Resistance experiments with divided ships models

MEDDELANDEN

FRAN

STATEN S SKEPPSPROVNINGSANSTALT

(PUBLICATIONS OF THE SWEDISH STATE SHIPBUILDING EXPERIMENTAL TANK)

Nr 30 GOTEBORG 1954

RESISTANCE EXPERIMENTS WITH

DIVIDED SHIP MODELS

BY

R. RODSTROM

GIIMPERTS FORLAG

Introduction

The resistance of a ship is influenced to a certain extent by flow interference between the fore and after bodies.

In the past, the

effect of this interference has been studied either by means of

resistance tests with a series of systematically varied models, or by measuring the normal water pressure at a number of points on the model hull surface. The latter method has only been employed to a very limited extent on account of experimental difficulties and excessive costs.

In the experiments described below, another method was adopted;

this involved measuring the resistance of the fore and after bodies

separately. In order to facilitate this, the model was cut transversely

at midships, both halves being fitted with watertight bulkheads at the division.

In the tests, the fore and after portions were

restrained vertically and transversely in relation to each other but were free to move independently in the longitudinal direction.

It

was hoped to be able to study the interference effects on the basis of the separate fore-body and after-body resistance results obtained in these tests.

Experimental Technique

Both the models employed in these experiments had a length

between perpendiculars of 6.096 metres or 20 feet. The testing arrangements, which were designed to suit this size of model are illustrated in Fig. 1.

As will be seen from this diagram, each model was divided at

section A-A at the middle of the length between perpendiculars

and the end-bulkheads (a) of the fore and after bodies were situated 50 mm from the division.

From each end-bulkhead to the 1.5 mm gap (c) between the fore and after bodies, the model hull was constructed of 1.5 mm brass

plate (b) and the outside of the plating was finished to conform

exactly to the hull lines. The plating thus enclosed a water-filled space 100 mm long between the end-bulkheads (see section D-D).

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Preliminary tests had shown this to be a satisfactory method of separating the two halves of the model. At the same time, it

was shown ,that the water pressure on the end-bulkheads when under

way could be assumed to be only that due to the static head. The latter was determined by measuring the water level between the bulkheads by means of a U-tube manometer.

As mentioned in the introduction, the fore and after portions

were controlled relative to each other and the guide-frame apparatus

is illustrated in Fig. 1. Two longitudinal steel girders (e) were

attached horizontally, one each side, to transverse beams (d) on the after-body. The girders (e) were also connected to each other by beams (f). A horizontal rectangular steel frame (h) was attached to transverse beams (g) on the fore-body and longitudinal guide rails (i) and (j) were fixed in pairs to the girders and beams (e) and (f). Rollers (k) and (1), mounted on ball bearings, were fitted on the

frame (h) and ran between the rails (i) and (j) respectively.' Sections

B-B and C-C show the construction of the guide apparatus and

rollers. Thus the two halves of the model were guided vertically and transversely relative to each other but they could move inde-pendently in the longitudinal direction. This relative longitudinal

freedom was necessary for the measurement of the resistance. The resistance was measured on the thrust balance of a Gebers-type propeller dynamometer (m), placed in the forward half of the model

(see Fig. 1). A light, stiff rod connected the after half of the model

to the dynamometer in such a way that there was a gap (c) of 1 5 mm

between the two portions. The movement of the dynamometer balance was adjusted for a variation of about ± 0.5 mm from the mean position, thus allowing the gap between the two halves of the model to vary between the limits of 1.0 mm and 2.0 mm.

By this means, if the model was towed by the forward half, the after-body resistance could be measured, while if the model were

towed by the after portion, the fore-body resistance could be

recorded. The actual towing force was provided in the usual way through the carriage resistance dynamometer and this, of course, recorded the total resistance at the same time.

The experiments were carried out at a draught corresponding to that of a fully loaded ship (L.W:L.). It should be mentioned here

that the two halves of the model were ballasted down to this

draught separately and then connected to each other; thus any

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Model Particulars

The two models used in these investigations, namely Nos. 72 and 74, were made of wood and were each 6.096 m in length. The body plans and general particulars of these models are given in

Fig. 2 and Table 1 (p. 13) while their stem and stern contours are

shown in Fig. 3. The models had been employed previously for

ordinary resistance tests and their form can be regarded as

repre-sentative of the fast cargo ship type.

The models were altered for some of the experiments by intro-ducing a parallel middle-body, 6.096 m in length (the same length

as the model), between the fore and after bodies. This was in order

to be able to determine the effect of a long after-body on the

resistance of the fore-body, since the result should be different from

that obtained with the normal after-body.

The determination of the effect of the parallel middle-body on the

resistance of the after-body was not considered to be of such _great

importance in this case. It would be expected, however, that behind

-a long parallel section, the frictional resistance would decrease _on

account of the increased thickness of the boundary layer.

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Fig. 3. Models 72 and 74.

The same apparatus was used for the tests with the extended

models as for the tests with the models of normal length. It should

also be mentioned that no turbulence-stimulatingdevices were

emp-loyed in any of the tests.

