THE SHIPBUILDING LABORATORY OF THE DELFT
UNIVERSITY OF TECHNOLOGY
by
Ir. J . G E R R I T S M A *)
Sniiiiiiary
I n the present article the general arrangement, the apparatus and the method of working of the modern Shipbuilding Laboratory of the D e l f t University of Technology are discussed.
1. Introduction
From 1937 onward until recently the Shiping Sub-department had, in the then existShiping building for Mechanical Engineerbuilding and Naval A r c h i -tecture, a laboratory at its disposal in which re-searches could be made into matters concerning hydrodynamics w i t h regard to ships.
The unpretentious equipment of this laboratory consisted principally of a modeUexperiment tank, having a length of 37 m and a breadth of 2.7 m. This tank was of semi-circular section and con-structed of steel plate.
Towing was done by an unmanned small carriage carried by a rail óver the tank.
This towing-carriage carried the resistance dynamometer, w i t h the aid of which the resistance of models up to approximately 1.5 m in length could be measured.
The equipment of the laboratory did not permit modél self-propulsion tests to be carried out; as a matter of fact, the restricted model length would imply such a small size of model propeller that serious difficulties could be expected in the inter-pretation of the results of the measurements.
The research work and the practical work done by the students of naval architecture was therefore mainly confined to measurements of resistances i n sniooth water.
Soon however the analysis of log-books of sea-going ships, in coniiection w i t h allowances required to be made on power and the loss of speed i n a seaway, created a need f o r model-experiment tests in artificially generated waves.
For obtaining some idea at least of the possibili-ties and difficulpossibili-ties met w i t h during such a research, a wave generator of the flap type was installed in 1953, by means of which regular waves having a maximum length of 3 m and a maximum height of 0.09 m could be generated.
A number of investigations were carried out w i t h this apparatus, such as those concerning the degree
* J Princtpnl Scientific O f f i c e r of the Siijpbuijding Laboratory at the University of Technology in Delft.
of pitching, heaving and rolling in waves o f differ^ ent dimensions, the disturbance of the wave profile by a vertical cylinder ( i n connection w i t h the construction of the seakeeping laboratory of the N e -therlands Ship Model Basin (N.S.M.B.) at Wage-ningen) etc.
For carrying out statical-stability tests a so-called moment indicator was procured in 1951.
W i t h the aid of this apparatus the statical sta-bility of a model of the ship to be examined can be measured; i n a few hours the curves of righting arms of statical stability can be determined up to angles of approximately 90". A more detailed des-cription of this moment indicator is given in the present article.
One of the opportunities given to the Shipbuild-ing Sub-department by the construction of a new building for Mechanical Engineering and Naval Architecture w i t h the attendant laboratories was to extend the facilities f o r experimental research in a way so far unknown.
To this end, two new laboratories were built, yiz. a. the Shipbuilding Laboratory, whose aim, general arrangement and method of working w i l l be discussed in the present article; and
b. the Laboratory f o r Ship Structure Research (see Part. I ) .
2. Purpose of (he Laboratory
The operations conducted i n the Shipbuilding Laboratory can be summarized in three groups; this classification w i l l also give an idea of the aim pur-sued by the laboratory.
The laboratory primarily provides mechanical aid to teaching matters concerning the resistance and propulsion of ships. The students of naval architect-ure are, in the third or f o u r t h year of their course; expected to carry out a simple test i n the laboratory and to work out the results obtained.
Besides, there is a possibihty of research work being done by student assistants under the direction of the staff.
For those who have chosen the theoretical course in their study of N a v a l Architecture, there is an opportunity of completing their studies of a hydrodynamical subject w i t h regard to ships, which i n -volves independent research work";
SECTIOH A.B
Fig. I . Gciicral-arriiiigciiiciil plan of llic Sliipbnililiiig Laboratory
Secondly, a considerable portion of the operations consists of research w o r k carried out under the direction of the staff of the laboratory.
