SMK 3522 SHIP DESIGN I
Dr Koh Kho King © 2013
POWER ESTIMATES
An estimate of power requirements forms one of the most important and critical steps in
preliminary design. The power derived has a direct and significant effect on the deadweight
which can be carried by a given ship. It is therefore important to keep engine weight and
volume as low as possible. However, power is also a controlling factor on ship’s speed and
severe penalties can be incurred for not achieving this speed. Thus the designer requires a
margin of safety in power estimates.
An estimete of power is one of the most complex factors and it is related to a large number of
design parameters. An additional complexity in the design is the final choice of machinery
based on the power estimates. Even when the machinery type is chosen, the range of units
available commercially is limited.
PROPULSION
The power needed by the main machinery can be divided into three groups:i)
Those affecting hull resistance, PE
ii)
Those affecting conversion of torque into thrust, PD
iii)
Loss of power during transmission from main engine to propeller,
PB
PS
PD
Stern Tube
PE PT PD
D
Gear
Bearing
PS
S
B
Machinery
PT
G , Gearing Efficiency
S , Stern tube Efficiency
B , Bearing Efficiency
D , Quasi Propulsive Efficiency
PB
G
Transmission Efficiency, et
PE , Effective power = RT.V
PT , Thrust power = T.VA
PD , Delivered power =
PE
PE
PE


D QPC  . .  2QN
H R O
Week 5-1
PE
SMK 3522 SHIP DESIGN I
H
 hull efficiency 
Dr Koh Kho King © 2013
1 t 
1 w 
O. R  propeller efficiency behind hull
Q
 shaft torque
N
 Shaft revolutions
QPC
 Quasi propulsive efficiency
QPC can be estimated from the formula due to Emerson:
QPC  K 
N L
10000
(0.02 reduction for controllable pitch propeller)
where,
K = 0.84
N = RPM
N can be estimated from propeller diam.:
Pr op. Diam  k' x
P 0.2
N0.6
P = Effective Power in kW
k’ = 15.5  17.5 (for metric units)
L = Length of ship (m)
Ps , shaft power = PD /(S . B)
S . B = 0.98 machinery aft
= 0.97 machinery amidships
PB, brake power = Ps G
G = 0.96 – 0.97 for medium speed diesel engines
Effective power, PE = RT.V
where,
RT = RF + RR + RAppendage + RAir
Week 5-2
SMK 3522 SHIP DESIGN I
Dr Koh Kho King © 2013
1
2
1
Residuary resistance, RR = SV2. Cr
2
1
Total resistance, RT = SV2. Ct
2
Frictional resistance, RF = SV2. Cf
 Ct = Cf + Cr + CAppendage + Cair
U sing ITTC (1957 ), Cf 
and R n 
V.L

0.075
 0.0004
(logR n  2)2
V, velocity of ship
L , length of ship
,1.1906 x 10 6 m s for salt water
2

Wetted surface area, S = 1.7 L.T + T
(Denny Mumford)
L = length between perp.
T = draught
 = moulded volume of displacement
Residuary resistance, RR or Cr is predicted using the results based from standard series.
1.
Taylor Standard Series
 VL , 0.5  2.0
Ref:- Gertler, M.,
A Reanalysis of the Original Test Data for the Taylor Standard
Series. DTMB Report 806, March, 1954.
Easy to use with wide applicability. Residual resistance ratio is plotted against speedV

length ratio  L  with varying parameters Cp, L/B, B/T, C  .
C 

0.01L 3
Week 5-3
SMK 3522 SHIP DESIGN I
2.
Series 60
Dr Koh Kho King © 2013
 VL , 0.4  1.0
Ref: Todd, F.H.,
Series 60 Methodical Experiments with models of Single-Screw
Merchant Ships, DTM Report 1712, July 1963.
Easy to use. Residual resistance plotted against
V
L
.
The variable parameters are CB, L/B, B/T, C  , LCB.
(3)

BSRA Methodical Series V
Ref: Lackenby, J
L
,0.4  0.85 

The BSRA Methodical Series - An overall Presentation, Variation
of Resistance with Breadth-Draught Ratio and Length-Disp.
Ratio, RINA 1966.
Single screw merchant vessel type hulls. Residual resistance plotted against
V
L
with
variables CB, B/T, C, LCB, Lp.
LP is length parallel midbody as % of LPP.
(4)
V
,0.5 1.3

