A comparison of resistance and sideforce measurements on a TEMPEST yacht in a water channel and a towing tank

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A COMPARISON OF RESISTArCE AND SIDEFORCE MEASIJREPENTS ON A 'TEMPEST' YACHT IN

A WATER CHANNEL AND A TOWING TANK A. MILLWARD

THE UNIVERSITY OF LIVERPOOL

DEPT. OF MECHANICAL ENGINEERING

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.-

-for a range of speeds and leeway angles on a 1/6th scale model of a Tempest class yacht in the recirculating water channel.

A comparison was made between the results obtained in the water channel and those measured previously in a towing tank using

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1. INTRODUCTION

The general practice of using models to predict the perfornance of a full size ship has been accepted in principle for many years both for conventional ships (see for example Ref. 1) and also for sailing yachts (Ref. 2) which have to be tested through a range of leeway angles in order to simulate the sailing condition.

The sailing yacht operates at the interface of the two fluids

air and water. The sails provide the propulsion and extract their

energy from the air while the hull, which provides the load carrying

capacity, generates forces in the water. Thus the general forces

on a sailing yacht are a conbination of aerodynamic and hydrodynamic forces which are in equilibrium under steady sailing conditions. If both the hull and sail forces are known then it is possible to

predict the performance of the complete yacht. In such cases as

the present tests where only factors governing the hull were varied then the performance of the full size yacht can still be predicted by using some generalised sail forces such as the Gimcrack

coef-ficients (Ref. 3) and predictions of the Tempest performance from the present tests are given in Ref. 4.

In the present tests the model size was similar to that used in many of the small towing tanks but with the major difference that in a recirculating water channel the model Is held stationary

while the water flows past. Since the use of a water channel for

performance measurements is unusual a comparison has been made with results obtained on the very same model in a towing tank. The use of a water channel is relatively unusual although it can offer considerable advantages in certain areas, particularly for example with flow visualisation, pressure measurements and oscillatory motion, where it Is an advantage to have an unlimited running time.

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It is important however to have a uniform velocity distribution in the working section and a horizontal water surface free from waves. This has been achieved in the water channel used in the present tests.

2. EXPERIMENTAL FACILITIES AND METHOD

2.1 The Recirculating Water Channel

The tests were carried out in the recirculating water channel

(flune) at the University of Liverpool. A detailed account of the

design of the flume has been given by Preston (Ref. 5) but since the use of such a facility is unusual as compared to the

conven-tional towing tank for ship model testing, a short description is included.

The channel, which is shown diagramatically in Fig. 1, has a capacity of approximately 90,000 litres (20,000 gallons) 0f water circulated by an impeller driven by a 100 HP electric motor. The

working section ïs l.4ni wide, O.84m deep and 4.Orn long (4.Sft x

2.75ft x l3ft). It has a moveable false floor which can be adjusted

both in height and inclination to give a flat, level water surface

at the required operating speed. After the working section the top

layer of water, which contains most of the air bubbles caused by the presence of the model in the channel, is separated off by an adjustable flap or 'splitter plate'. This water is slowed down by a deepening of the local section and passes through several gauzes allowing the air time to settle out before the water is

re-intro-duced to the main circuit upstream of the impeller. The water then

passes through the impeller and along a circular section diffuser after which the cross-section becomes rectangular. The flow goes through two sets of guide vanes and a honeycomb to minimise swirl

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and is finally accelerated through a contraction into the working section.

The flow velocity in the working section can be set within

the range 0.03 to 6.1 rn/s (0.1 to 2Oft/sec). Owing to the

adjust-able floor any speed can be maintained without the presence of standing waves or a hydraulic jump - the critical speed with the floor in Its lowest position would be in the region of 2.7 rn/s

(9 ft/sec).

Preliminary work on the flume (Ref. 6) showed that there was an appreciable wake in the free surface caused by the boundary layer

on the upper surface of the contraction. In order to correct this

a jet injection system was installed - water is bled off from the

lowest part of the return circuit and pumped through a 1m wide slot running the full width of the channel at the upstream end of

the working section. The pump speed is controlled so that the

velocity defect at the free surface is corrected with the beneficial

side effect of further improving the flatness of the free surface. Although not relevant to this series of tests, two covers are

available for the water channel. One cover converts the open

channel into a closed water tunnel while the other cover fits over

the

whole working section allowing the air pressure above the water to be reduced to approximately 0.03 atmospheres thus creating a

free surface cavitation channel. In addition both covers can be

used together to provide a cavitation tunnel.