I n this connection the cooperation w i t h the Ne-therlands Ship Model Basin at Wageningen must be mentioned:
The contact between the two institutions is evi-dent, f o r instance, f r o m the monthly meetings between the scientific workers of both laboratories^ where Views are exchanged upon work that is being or going tp be done.
I n addition, the Shipbuilding Laboratory is re-presented in the group of workers making invest-igations into the behaviour of ships in a seaway ( „ Z e e g a n g s o n d e r z o e k " ) , which was established by the N.S.M.B. f o r considering the problems arising f r o m a ship's behaviour i n .a seaway.
T h i r d l y , the laboratory may accept commissions f r o m industry.
I n order to avoid undesirable competition between the D e l f t Laboratory and the N.S.M.B., a specified research which falls w i t h i n the sphere of both, institutions is to entail equal charges,
3. The construction and general arrangement of the laboratory
Fig. 1 represents the general-arrangement p k n of the laboratory.
The length of the building is 117 m and its breadth 13.J m , the direction of the longitudinal axis being f r o m east to west.
The foundation consists of 88 Franki piles having lengths of from* \ 6 to 17 metres below ground level. O f these 88 piles, 30 are placed under the model experiment tank.
The passage connecting the four f r o n t blocks of the building, f o r Mechanical Engineering and Naval. Architecture leads to the ship-model workshop (1) w i t h the adjoining carpenter's workshop ( 5 ) .
O n the south side of the building there are in succession: the instrument^making establishment
( 6 ) , the store room {?)., the entrance of the south f r o n t ( 8 ) , the cloakroom and lavatories ( 9 ) , the drawing-office ( 1 0 ) , the scientific officer's room ( 1 1 ) , the rooms f o r the assistants ( 1 0 ) , the com-partment f o r carrying out the statical-stability tests (12), the electronics workshop which is also used as the switch room ( 1 3 ) . Behind these spaces is the flow canal ( 1 9 ) . O n the north side of the building the large model experiment tank (1'6) is situated w i t h the cavitation tunnel (22) at its end. Daylight enters the laboratory through small windows in the north f r o n t .
The second storey is partly used as the mould l o f t ( 2 3 ) , where on a floor having an area of 1000 sq. m lessons are given in development technique,
and partly ias a drawing office (412 sq. m ) f o r students of naval architecture.
This second storey, therefore, does not belong to the laboratory proper, but ensures an excellent, i n -sulation against changes i n temperature, as also do the series of rooms on the south side of the building. The heating elements provided haye ample capa-city for ensuring a temperature of H ° C in the hall during the winter months. Fig. 2 is a photograph showing this hall together w i t h the model^experi^ ment tank and the^ f l o w canal.
4. Apparatus and method of working A. The ship-model workshop
I n this space the ship models tO' be tested are ma-nufactured f r o m p a r a f f i n wax ( i n special cases f r o m wood). T o the p a r a f f i n wax 3 per cent, of bees wax is added so as to obtain a somewhat less brittle material; the melting point of the mixture is approximately 65° C.
Melting is done in an electrically heated furnace ( N o . 3 of f i g . 1) of 9 k W , having a capacity o f 300 litres, a thermostat keeping the temperature constant at 7 5 ° C.
The casting mould is modelled in a clay trough
Fig. 2. Hall Wllh luojcl-cxpcriiiieiil lank and jlow channel
(No. 4 of Fig. 1 ) , during which an allowance of about 1 cm is required.
A core of lathwork, covered w i t h canvas leaves a thickness for the sides of the model of f r o m 2 to 4 cm, depending on the size of the model.
The casting-mould, which is provided w i t h the required risers, can next be filled w i t h liquid pa-r a f f i n wax f pa-r o m the fupa-rnace, so that aftepa-r the coagulation of the material the model is ready f o r further treatment. This is done i n the milling machine ( N o . 2 of Fig. I ) ; the milling machine cuts waterlines into the casting, and f o r this purpose a waterline drawing of the model is used. This machine, therefore, is essentially a copying machine transferring a waterline f r o m the drawing to the model by means of two mills (one on the port and one on the starboard side).