Guldhammer’s and Harvald’s Diagrams  L
Ref: Harvald, S.A.,
Resistance and propulsion of ships pg. 118-126.
Vessels of standard form i.e. LCB, B/T, normally shaped sections, moderate cruiser
stern, and raked stem.
Cr versus V
L
with varying Cp and
L
1
 3
Other variations in the estimation of residuary resistance, includes:
(i)
High speed displacement hulls
Eames, M.C.,
Concept Exploration - an approach to small warship design.
Trans RINA, 1976.
Week 5-4
SMK 3522 SHIP DESIGN I
(ii)
Dr Koh Kho King © 2013
Planing Hull
Clement, E.P and Blount, D.L.,
Resistance Tests of a Systematic Series of Planning
Hull forms, SNAME 1963, Series 62.
(iii) Trawler
Ridgely-Nevitt, C,
The Resistance of a High Displacement - Length Ratio Trawler
Series. SNAME 1967.
Doust, P.J.
Statistical Analysis of resistance data for trawlers.
- Fishing Boats of the World 2.
Patullo, R.N.M. and Thomson, G.R.,
The BSRA Trawler Series Beam-Draught and
Length-Displacement Ratio Series resistance and propulsion tests.
Part I - RINA 1965, Part II - RINA 1968
Air Resistance
Air resistance is small i.e 2-3% of total resistance.
V air , ship plus wind velocity
Cair  1
R air
3
air  1.23 kg / m
 S V air2
2 air f
S f , frontal area
For hulls, Cair = 0.33 to 0.50
For flat superstructures, Cair = 0.67 to 1.00
For flat plate, Cair = 1.00
Appendage Resistance
Appendage resistance is usually some percentage of bare hull resistance
RF + RR.
From Watson and Gilfillan:
Shaft “A” brackets
5%
Week 5-5
SMK 3522 SHIP DESIGN I
Bow thruster
2-5%
Twin screw bossings
Twin rudders
Single screw
3-10%
3%
3-5%
Dr Koh Kho King © 2013
Power Determination and System Selection
Power Determination
In the earliest conceptual design power determination depends largely on V
L
, type of hull
and displacement.
For comparison, figure below shows the plot of EHP/ton against V
hull.
Week 5-6
L
for different types of
SMK 3522 SHIP DESIGN I
Dr Koh Kho King © 2013
Harvald (Resistance and Propulsion, 1983)
2
 3. V3
Ps 
AC
Ps [KW]
L [m]
V [m/s]
where,

