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ShipResistance

Calculations

Chapter 05(Harvald)

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Telfer's Method:

Consider a family of geometrically similar models

Situation 1

1. Keep Froude Number Constant

2. Determine the specific resistance by varying Re

Situation 2

1. Keep Re constant

2. Determine the specific resistance by varying Fr (Speed

Length ratio)

3. Since Re is constant, Specific Frictional resistance is

constant. So chan e occurs onl due to wave makin or


more generally inertia resistance.

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3. All contours of constant speedlength ratio will be mutually

parallel to a base of Re

4. To a base of speedlength ratio, all contours of Re will be

parallel.

5. This principle of parallelism was brought out by Telfer

6. To make use of this principle as a practical means of

extrapolating model specific resistance, it was essential to

determine the law of variation of the constant speed length

contours with Re.

7. Telfer Proposed the function


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8. Here 'a' for total specific resistance depends on the speed

length ratio and is constant for constant speedlength ratio

and 'b' depends on the amount of total resistance subject to

scale effect.

9. The value of 'b' was found for very fine forms to be the same

as that derived from plank tests.

10. The ship extrapolators will have a slightly greater slope

than that of the plank and in general every form of model

will have a different extrapolator.

11. The extrapolator for any form can be determined when a

number of eometricall similar models are tested and

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12. The Figure shows Telfer's Method for a fine form. By using

as abscissa

the extrapolator will be a straight line


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13. The first condition that an extrapolation method has to fulfill

s t at t s ou ena e t e exper menta resu ts o ta ne w t

models of the same ship to various scales to be derived from

one another.

14. Therefore by using the results obtained from experiments

with a number of geometrically similar models a so called

model family, this condition is automatically satisfied.15. The slope of the extrapolator is well determined in the region

covered by the experiments carried out with the model family.

However even with reliable results from experiments with a

large model family at one's disposal, extrapolation outside the


experimental region of the Re remains a risky affair.

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Resultsfromexperimentswithamodelfamily

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16. The total resistance coefficient is

given as function of Re for different models of the ship

17. The above equation gives the total resistance coefficient forthe underwater part of the hull of the smooth ship.

18. If the coefficient CTS for the rough ship is wanted, a

roughness allowance CA ( in general called the incremental

resistance coefficient for modelship correlation) has to be

added.

19. An air resistance coefficient can also be added if this

correction is not included in the CA.


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20. The curves for constant Froude numbers are nearly parallel to

Shoenherr's flat plate friction drag formula.

. .

22. The resistance of the ship is then determined by

TS


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Problems With Telfer's Method:

1. Even when using a large model family the distance from the

model region to the ship region is very large. Minor

inaccuracy on the extrapolator can imply a large inaccuracyon the resistance forecast.

2. One of the conditions to be met for obtaining satisfactory

results from experiments with model family is complete

similarity. This means that the ships model as well as the

surroundings have to be similar.

3. When performing experiments with big models in the

famil , the towin tank boundar will often be at a distance


that it can give rise to interfering influence.

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4. Usually the wall results in increased model resistance.

5. When testing the small models in the family the flow over a

large party of the models can be laminar. If laminar flow

occurs along part of the model, the result with be that a

resistance is measured which is low compared with those in

turbulent flow.

6. To perform experiments with a model family is expensive and

time consuming.

7. Some of the largest families have been that of Simon Bolivar

model family (Lammeren 1938) and the series in the socalled

Victor ship research program (Lammeren et 1l 1955). In this

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last family a 21 m model model was also includede

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ITTC Method:

"how to transform the model test result from model to full

.

2. This method (ITTC Method), is based on Froude's principle

3. In 1957, ITTC decided that the line given by the formula be

.

. .coefficient for the model is determined by the towing test


an rom e ormu a

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5. The residuary resistance coefficient for the model is then

calculated by CRM = CTMCFMwhere the frictional coefficient

resistance is calculated from

6. Now it is supposed that the residuary resistance coefficientfor the ship at the same Froude number as for the model and

at the corresponding Re number is CRS = CRM

7. Using ITTC 1957 modelship coefficient for a smooth ship

can be determined by CTSS = CFS + CRM and CTS = CFS +CRM + CA

8. CA can be taken same for all ships or

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Hughes's Method:

1. In Hu he's Method we use

2. There was a good agreement of this formula

with the ex eriment curve.

3. Hughes Proposed that the hull resistance as


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.

. .

edge effect) of a plane surface area and the same mean length

as e u .

b. The form resistance, being the excess above (1) that would be

exper ence y t e u eep y su merge as part o a

double model.

c. The free surface resistance, being the excess of the total

resistance of the surface model above that of a deeply

submerged hull when part of a double hull

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4. This division is only for analytical purposes only; these three

.

5. On the other hand it is a logical one since (1), The sum of (1) + (2)

.

6. Hughes meant that there must be a universal law governing the

symmetrical form when towed at zero incidence submerged in a

fluid without boundar interference.

7. Streamlining implies that there is no separation of flow at any

oint.

8. Symmetry about two planes at right angles is essential to ensure

no lift in an direction when the bod is towed in the direction of


its axis.

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9. The law proposed by Hughes was For a given body, the mean

specific resistance is a constant ratio of the specific resistance of

a plane surface of infinite aspect ratio at the same Re. The ratio

is independent of Re and depends only on the form of the body.

10. The resistance equation could be written as

Total resistance = Base friction resistance + form resistance +

free surface resistance.

Using the law, this now becomes

Total Resistance = (Basic friction resistance)* r + free surface

resistance where r is the resistance ratio and is constant

factor for a given hull form or r = 1+k where k is the form


factor

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8. For basic friction resistance coefficient, one can use the

9. The curve of CF together with the curves of CF *(1+k) for


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10. The value of r or k can be determined from the low speed test.

a resistance curve CT is drawn, and the curve CF (1+k) having

.

11. Thereby k is determined and the CF(1+k) curve can be used as

.

12. The free surface resistance can be found from the model tests

resistance. It will be assumed that this scales up according to

' .13. A correction CA taking into account the roughness of the hull


sur ace can e un er a en an e o a res s ance or e s p

can be calculated by

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14. With regard to the decisions made at ITTC ( after discussing

single line (the ITTC Model Ship correlation line) owing to the

' '.

15. Many towing tanks have used Hughes method with good

,.

16. Often this method is combined with Prohaskas method.

.

parameters has been carried out at NPL.

. , ,the block coefficient and with the length displacement ratio


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19. For shi s below 100m k is ver difficult to determine.20. Many of these small ships have sharp shoulders and shapes

leadin to se aration and hi h ressure dra .

21. Owing to the procedure normally used, the high resistance

form factor k.

. . . ,

the highest being for full forms. It is unrealistic to assume

factors.

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23. In cases where strong vortices are created owing to

sharp shoulders and where the model tests have been

given a form factor that is much higher than that of aconventional ship of similar dimensions, then some

towing tanks discard the form factor assumption and

instead treat form drag in the same way as wave drag.

24. This means that the form drag coefficient is assumed to

the same in model and full scale.

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