The waterlines are so many i n number that they practically determine the shape of the model.
The material that is l e f t between the waterlines cut into the model can next be touched up by hand until a smooth surface is obtained.
D u r i n g this treatment transverse templates are used in some places to indicate the correct shape. The maximum length of the model is about 3 . 1 m . For stimulating turbulent flow past the model a so-called trip wire 1 m m diameter is provided at 5 per cent, of the length of the model f r o m the stem. A f t e r the model has been ballasted to the required displacement w i t h the aid of a weigh bridge (having a limit of 25 0 k g ) , the model is ready f o r carrying out resistance and self-propulsion tests in still water. W i t h tests carried out in longitudinal waves, when the model is subjected to pitching, heaving
and surging motions, care has to be taken that the longitudinal moment of inertia of the model has the correct ratio w i t h respect to the prototype. W i t h a scale a f o r linear dimensions, the scale f o r the moments of inertia w i l l be a'.
I n the laboratory an apparatus has been con-structed, the so-called inertia table (Fig. 3 ) , w i t h which the moment of inertia of a model about its centre of gravity can be determined quickly. By shifting ballast weights longitudinally w i t h respect to the centre of gravity, a certain moment o f inertia can, therefore, be established.
Fig. 4 shows the principle of the inertia table diagrammatically. I n the first place the hinge shaft (1) is, w i t h respect to the table ( 2 ) , adjusted to a height a, which is equal to the height of the centre of gravity of the model above the keel.
The table, on which the model is placed i n such a way that its centre of gravity is situated i n the
15
hinge shaft, is at one of its ends connected to the foundation (4) by means of springs (3) capable of being disconnected. "When the table is made to Oscillate about the hinge shaft, the period T of this oscillation w i l l be a measure of the moment of inertia of the model.
The relation between period and moment of i n -ertia can be established experimentally by placing known weights on the extremities of the table; the moment of inertia of these weights and the period of the oscillation can be easily determined. This calibration has been carried out f o r different values of Ö .
I t w i l l be clear that during the shifting of the weights the centre of gravity of the model should remain in the same place w i t h regard to both length, and height. This can be verified by uncoupling the springs ( 3 ) : the inertia table w i l l then have become a balance, and a t i n y spirit level on the table i n -dicates the horizontal position i f the centre of gra-v i t y is in its correct position in the length of the model.
I f this is the case, a light weight at one o f t h e ends of the table w i l l result in an angular rotation which is independent of whether the model is present or not, i f the centre of gravity of the model lies in the hinge shaft.
From the above, i t w i l l be evident that, w i t h the aid of the inertia table:
1. the required moment of inertia; 2. the correct t r i m , and
3. the height of the centre of gravity of the model can be fixed.
Adjacent to the model workshop is the carpent-ers' workshop, i n which some wood-working machines are installed f o r the construction of mo-dels (e.g. f o r wooden momo-dels, cores etc.).
B. The instrument-making establishment
This establishment is f o r the purpose of manu-facturing measuring instruments and model pro-pellers, f o r which a number of metal^workirig machines are installed. Here the greater part of the measuring apparatus is manufactured.
The model propellers to be used in the cavitation tunnel and f o r carrying out self-propulsion tests are first cast in a plaster mould, due allowance being made during casting. The material used is a mixture of antimony, t i n and bismuth. O n the propeller workbench a large number of little holes are drilled in the high- and low-pressure sides of each blade, the greatest depth of these holes extending as fair as the required surface of the propeller. The super-fluous material is removed by handfiling, so that all holes w i l l just have disappeared. N e x t , the dimens-ions of the propeller, when finished, can be checked on the measuring bench.