2.2 Model and Test Programe

The model was a Tempest to a one-sixth scale and made in

glass reinforced plastic. The hull lines are given in Fig. 2 and

other particulars of both the model and the full size yacht are

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manoeuvring tests were made, because the rudder area Is

approxi-mately 33% of the total lateral area and would therefore be exoected to contribute a significant proportion of the side force and hence of the induced resistance.

The model was floated in the channel attached to a dynanometer which allowed the model to heave and trim while measuring resistance,

side force, trim and heave for various leeway angles. The trim

and heel angles were measured and could be either free or fixed in suitable increments.

A series of tests was carried out over a range of speeds from

approximately 0.3 to 2.5 rn/s ₍ i ft/sec to 8.5 ft/sec) corresponding

to speeds on the full size yacht of between 1.4 knots and 12.2 knots. Measurements were nade of resistance and side force for a range of

leeway angles In the unheeled condition at the design displacement with the model free to trim.

Several further tests were also made to determine the effects

of changes in trin, displacement and heel. These are reported

separately in Ref. 4.

Turbulence stimulators in the form of sandstrips were fitted parallel to the leading edge of the keel and rudder and vertically in two positions on the force part of the hull following the

standard practice at the University of Deift where the cornoarative

tests were done (Ref. 7). Flow visualisation experiments were

made at the lower speeds to check that the turbulence stimulators were indeed causing transition.

3. DISCUSSION 0F RESULTS

As mentioned previously the general practice of using tests of models to predict the behaviour of a full size ship has been

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accepted for many years both for conventional ships

(see for

example Ref. 1) and for sailing yachts (Ref. 2).

However

recently Kirkman and Pedrick (Ref. R) have suggested that

tank

tests of small yacht models are less reliable,particularly

at

low speeds, than those on bigger models in predicting full scale

performance.

In the present tests the model size was similar

to that used In many of the smaller towing tanks and

could

there-fore be

subject to similar criticism with the possible alleviation

that turbulence in the water channel can be expected to ensure

turbulence stimulation at lower speeds than in a towing tank.

Unfortunately Kirkman and Pedricks deductions appear to be based

ori comparisons with full scale towing tests in the field together

with the only test of a full size yacht in a towing tank,neither

of which are wholely satisfactory.

In particular the scatter of

data from field trials Is comonly of the order of 20%, thus

making any deductions of scale effects of dubious value.

It is however, no part of the present report to discuss the

validity of their work or their conclusions but rather to show

that measurements obtained in the water channel are substantially

the same as those obtained in a towing tank.

Side Force

Figs. 3 and 4 show the side force (FH) obtained on the model

at different leeway angles (x) for a range of speeds for both the

towing tank and the channel.

It can be seen that in both cases

the side force varied linearly with leeway angle over the range

of angles tested.

This can be expected sin the Tempest has a

shallow hull so that the side force is predominantly developed

by the keel and rudder which reseITtle conventional aerofoils

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facilities the speeds were not identical so that a clearer comparison

is obtained from Fig. 5 where the results are given In the form of

side force per unit leeway angle against speed. The two sets of

results are very similar with slightly higher values of

_T

in the

channel at the higher speeds.

It was noted however, that in the

towing tank the rudder was inadvertently set to a small angle with

the result that the angle at which no side force was obtained

differed from the geometric zero leeway angle by 0.7 degrees.

Although this would not affect the value of

_T

equivalent to

the lift curve slope, for an isolated aerofoil shape the presence

of the hull has been shown to produce a crossflow (Ref. 9) which

would affect the sideforce on the rudder differently in each test

facility because of the difference in rudder setting and may

therefore account for the small difference between the two curves.

Upright Resistance

The measurements of model resistance for the tank and channel

are shown together in Fig. 6 in the form of -

against V where FR

V

is the total model resistance and therefore includes stimulator

resistance.

The curves show reasonable agreement but with a lower

result in the channel at higher speeds.

It was noted that the

towing point in the two tests was at a different height - in the

channel It was approximately at deck level whereas in the towing

tank the towing point was significantly higher.