Experimental Results

The values given herein are the results of only a part of all the experiments which were carried out. It has already been mentioned that extensive preliminary tests had to be conducted to determine,

for example, the most suitable construction for the dividing section

and the correct adjustment of the dynamometer,

etc. The latter

was necessary in order to obtain satisfactory damping and stability in the resistance measurements. The manometer apparatus for measuring the water pressure in the space between the bulkheads gave rise to certain difficulties and some of the measurements had

to be disregarded on account of innaccuracies.

The measured resistance of the fore and after bodies consists

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due to the water pressure exerted in the _{space between the}

bulk-heads. This pressure has been assumed to be of equal magnitude on the two bulkheads but opposite in direction on the two halves

in relation to their direction of motion. Thus if all the resistance components are added, the bulkhead pressures are eliminated and

the total resistance of the complete model is obtained.

Figs. 4 and 5 show the various resistance results for Models 72 and 74 respectively on a basis of model speedvn, and FROUDE number

v.

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the length of the model between perpendiculars). The results of

trim measurements are also shown in these diagrams Fair curves

have been drawn. through the experimental spots.

It is evident from Figs. 4 and 5 that the resistance of the after-body is smaller than that of the fore-after-body, particularly at the lower

speeds. Furthermore, the resistance curves show pronounced humps and hollows. In general the humps on the fore-body curve occur at the

same speeds as the hollows on the after-body curve and vice versa.

The sum of these resistances or the resistance curve for the complete model is therefore fairly even in character.

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In the case of Model 72 (see Fig. 4), the total resistance was also measured and the values found to agree closely with the sum of the fore- and after-body resistances, thus showing the experimental technique to be satisfactory. It should be mentioned that these

three tests were carried out at different times.

Figs. 4 and 5, referring to Models 72 and 74 respectively, also each

include a curve for the resistance of the fore-body of the extended

model. These two curves have the same character as the correspon-ding curves for the normal-length model, but they both lie somewhat

lower.

The measurements of the water pressure on the end-bulkheads are plotted for Model 72 in Fig. 6. One curve refers to the normal-length model and the other to the extended model. In the case of the extended model, of course, the water pressure vvas measured in the space between the fore-body and the parallel middle-body.

Similar curves could not be obtained for Model 74. The water pressure is expressed as the change in the water level between the

bulkheads, 4 T, relative to the model at rest, a fall in the level being

denoted as niinus.

It is evident from Fig. 6 that there was a drop in the water

pressure in both cases, the decrease being greater with the extended

11

model. The humps and hollows of these curves appear to occur

with about the same frequency as those of the resistance curves. For the purpose of enabling the wave profiles to be studied, the models were photographed when under way. The photographs are

shown in Fig. 7. The models were divided at Station 10 and the longest horizontal line on each station denoted the waterline at

rest; the vertical distance between the horizontal lines was 50 mm. In order to obtain a coriect resistance value, an allowance must be made for the change in the pressure on the bulkheads. The cor-rection is obtained by taking the difference between the thrust on

the whole bulkhead when the model is in motion and that when

the model is at rest. This difference in thrust is given in kg by

(Bx

AT

LI

\

AP=T x0 1 4-

_{2 x 1000C)/}

where AT is the change in water level in mm, C) is the area of the bulkhead in m2 and B is the breadth of the model in m. The plus sign is used for a rise in the water level and the minus sign for a fall. If the level drops, as in this case, the measured resistance of

the fore-body is reduced by AP and that of the after-body is

increased by the same amount. In the models in question C) =

0.2962 m2. Thus, a change of 1 mm in the water level corresponds to a correction of about 0.3 kg.

According to Fig. 6, in the case of Model 72 the maximum drop in the water level was about 9 mm at a speed of about 2.0 m/sec.

and this gives a value of AP= 2.7 kg. The minimum change in water level was 1-1.5 mm at the lower speeds corresponding to

AP= 0.3-0.45 kg.

The correction for the higher speeds seems excessive and this may be due to errors in measurement. It appears from the photo-graphs, however, that the water level between the bulkheads was about 25 mm lower than that outside the hull. This indicates that there was an inflow of water through the gap at the surface and a corresponding outflow probably in the region of the bilge.

It will be seen from Fig. 6 that the resistance correction for Model

72 extended is qualitatively similar to that for the normal-length model. Quantitatively, however, there is a difference of 1 to 3 mm in AT corresponding to an increase in A.P of 0.3 to 0.9 kg. In this

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connection, it should be mentioned that an examination of the

photographs in Fig. 7 shows that the fore-body wave profiles on the normal-length model have a different shape from those on the extended model. This should be borne in mind when interpreting the results.

Table I

Principal dimensions of the models

Acknowledgements

The author wishes to express, his gratitude to the Martina

Lundgren Foundation for Maritime Research for

financing these investigations.

The author is also very grateful to Professor H. F. NoRnsTRom,

the Director of the Swedish State Shipbuilding

Experi-mental Tank, for his kindness in enabling the experiments to

be carried out there.

Thanks are also due to Mr. DACRE FRASER-SMITH, B. Sc., who

has translated the paper from the Swedish.

Unit Model 72 Model 74

Length pp ... ...

m 6.096 6.096

Breadth m 0.8535 0.8535

Draught ... ...

. m 0.3556 0.3556

Displacement kg 1217.2 1226.5

Immersed midship section area m2 0.2962 0.2962

Block coefficient 0.658 0.663