The instrument-makirig establishment also serves as a service workshop f o r the Laboratory f o r Shipstructure Research. For m.anufacturing larger jobs the facilities of the Central Workshop of the D e l f t University of Technology are available, where a comprehensive collection of machine tools is found.
C. The model experiment tank and the towing carriage
The model experiment tank has the following dimensions:
length 96.80 m ; breadth 4.28 m ; depth 2.70 m ; the ordinary depth of water is 2.50 m. The tank, which is of rectangular section, was built in 5 sections, which were afterwards inter-connected.
The first section of the tank consists of a little harbour shut o f f b y a removable wooden partition to protect the models against the waves; when they are not being used.
I n f r o n t of this partition is a beach f o r the pur-pose of damping the waves being generated on the opposite side of the tank. The beach consists of an angle-bar structure covered at the top by wood on which gravel has been provided. By means of four screw spindles the. whole beach can be raised or lowered and be adjusted longitudinally at the angle desired (usually at an angle ^ 1 0 ° ) .
FiK. 4. Principle of lhe incrlin table shown liia^ramiiialically
lUCTtOH PdlUUflC
Fig. 5. Principle of lhe pncunialic wave generalor shown diagramniaikaUy
Its section is parabolic longitudinally w i t h a cir-cular portion at the top, which touches the surface of the water.
A t about half the length of the tank there is a pump room,, where the water can be pumped out of the tank into the sewer or into the f l o w canal.
The tank is filled w i t h ordinary company's water. N o chemical substances for preventing the growth of algae are added. Hitherto i t has appeared that the growth of algae does not occur, which is pro-bably due to the small amount of daylight entering the tank f r o m the north.
A t the end of the tank a wave generator of the pneumatic type is installed,
Some time ago, dr. Todd of the Taylor Model Basin at Washington placed the design of such a wave generator, to be used f o r a tank of smaller dimensions, at pur disposal.. W i t h the aid of the data obtained the installation represented diagram-matically in Fig. 5 was constructed.
Via a valve casing a fan engine alternately i n -duces an increased and a reduced pressure in a dome which is placed above the water, The changes caused by this in the level of the water are propagated in the tank as waves.'
A t its bottom the dome can be partly shut o f f by a steel platé: the small opening l e f t is used f o r pro-ducing waves of very short lengths.
Behind the wave generator a filter is provided, which causes slight disturbances in the wave profile to disappear.
The period of the valve determines the period, hence the length of the waves since i t can be stated w i t h fair approximation that w i t h a depth of water of at least half the wave length:
2 n X g where:
T = the period,
A = Éhe wave length, and g = the acceleration due to gravity.
The drive of the valve is provided by an elec-tronically controlled electric motor of 1.2 k W , w i t h which periods of f r o m 0.7 to 2 seconds can be ob-tained w i t h corresponding wave lengths óf approx-imately 0.80 and 6.25 m respectively.
The wave height is regulated by adjusting the supply óf air by the f a n engine to a specified value
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by means of a sliding bottom in the fan-engine casing. The size o f the f a n engine is thus, as i t were, either increased or reduced at a constant number of revolutions, so that in- this way the supply of air can be varied. Waves as high as about 30 cm can be obtained.
The principal data of the fan engine aie: Maximum amount of air supplied 9,000 cu. m / h ; maximum difference of pressure on high- and low-pressure sides 60 cm of water; number of revo-lutions 1,500 r e v / m i n ; and maximum power re-quired 2 5 hp,
Fig. 6. Towing carriage
The advantages of a pneumatic wave generator are:
1. rapid control and httle power for regulating period and amplitude;
2. the possibility of first adjusting the period and switching in the fan engine afterwards, so that waves of the correct length can be produced directly;
3. the absence of moving parts under water. The installation discussed above is suited f o r generating regular waves.
I f the period of the valve is varied when the wave generator is being used, the latter w i l l emit waves having different lengths and different velocities.
