ship design notes

SMK 3522 SHIP DESIGN I Dr Koh Kho King © 2013

Week 5-1

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 PT PE

iii) Loss of power during transmission from main engine to propeller,

PB PS PD

D S B G

Transmission Efficiency, et

PE , Effective power = RT.V

PT , Thrust power = T.VA

PD , Delivered power =

PED

PE

QPC

PE

H.R .O 2QN

PE PT PD

Stern Tube Bearing

Gear

r Machinery G , Gearing Efficiency

S , Stern tube Efficiency

B , Bearing Efficiency

D , Quasi Propulsive Efficiency

PS PB


Week 5-2

H hull efficiency1 t 1w

O.R propellerefficiency behind hull

Q shaft torque

N Shaft revolutions

QPC Quasi propulsiveefficiency

QPC can be estimated from the formula due to Emerson:

10000

LNKQPC

(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

P0.2

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 = GsP

G = 0.96 – 0.97 for medium speed diesel engines

Effective power, PE = RT.V

where,

RT = RF + RR + RAppendage + RAir


Week 5-3

Frictional resistance, RF = 1

2SV

2. Cf

Residuary resistance, RR = 1

2SV

2. Cr

Total resistance, RT = 1

2SV

2. Ct

Ct = Cf + Cr + CAppendage + Cair

U sing ITTC (1957 ), Cf 0.075

(logRn 2)2 0.0004

and Rn V.L

V, velocity of ship

L , length of ship

,1.1906 x 106 m2

sfor salt water

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 V

L , 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-

length ratio

VL

with varying parameters Cp, L/B, B/T, C .

C

0.01L 3


Week 5-4

2. Series 60 V

L , 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

VL

,0.40.85

Ref: Lackenby, J 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) Guldhammer’s and Harvald’s Diagrams

VL

,0.51.3

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 L

V with varying Cp and 31

L

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 5-5

(ii) 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.

Cair Rair

12air Sf Vair

2

Vair , shipplus wind velocity

air 1.23 kg /m3

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 5-6

Bow thruster 2-5%

Twin screw bossings 3-10%

Twin rudders 3%

Single screw 3-5%

Power Determination and System Selection

Power Determination

In the earliest conceptual design power determination depends largely on

VL

, type of hull

and displacement.

For comparison, figure below shows the plot of EHP/ton against

VL

for different types of

hull.


Week 5-7

Harvald (Resistance and Propulsion, 1983)

Ps

2

3. V3

AC Ps [KW]

L [m]

V [m/s]

where,

AC 3.7 L 75

V

[tonnes]

Watson, Trans. RINA 1960

Ln

CKLV B

1101500

121400017.0400.5 P

23

S

32

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

VL

) then:

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:

BS

E

SQPC

PP

(Refer to previous notes)


Week 5-8

More detail estimates relates basic parameters of , L, B, T, CB

, CP and speed with power

required (PE

, PS, P

B).

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 5-9

5

3

Q

Q

.2

.

A

n

A

D

To

Vb

nQ

VT

PP

where: T = Thrust

VA = Advance Speed

n = Rate of Revolutions

Q = Torque

Note: 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 5-10

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 : 35,000 - 100,000 +

Gas turbine : 500 - 40,000

Diesel : 25 - 25,000

Otto (gasoline) : 10 - 500

Turbo jet (aircraft type) : - 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 5-11

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 5-12

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 5-13

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

50001.40 3

1.413 1.1187

L*

L

L* 100.68 m

Corresponding speed;

V

L

V

L

V VxL

L

knots

Then PP x x V

x V

SP x x

SP x

kW

ss

*

*

**

**

.12 43

2590

23

3

23

23 2

3

35

3

33

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