fn
L
V

3
.
33

1
.
67
,
1
L
3

2
1
 V 
3,
LC 


V

2


Posdunine:
VÖlker's:
L

1
 3.5  4.5
V
g
3
1
3
Cube Root Format:
   
1
 dwt  L 2  B  3
B
T 
L
   Cb  C D



Knowing the length of the new tug boat the
other main dimensions can be determined
through detailed calculations regarding
buoyancy, stability, flooding, statutory
freeboard, powering,…..etc.
Proposed Design Charts For New Ship's
Length
It is difficult to say that the lengths
determined for current new tug boats using
the above mentioned methods are still
optimum. That is because technology and
economy may be changed. Therefore it is
necessary to obtain such formulae based on
statistics data for modern existing tug boats.
This job was done in this thesis for the
purpose of picking up new formulae for
determining the length as well as other main
dimensions for new tug boats. For this
purpose the main particulars of about one
hundred existing tug boats were collected
and analyzed. The results of such analysis
are presented in the form of group of
formulae for determining the length of a
new tug boat (L) as a function of tug boat
bollard pull which is one of the important
characteristics of tug boats.
The standard for judging a tug’s ability is
usually based on its “bollard pull”. This is
the thrust in pounds or in kilograms
delivered by the engine under static
conditions (pulling against a dock or other
fixed structure).
Bollard pull is an acceptable criterion for
judging a tug’s capability in some respects,
but there are other factors that must be
considered when evaluating a tug’s
suitability for ship work. These include its
maneuverability, stability, and backing
power. (GEORGE H. REID, 1975)
Table (1) presents 32 samples of a group of
tug boats, where the lengths range from
about 20 m to 60 m, with a bollard pull
range from about 10 tons to 65 tons.
Table (1): Tug boats Characteristics
No
1
2
3
4
TUG
NAME
MOAWEN
3&5
MOAWEN
4&8
MOAWEN
6&7
ELSHEKH
ZAYED
Length(L)
m
Breadth (B)
m
32.8
9.5
31.6
8.6
32.83
9.5
32.8
9.5
5
ANTAR
46
12
6
MARED &
SHAHM
50
12
7
SALAM 5
35
11
8
SALAM
35
11
9
A.BAHGAT
35
11
10
FAHD
34.4
9.57
52.7
11
53.47
11
53.3
11.6
55.9
12.25
55.77
12.25
55.77
12.25
11
12
13
14
15
16
MARIDIVE
3
MARIDIVE
4
MARIDIVE
5
MARIDIVE
7
MARIDIVE
8
MARIDIVE
9
Depth (D)
m
Draught (T)
m
Bollard Pull
ton
4.5
3.21
43
4.3
2.7
30
4.5
4.3
42
4.5
3.3
39
4.4
5
50
4.4
5
60
3.95
5
50
3.95
5
40
3.95
5
40
4.5
3.8
36
3.95
3.35
18.7
3.95
3.35
34
4.6
4
40
4.6
4.05
63
4.6
4.05
63
4.57
4.05
65
17
MARIDIVE
94
55.47
11.6
18
MZ 104
56.39
12.19
19
ocean
drum
57.7
12.2
20
ahmos
26.2
8.3
21
abu el
abbas
40.5
9
22
dekhela
30
8.5
23
yousef
elserafy
22.5
6.5
24
BASEL 2
35
11
25
tradewind
service
31.91
9.14
26
janus
24.28
8.52
27
w.h.parr
20.12
8
28
bugsier 1
27.1
8.8
29
karl
28.75
9.1
30
maridive
519
59.25
14.95
31
bugsier 8
34.64
9.3
32
greatham
cross
28.28
8.8
According to the above statistic for the tug
boats, four charts can be represented
showing the different relationships between
the main dimensions. Figure (1) shows the
relation between the breadth (B) and the
length (L) of tug boats, where Figure (2)
represents the relation between the depth (D)
4.88
3.86
55
4.27
3.65
39.4
4.5
3.9
53
4.5
3.15
20
4.5
3.84
30
4.3
3.6
25
3.8
1.8
10
3.95
5
40
4.39
3.9
33.3
4.3
3
12
3.66
2.82
19.8
3.6
4.8
29.2
3.65
4.75
35.5
6.1
4.95
60
4
5.6
42.6
3.55
4.8
30
and the length (L) of tug boats. Similarly,
Figure (3) shows the relation between the
draft (T) and the length (L) of tug boats,
where Figure (4) shows the relation between
the bollard pull and the length (L) of tug
boats.
16
14
12
B (m)
10
8
6
4
2
0
15
25
35
45
55
65
L (m)
Figure (1): Relation between the breadth (B) and the length (L)
7
6
D (m)
5
4
3
2
1
0
0
10
20
30
40
50
60
70
L (m)
Figure (2): Relation between the depth (D) and the length (L)
6
5
3
2
1
0
0
10
20
30
40
50
60
70
L (m)
Figure (3): Relation between the draft (T) and the length (L)
70
60
BOLLARD PULL (TON)
T (m)
4
50
40
30
20
10
0
0
10
20
30
40
50
60
70
L (m)
Figure (4): the relation between the bollard pull and the length (L)
Analysing of these charts, the following
formulae may be obtained:
B = 0.127 L + 5.245
D = 0.0239 L + 3.306
T = 0.0135 L + 3.398
B.PULL = 0.852 L + 5.153
(1)
(2)
(3)
(4)
Proposed Design Method for Preliminary
Estimating of Main Dimensions of New
tug boats
The following steps are proposed to
determine the main dimensions of a new tug
boat:
1- Knowing the bollard pull required to
be carried by the new tug one can
find the approximate length for such
a tug using the suitable formula (4).
2- The other main dimensions breadth
"B", depth "D" and draught "T", are
determined using formulae (1), (2)
and (3), respectively.
3- It must be noticed that the above
obtained main dimensions are
preliminary values and to be
rechecked during the design
procedure to reach the final design.
(WATSON, 1998)
Conclusions
The main dimensions decide many of ship's
characteristics, e.g., stability, hold capacity,
power requirements and even economic
efficiency. Therefore determining the main
dimensions is a particularly important phase
in the overall design procedure. A quick
method based on statistical analysis of the
data of existing tug boats of different sizes
has been proposed and represented in simple
formulae ready for use. The results obtained
by the proposed method were fair enough
and compatible with real figures than those
obtained by other design methods.
References
1. Munro-Smith, R.(1975), Merchant ship
types, Marine Media Management
Limited
2.
REID, GEORGE H.(1975), Ship handling
with tugs, Cornell Maritime Press,
centreville, maryland .
3.
BARRAS, C.B (2004), Ship Design and
Performance for Masters and Mates.
Amsterdam: ELSEVIER
4.
Schneekluth, H. (1998), Ship Design
for Efficiency, Economy\ H.
Schneekluth and V. Bertram BH, 2nd
ed.
5.
WATSON, DAVID G.M. (1998), Practical
Ship Design.-Amsterdam:ELSEVIER
BRADY, EDWARD M. (1977), TUGS,
TOWBOATS AND TOWING, Cornell
Maritime Press, INC., Cambridge, Maryland.
6.
‫مستخلص‬
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‫ ولذلك كان من‬.‫هذه الطرق اصبحت قديمه‬
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‫تصميميه حديثه لتعيين األبعاد الرئيسيه‬
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‫االبتدائي لهذه النوعيه من‬
ٍ ‫مرحلة التصميم‬
.‫السفن‬