By superposing the single components an irre-gular wave pattern is obtained; in this way specified wave spectra can be generated corresponding w i t h wave systems at sea.
To enable such a spectrum to be produced i n a tank the valve is provided w i t h an electronic pro-gramme regulator, capable of switching in 100 periods successively, whose values may, i n a special sequence, be chosen f r o m the progression 0^8, 0.9, . . . 2.0 sec. This sequence can be adjusted at w i l l , but is subsequently constantly repeated after every ioo periods. The length of time between two suc-cessive periods can be regulated f r o m 1 to 10 sec.
I t w i l l be evident that the use of this wave gener-ator is restricted to the production of long-crested waves, the direction of their propagation being parallel to the longitudinal axis of the model-expe-riment tank.
The towing carriage^) (see Fig. 6 ) riins w i t h f o u r wheels (of 60 cm diameter) on rails having a perfectly smooth planed surface. T w o sets of four horizontal roller guides prevent the carriage f r o m leaving the rails.
The principal data of the towing carriage are: 1. Weight, inclusive o f that of the measuring:
apparatus 5 tons;
2. Speed range I 0.4 -7.J m/sec; I I 0.03 5-0.75 m/sec;
3. Drive: speed range I : on each of the wheels a direct current shunt motor of 5 k W 110 V ; speed range I I : a 1 k W motor on one
of the wheels;
4. Supply of current: a Ward-Leonard set of 2 5 k W , installed in the tank hall.
Current is supplied to the carriage via sHding. contacts.
The field of the f o u r carriage motors is excited by a generator installed on the towing carriage.
' ) T h e towing carriages, shaping machine, propeller dynamo-meters and the propeller workbench were siipplita by Messrs. Keiiipf und Remmers of Hamburg.
For the speed range N o . I I the carriage speed can be regulated as follows:
1. hand control: the excitation of the field of the Ward-Leonard generator is done by a small Ward-Leonard set on the carriage;
2. electronic control: an electronic arrangement providing the excitation. This electronic ar-rangement is controlled by a compensating step, whose output voltage is proportional to the deviations f r o m the speed adjusted, which results in stabihzation of the carriage speed. For purposes of measurement there are 220 V direct and alternating current and 12 V direct cur-rent available on the towing carriage.
Besides the electro-magnetic braking system, which operates automatically at the ends of the tank, there is, on the side of the wave generator, an intercepting device, consisting of a steel wire rope stretched across the tank. When the carriage is intercepted by the wire rope the latter raises, by means of guide pulleys, a heavy weight suspended f r o m i t in a pit at the end of the tank.
To the f r o n t of the carriage a plank is attached, 4:2 by 0.7 m and capable of being raised. A t the end of the measuring r u n the plank is lowered on to the water, and when the carriage returns smooths down, as i t were, the surface of the water. O w i n g to this, the waiting-time between two successive runs is considerably reduced. I f this were, not done the transverse waves caused by the model might, particularly in carrying out tests in waves, keep the water in a disturbed condition f o r a very long time. D ; Measuring arrangement of the model (see Fig.
7) and measuring apparatus
A set of light horizontal rollers ( 1 ) are provided on the model forward and a f t of the midship section, and a set of vertical rollers ( 2 ) , f i t t i n g between them, are attached to the carriage. By the adoption of ball bearings the f r i c t i o n is reduced to a m i n i m -um, while the model is free to pitch, to heave and to surge. I t is impossible f o r the model to swerve to the right or l e f t .
I n order to enable the carriage to give the model its required speed, a clamping arrangement is f i t t e d on the model, consisting of two horizontal rollers capable of being pressed against either side of a wooden box which is fastened to the model ( 3 ) . When the carriage has' gathered speed the driver can, f r o m his position, move the two rollers apart w i t h the aid of a hand wheel, so that the model w i l l be capable of moving independently of the carriage. For carrying out resistance tests in smooth water and in waves, a gravity dynamometer ( 4 ) has been constructed, exerting a constant pull on the model through a transmission gear of ratio 1 : 5 . The transmission reduces the acceleration forces of the
Fig. 7. Measuring arrangeiiieiil of lhe viodiil
weight which are caused by the surging movements of the model, f o a negligible amount.
