experimental methods for calculating ship resistance.pdf

8/12/2019 Experimental Methods for Calculating Ship Resistance.pdf

1/23

Experimental Methods for

Calculating Ship Resistance


2/23

Methodical Series Experiments

A methodical series consist of a family of geometrically related models in

which more than two main hull parameters are systematically varied.

The most common hull parameters which are varied in different series are

CB, LCB, B/T and L/B or L/1/3

.


3/23

Taylors Standard Series

Admiral Taylor in the Experimental Model Basin (EMB), Washington,

investigated the effects of altering proportions using a single parent form

CP 0.48 to 0.86B/T 2.25, 3.00, and 3.75

W/(L/100)3 English 20 to 250

0.7 to 8.75 x 10-3

CM 0.925

3)/( WLL


4/23

Data appeared as contours of residual resistance per tone of displacement

against prismatic coefficient and displacement length ratio, each chart

being for particular values of B/T and .

),,,(

,

3 p

R

R

CLT

BFnf

R

tsCoefficienandRatioslGeometricaFnfR

LV/


5/23

Gertler

Reanalyzed Taylors data

Used ATTC standard method

Added Schenherr roughness allowance

New contours published in (1954).

),,,(

,

3 pR

R

CLT

BFnfC

tssCoefficienRatiosandlGeometricaFnfC


6/23

Series 60

The series concerns the design of normal-bow single-screw ocean-going

forms. The varied parameters are CB, LCB, L/B and B/T, and their ranges

are as follows:

CB 0.60 to 0.80

LCB -2.48 to + 3.51% L

L/B 5.5 to 8.5

B/T 2.5 to 3.5


7/23

B.S.R.A. Series

A chart giving the resistance coefficient, at different , faired to the

base of block coefficient for the parent forms having standard values of

LCB, B/T and L/1/3 or L/B. Correction factors for variation in the

aforementioned standard values are provided in sets of charts.

LV/

CB 0.65 to 0.80

LCB -2.0 to +3.5% L

4.232 to 6.361

L/B 5.0 to 8.875

B/T 2.12 to 3.93

LPMB 0 to 50% L

3)/( WLL


8/23


9/23

Guldhammer and Harvald

Collected results from model tests, (NPL and SSPA series) in addition to

Taylors series and others,

Prepared diagrams which give the residual resistance coefficients CR for

merchant ship forms.

Provided correction diagrams for difference from standard.

rakedstem

andsterncruisererate

tionshapednormal

LCBdards

T

Bfor

CL

FnfC

tssCoefficienRatiosandlGeometricaFnfC

pR

R

,,mod

,sec

,tan

,5.2

),,(

,

31


10/23

CR

Fn

CP=0.8

CP=0.5

L/V1/3=constant


11/23

1- Corrections of CR for ships having :

2. Corrections of CR for ships having non standard LCB

5.2

101010

3

5.2/

33

T

B

TB

CCC R

TBRR

5.2TB

dards

R

dardLCsRR LCBLCB

LCB

CCC tan

3

5tan

33 101010

LCBLCB

CCC R

dardsRR

3

tan

33 101010


12/23


13/23

4. Corrections for bulbous bow

For a vessel with bulbous bow having (ABT is the sectional

area of the bulbous bow at the fore perpendicular an Ax is the area of the

midship section) the following corrections to 103CR are suggested:

With ABT/Ax=0.10 the bulbous bow is rather pronounced. For

the corrections are assumed to be proportional with size of bulb.

1.0/ xBT AA

Fn. 0.15 0.18 0.21 0.24 0.27 0.30 0.33 0.36+0.2 0 -0.2 -0.4 -0.4 -0.4 0.5

+0.2 0 -0.2 -0.3 -0.3 0.6

+0.2 0 -0.2 -0.3 -0.3 0.7

+0.1 0 -0.2 0.8

1.0/0 xBT AA


14/23

Schneekluth Formula for Residuary Resistance

Schneekluth developed Simple formula for CR based on Taylor-Gertler and

Guldhammer-Harvald data with the following limits :

The block coefficient CB should be less than (CB)Ayre where (CB)Ayre is

Froude Number Fn 0.17 to 0.3

Volume length Coeff. 2 x10-3 to 11x10-3Cp 0.5 to 0.8

L/B 5 to 10

B/T 2 to 4.5

3L

FnCBAyre 68.108.1


15/23

The approximate residuary resistance coefficient is

17.05.22.01005.00012.04103.3108.010103

3

3

3243

T

B

LLCFnC PR


16/23

Example

Consider a vessel with the following particulars:

LWL 123 m

B 17 m

T 7.083 m

8308 m3

LCB 1.5 m (aft)

CB 0.575

Cm 0.96

Estimate the ship's residuary resistance coefficient for the speed of

18 knots using Guldhammer and Harvald approach


17/23

for constant B/T value of 2.5

Using the above values and the appropriate chart we get a value of

CR=1.25x10-3

),,/( 31

PR CgLVLfC

6.05989.0

96.0

575.0

90.0400

18

264.012381.9

5144.018

0.68308

123/

3333.0

31

M

B

p

C

CC

LV

xxgLV

L


18/23

0.15 0.25

0.2 0.

Fn

0.00

4.00

8.00

12.00

16.00

1000CR

Fn=0.246

CP=0.6

103CR=1.25


19/23

Corrections

This value is based on B/T=2.5, however the actual ,

so a correction is required as:

The above value is also based on a standard longitudinal center of

buoyancy which can be found from figure below

5.24.2083.7

17

T

B

5.2

101010

3

5.2/

33

T

B

TB

CCC R

TBRR

234.1)5.24.2(16.025.110

5.216.01010

4.2

3

5.2

33

R

TBR

TBR

C

TBxCC


20/23

0.15 0.25

0.1 0.2 0.3

Fn

-4.00

-2.00

0.00

2.00

4.00

LCB

%

ofL


21/23

for , we pick

So,

The actual LCB =-1.5 m which is forward of the "standard" LCB,

hence correction is needed. The correction factor is a function ofboth Froude's number and prismatic coefficient.

Other corrections are required for different section shapes, bow,appendages, ..etc.

264.0gLV LLCBdards

022.0tan

mxLCBdards

706.2123022.0tan

23.0103

LCB

CR

511.15.1706.223.0234.110

101010

3

3

tan

33

LCBR

R

dardLCBs

R

LCB

R

C

LCB

LCB

CCC


22/23

Using Schneekluth formulaAt first check the limits of the formula applicability:

Fn=0.264 (within limits)

(within limits)

Cp=0.600 (within limits)

B/T=2.4 (within limits)

(within limits)

3

33 1046.4

123

8308 x

L

6364.0264.068.108.168.108.1 xFnCBAyre

BAyreB CC 575.0


23/23

Substituting these variables in Schneekluth formula

we get CR=1.254x10-3:

17.05.22.01005.00012.04103.3108.010103

3

3

3243

T

B

LLCFnC PR

17.05.24.22.046.405.00012.041046.4*103.36.0*108.0264.0*1010 33243

xCR

experimental methods for calculating ship resistance.pdf

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