75 
  [tonnes]
A C  3.7  L 

V 
Watson, Trans. RINA 1960
2
PS 

5.0  3 V 3 400.017 L  400K  1 12C B
2

1500 110n L
Ps, KW
n, rev/s
K, Alexander’s formula
L, m
 , tonnes
V, m/s
Basis Ship
If two ships have geometrically similar underwater forms (i.e. same Cb) and run at
corresponding speeds (i.e. same V
) then:
L
PE   2/3 x V3
If propeller efficiency and transmission efficiency are assumed to be the same, this could
be written as:PS   2/3 x V3
Note: Relation between PE, PS is:
PS 
PE
QPC S B
(Refer to previous notes)
Week 5-7
SMK 3522 SHIP DESIGN I
Dr Koh Kho King © 2013
More detail estimates relates basic parameters of  , L, B, T, CB, CP and speed with power
required (PE, PS, PB).
These estimates usually uses series charts and diagrams to estimate PE. The series which are
commonly used are series 60, BSRA methodical series, Taylor series and guldhammer’s and
Harvald’s diagrams.
Choice of Propulsor
One of the factors that determines the type of propulsor is efficiency. Fig. below shows
optimum efficiency against propeller loading for different type of propulsors.
Week 5-8
SMK 3522 SHIP DESIGN I
 o  PT P 
D
Dr Koh Kho King © 2013
T .V A
2Q.n
3
bQ 
Qn
V A 5
where:
Note:
T
VA
n
Q
=
=
=
=
Thrust
Advance Speed
Rate of Revolutions
Torque
There are also other types of propulsors such as the jet propulsion system.
In the case of screw propellers, theory dimensions are determined by various theories such as
Momentum theory, Blade Element theory, Lifting Surface theory, etc.
Series charts such as K-J and Bp-could also be used.
Model Tests
Model tests such as resistance test, open water tests, self propulsion, etc are conducted to
check/confirm/modify the values obained by calculations [PE, PS, propeller efficiencies and
dimensions, propeller revs. etc].
Ref: Femenia, J (1973) - Economic comparisons of Various Power Plants Trans. SNAME.
Main Machinery [Prime Mover] Selection
The factors that mainly determines mainmachinery type.
i)
Operational
-
Power requirements [Speed, Displacement, Length, etc]
Weight limitation (also space)
Range
ii)
Economic
-
Cost of Engine
Fuel consumption
Week 5-9
SMK 3522 SHIP DESIGN I
Dr Koh Kho King © 2013
Other factors
-
maximum to continuous power ratio
maintenance and repair requirements
lubricating oil consumption rate
life-cycle costs
Basic main-machinery types and their power ranges is given below:
Horse power (HP)
Steam turbine
:
Gas turbine
:
Diesel
:
Otto (gasoline)
:
Turbo jet (aircraft type)
:
35,000 500 25 10 -
100,000 +
40,000
25,000
500
3,500 (max)
In recent years, CODAG (combination diesel and gas turbine) system has been introduced and
now increasingly being in used in most modern ships from comparatively small patrol craft to
larger destroyer type escort craft, coast guard cutters, rescue craft etc.
The system user diesel for continuous lower power cruising requirements and the gas turbine
for the high power more intermittent full-speed requirements.
Fig. below shows the usage of different systems depending on speed and vessel range
requirements.
Week 5-10
SMK 3522 SHIP DESIGN I
Dr Koh Kho King © 2013
Two figs. below shows the characteristic ranges of specific weights for gas turbine, diesel and
otto cycles compared with their maximum power ranges.
The next 2 figs below show the relative fuel consumption characteristics of various types of
main machinery.
In summary, we can say that for high powered machinery at intermittent usage (small range)
we use steam turbines and gas turbines.
Diesel and Otto (gasoline) engines are suitable for lower powering and longer range.
Week 5-11
SMK 3522 SHIP DESIGN I
Dr Koh Kho King © 2013
Otto cycle engines has lower specific weight at low powering and gas turbines specific weight
are comparatively consistent at high powering.
Specific fuel consumption in the order from low to high Diesel and otto
Steam turbine
Gas turbine
CODAG system utilities both advantages of diesel and gas turbine.
TRIAL AND SERVICE MARGINS
If speed penalty is high then it is wise to keep a margin of about 5%.
Service speed is lower than trial speed due to fouling of the hull, increased roughness etc.
Percentage allowance will depend on paint system, cathodic protection system, voyage
pattern, weather conditions, etc. The usual practice is the service speed to be 85 - 80% of the
trial speed.
Week 5-12
SMK 3522 SHIP DESIGN I
Dr Koh Kho King © 2013
MCR
= Maximum Continuous Rating of Engine
Trial Power
= The power produced by engine to run the vessel at speed Vt (trial speed)
during trial run.
Service Power =
Usually is 85% ---> 80% of Trial Power. Using the Power Curve, the
Service Speed (VS) is predicted at the selected Service Power.
Example:
If a ship of 90 m length requires an SP of 1750 kW for a trial speed of 11.75 knots when the
full displacement is 5000 tonnes, then the size, speed, and required trial power for 7000
tonnes geometrically similar ship at corresponding speed can be approximated by:-

7000
*

 1.40  3
5000

 
 1.4
1
3
L*
 1.1187 
L
 L*  100.68 m
Corresponding speed;


V*
*

V
L
L
V*
 Vx
L*
L
 12.43 knots
Ps x  3 x V*
2
Then Ps* 
3
 3 x V3
2
  x  
 SP x 3
2
3
2
3
3
 SP x  5
3

2590 kW
Week 5-13