The towing force is transmitted to the model via a cross (5) and a vertical rod which is, by means of a cardan joint, secured at the centre of gravity of the model; The vertical rod, guided by ball bearings in the cross, is free to follow the heavirtg motions of the model.
The cross itself can move freely over the hori-zontal rod by means of ball bearings.
I f the motion of the model in waves is split, up into a motion o f the centre of gravity (heaving ver-tical, and surging horizontal) and an angular ro-tation around the centre of gravity (pitching), i t w i l l be clear that this arrangement also enables these motions to be measured.
T o this end, precision potentiometers w i t h little f r i c t i o n are used which convert the displacements and the angular rotations into voltage variations; by means of a three-channel amplifier these varia-tions can be simultaneously registered on a pen recorder;
— T h e pirrhing
motion-is-iacceased-^-teen=fold-by-means of toothed gearing; the heaving motion is, w i t h the aid of rack and pinion. Converted into a rotary motion, while f o r the surging motion the potentiometer is connected to one of the wheels of the resistance dynamometer.
I f self-propulsion tests have to be carried out,, the resistance dynamometer is replaced by an indepènd-ent means of propulsion of the model. There are two propeller dynamometers (Kempf und Rem-mers' system) available, w i t h which torque and thrust can be measured. The propeller dynamo-meters are direct-current shiint motors of 50 W at 2,000 r e v / m i n ; they are of the suspension motor design, so that the torque exerted can be measured in a simple manner.
Preparations are being made to construct an elec-tronic dynamometer f o r momentarily measuring torque and thrust during self-propulsion tests i n waves and to design the motor in such a way that either the torque or the power can be kept con-stant. I t is possible to measure the linear acceleration at the ship's bow and stern ( 6 ) .
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(Vibrometer) can be used for different purposes (measurement of forces, displacements etc.), its operation being based on the conversion of the quantities to be measured into changes of induct-ance which can be recorded. Together with the Laboratory f o r Shipstructure Research four mea-surement amplifiers are at the disposal of the Ship-building Laboratory.
For measuring the height of waves, a wave-height meter is attached to the f r o n t of the towing car-riage (Fig. 8 ) . W i t h this apparatus the changes in resistance due to the changes in the level of the water between two conducting wires ( o f 0.4 mm diameter) are measured. These wires are secured to a streamlined brace. The changes i n resistance vary linearly w i t h thé height of the waves. The wave profile and the motions of the model can be re-corded simultaneously. Besides the two conducting wires there is a polythene wire going round, on which two silver contact points are provided. Whenever the water level passes one of these contact points a " p i p " is caused in the recording of the waves. W i t h a known Vertical distance between the points of contact, this results in a continuous cali-bration of the measurement.
The speed of the tmving carriage can be measured in either óf two ways:
1. Via a transmission of 10 : 1 a measuring wheel of exactly 1 m circumference, which runs on one of the rails, drives a generator réquiring a very low torque. D u r i n g one revolution of thé measuring wheel this generator produces 1000 impulses, which are counted by an electronic counter every 0.001, 0.01, 0.1, 1 b r ' l O seconds as desired.. The counting is .automatically, re-peating.
The speed of the model is found by increasing the speed of the carriage algebraically by the displacement, in u n i t of time, of the model w i t h respect to the carriage. This displacement can bé measuréd w i t h the "surge" potentiometer and by recording time. I t has been f o u n d , how-ever, that w i t h a correct adjustment o f the speed of the carriage the speed w i t h respect to the towing carriage may be néglected. The counting of the electronic counter can also be recorded.