Thus the nose

down pitching moment in the channel would have been smaller and

the trim angle under test would have been larger.

This has been

shown in the later tests on the effects of trim (Ref. 4) to result

in a trim closer to that required for minimum resistance, an

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it seems likely that the difference in resistance obtained in the channel and the towing tank is most probably caused by differences in trim as a result of the variation In towing height.

A second possibility for the difference since it is more noticeable at the higher speeds is that there is a reduction in

resistance due to shallow water effects in the channel. The

speed where the difference occurs (Fig. 6) corresponds to a Froude number, based on water depth, above 0.7 so that shallow water

effects may be present. However, it has previously been shown

(Ref. 10) that shallow water effects do not appear to be

signifi-cant In the channel on planing craft at such a light displacement. This point Is being further investigated but in the present tests

it seems more likely that the difference in towing height is the explanation.

Yawed Resistance

The corresponding curves of resistance, plotted as - , against V

model speed V for the channel and the towing tank are shown in Figs

7 and 8 respectively. The agreement is less satisfactory than for

the upright resistance.

It is noted however that the results obtained in the towing

tank are higher as might be expected with the added Induced resistance of the incorrectly set rudder referred to previously. An attempt was made to simulate this alignment error in the channel for a few selected speeds and the results indicated that this explanation could account for the majority of the difference found between the two Sets of resistance curves.

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and the channel showed reasonable agreenent although with differences which were attributed qualitatively to minor differenoes in

experi-mental technique and experiexperi-mental errors. These tests confirm that

results obtained in a good water channel can be expected to be the same as those obtained in a towing tank.

ACKNOWLE DCEMENT

The author is very grateful to Professor Gerritsma for permission to use the towing tank at the University of Deift and to Mr. Moeyes

for carrying out the tests. The author wishes to thank the British

Council for the gift of a Young Research Workers Award to enable

him to visit Deift and also wishes to thank Mr. Ian Proctor, the designer of the Tempest, for supplying the hull lines and much other Information.

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a

RE FE RE1'CES

Cornstock J.P. "Principles of Naval Architecture". SN.A.M.E.

(1967).

Davidson K.S.M. "Some experimental studies of the sailing

yacht11. Trans. S.N.A.M.E., Vol. 44, pp. 288-334, (1936).

Spens PG. "Sailboat test technique". Stevens Institute TM 124, (October, 1966).

Miliward A. "The effect of trim, heel and displacement on a

'Tempest' Class yacht'. Liverpool University, Mech. Eng. Dept.,

Report fM/A18/75 (1975).

Preston J.H. "The design of high speed, free surface water

channe s'. NATO Advanced Study Institute on Surface

Hydrody-namlcs, (1966).

Nicholson K. "Measurements of hovercraft wavemaking drag on

a circulating water channel". Liverpool University Ph.D.

Thesis, (1972).

yes G. "Measured and

calculated

side force on a Tempest

modéTjrforman

prediction

of the ship". Technische

Hogeschool Delft, Laboratorium voor Scheepsbouwkunde Report No. 346, (February 1972).

Klrkman K.L. and Pedrick D.R. "Scale effects In sailing yacht

hydrodynar!ric testing". Trans. S.FLA.M.E., Vol. 82, pp 77-125,

(1974).

Miliward A. "The Induced drag of a yacht's hull". Southampton

University Yacht Research Report No. 24, (1968).

Miliward A. 'The use of a water channel for model tests on

planing hulls". Liverpool University, Mech. Eng. Dept.,

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side force on

model FR

resistance on

model V

model speed

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Yacht and model details

TABLE 1

Symbol Description

LOA length overall

L0w1 length on design waterline

BD beam on design waterline

T total draught

*total displacement *design crew weight

Sw total wetted area

SA effective windward sail area

SAD effective downwind sail area

* _{It Is understood that the average crew weight in competition}

is 1.668 KN and with an increase in boat weight the total would

currently be 6.318 KN. The tests and predicted performance were

made with the design weight so that some alteration In absolute values can be expected but the relative values obtained in the effects of increased crew weight will remain valid.

Units Yacht Model

m 6.70 1.117 m 5.87 0.978 in 1.44 0.240 in 1.10 0.183 KN 5.690 KN 1.619 in2 5.90 in2 18.74 m2 31.97