2. A simpler method of measuring the speed of the carriage is obtained when a measuring wheel is made to yield an electric impulse f o r every 0.1, O.J or l.J m travelled, resulting in a marking of distance on the recorder. Together w i t h a time base the speed over a specified distance w i l l then be obtained.
E. The flow channel
For carrying out model tests in restricted depth of water, during which the water may be either stationary or running, the laboratory has a so-called f l o w channel at its disposal ( f i g . 9 ) .
The principal dimensions of this tank are: length 44.7J m , breadth 2.80 m , and depth 0.60 m, the depth of water being f r o m 0 to JO cm.
The f l o w channel has a horizontal bottom w i t h a toleration of 1 m m .
Via a channel provided under the bottom, the water flows back to the end of the tank where the flowed started. The pump consists of a f o u r -bladed propeller having a diameter of 0.8 m and driven by a controllable three-phase current com-mutator motor of 30 hp. The number of revolu-tions of the motor can be continuously varied w i t h the aid of an induction regulator; the correspond-ing number of revolutions óf the pump varies f r o m 13 to 400 r e v / m i n . The output of the pump is 1.4 cu. m/sec, so that a mean water velocity of
1 m/s can be obtained.
A t the end of the tank, where the f l o w starts, a rectifier has been built i n , which eliminates the large vortices of thé arriving water.
A t either end of the tank resistance elements have been built i n , consisting of gauze at the starting end of the f l o w , while at the other end a number of vertical stiffeners are provided.
These resistances are f o r the purpose of damping longitudinal oscillations caused in the water, f o r instance when the pump is being started.
The towing carriage of the flow canal is capable of developing a speed of 2,J m/s in either direc-tion. I t is driyen by means of a 2 - k W direct-current motor, again provided w i t h an electronic arrangement^ f o r stabilizing the speed of the car-riage.
21 1 i 1 !
1
i 1 ! : • • • • •. . • • • •. . • • i 1 ! 1 ~1M;„. S E C T I O N A .. Fig. 9. F/o)i' channel
O w i n g to the limited dimensions of this tank the tests w i l l , i n general, be restricted to measuring resistance, investigating phenomena which occur when two ships are pasising each other or the case of narrows i n a f a i r w a y etc.
The resistance dynamometer works on the grav-itation principle, a spring resisting .the remainder of the towing force. The elongation of the spring is recorded on a recording drum, together w i t h the marks f r o m contacts Which are secured to one of the rails at intervals of 1 m , and a time base, so that resistance and speed can be determined.
There are t w o methods which may be used for measuring the velocity of f l o w , viz.,
1. the use o f a pitot tube w i t h a liquid mano-meter. The disadvantages of this method are the high inertia, and the low degree of accuracy at low velocities.
2. the use of a small .propeller whose number of revolutionsj which can be rheasured electro-nically w i t h o u t any contact, is dependant on the velocity of the water. This method of measurement implies a very high reaction veloc-i t y (the propeller beveloc-ing made of plastveloc-ic and having a diameter of 1.5 cm) while the result can be recorded.
The pitot tube as well as the propeller can be cahbratcd in the model-experiment tank.
F. The cavitation tunnel
There is a small cavitation tunnel, in which cavitation phenomena can be investigated ( f i g .
The principal data o f this tunnel are: length between the vertical legs J.O} m distance between the horizontal
legs 1.70 m size of the measuring section , . . . 0.3 X 0.3 m power of the screw pump • 20 hp maximum water velocity in way
of the measuring section . . . . 9 m/seC maximum torque of the propeller
dynameter 31 kgm maximum thrust of the propeller
dynamometer 40 kg maximum number of revolutions
of the model' propeller 3,000 r e v / m i n maximum output of the propeller
motor 4.3 hp
The screw pump can be continiuously adjusted f r o m 0 to 700 r e v / m i n by means of a hydraulic
variator. • • , The cavitation tunnel enables the following i n
-vestigations to be carried out:
I . the analysis of a propeller iri a homogeneous ' ' velocity field. I n this case the elbow ( 1 ) is
used, which is of ordinary construction.
Fig. 10. Cavhlioii liiniiel
2. w i t h the elbow ( 2 ) a specified velocity distribu-tion over the screw disc can be obtained ana-logous to that occurring behind a ship.
The nonhomogeneous field is produced by a velocity regulator, containing 146 elements. By means of some type of check valve each of the elements can be more or less shut o f f , which affects the velocity of the water f l o w i n g through fhe elements.
The velocity field in the vicinity of the propeller can be measured w i t h a so-called pilot comb. To enable this to be done 1} pitot tubes have been fitted on a rotating arm, w i t h which the velocity field can be explored radially and peripherally.
A detailed description of this cavitation tunnel w i t h f l o w regulator has been given by Prof, dr ir W. P. A . van Lammeren in International Ship-building Progress, N o . 16, 195J.
To enable the cavitation phenomena in a two-dimensional f l o w to be studied, a new measuring section has been constructed, in which the profile, having a chord of approximately 15 cm, can also make a transverse movement, on fhe analogy of that of a blade profile rotating i n a peripherally unequal velocity f i e l d ; fhe propeller blade w i l l then, as if . were, be oscillating w i t h respect to the direction of fhe intake velocity.
G. The moment indicator
W i t h the aid of this instrument, already referred fo above, the statical stability of a ship model can
be measured at angles of inchnation of f r o m 0 to 9 0 ° (see f i g . 11).
The moment indicator is, in principle, a balance for moments (see f i g . 1 2 ) .
The torque required f o r balancing a model at a certain angle of inclination, is:
w here:
p . ng sin 9 = ^ {mg + mn) sin f ,
p — fhe displacement;
m = fhe true metacentre; and n = the false metacentre.
Fig: 12. Principle of the moment imlicalor
The moment indicator is unloaded in neutral equilibrium w i t h respect to the axis A. By means of the weights Q and q the apparatus exerts on the model a torque which is equal and opposite to the stability torque of the model. By balancing the moments f o r increasing ang;les, the statical stability of the model can be determined readily.
The metacentric height mg of the model can be determined in two ways:
1. The more accurate method is by carrying out a stability test, during which the angle of i n -clination is measured w i t h a high degree of accuracy by means of an optical method. 2. I f the values {mg + 7im) measured are plotted
against ip as base, fairing of this curve to <p = 0 w i l l yield the value of mg, since, i f (p = 0, m and n w i l l coincide.
W i t h vtg, known mn can now be found;
The righting arms of statical stability f o r the ship w i l l foUpw f r o m :
( M G +MN) sin rp = {MG -f- a pm) sin cp,
where:
a ' the model scale; and
MG = the metacentric height of the ship. The advantages of this method of measurement over a calculation are the following:
1. The model is free to t r i m , so that alterations in t r i m during heehng can be automatically taken into account.
2. The influence of forecastle, bridge, poop and any other parts of the superstructure that may have to be included in the measurement can be taken into account in a simple manner. 3. When a model is once available, different
con-ditions can be examined in a short time. 4. Measurement is done w i t h a high degree- of
accuracy; a calibration of the apparatus w i t h a rectangular model, whose stability is easy to calculate, showed that the maximum error of the stability torque measured is léss than 1 per cent.
V. Conclusion
From the aboye i t w i l l be evident that the Sub-department of Naval Architecture of the D e l f t University of Technology bas been enriched w i t h a laboratory which is in every respect excellently equipped f o r researches to be made into matters concerning hydrodynamics i n connection w i t h ships,
The author is convinced that this laboratory w i l l render satisfactory service in teaching and research work. I n both cases the results of the work con-ducted in this laboratory w i l l eventually benefit the D u t c h shipbuilding industry.
