SflIPßU[OIN6 RESEARCh IS1I1UT[ 1AGRE
A Review of
Facilities and Ship-Model Instrumentation
al the Admiralty Experiment Works, Haslar
by
A. J. Vosper, M. I. N. A., R. C. N. C.
Admiralty experiment Works, Haslar
Delt
No j
'/'/3'W /r'/b'th)
Ï'aper to be presented at the Symposium on the
Towing Ta;k Facilities, Iistrumentation and
by
A.J. Vosper Summary
Phis paper reviews the new and. modernised facilities at
Admiralty Experiment Works, Haslar. The expansion in recent years has led to the development of new or improved experiment tech-nique and instrumentation methods. Regard has been paid to
automaticity in recording results with the object of saving manpower.
Mention is made of the new carriage and track for the exten-ded No.1. Ship Tank and the method adopted for speed control. Examples are given of the development of propulsion dynamometers
for surface and submerged models and the uses of electric strain gauges in instrumentation.
Installation of a new Manoeuvring Tank has required develop-ment of wavemakers and beaches and improvedevelop-ment in wave height recording apparatus and other instrumentation connected with seaworthiness experiments.
The large Cavitation Tunnél capable of carrying out tests on hull-propeller combinations is described.
The uses of digital and analogue computers in the work of the Establishment are mentioned and. the application of digital and. analoque recording techniques is discussed.
Finally, there are references to instrumentation used in connection with turning experiments and vibration surveys.
Introduction
A large expansion of the facilities at the Admiralty Expe-ment Works, Haslar, has occurred since the late Dr. Gawn's paper was presented to the Institution of Naval Architects
in 1954..'
A second cavitation tunnel - capable of testing hull-propeller combinations on a comparatively large scale - has been instal-led, and a manoeuvring tank of overall dimensions 11-oo-ft. x 2oo-ft x 18-ft. depth, combining a rotating arm and a sea-worthiness basin is well on the way to completion.
This Symposium, which marks the opening of the new Ship-building Research Institute, seemed an opportune moment to bring up-to-date the information published in 19514. It is
hoped that- it will be of some general interest to other workers in the field of lxydrodynainic research and perhaps of direct assistance to other authorities contemplating the construction of new facilities. The value of a mutual exchange of mf orma-tion has been borne in mind in submitting this paper, which is in the main, factual.
No. 1 Ship
Tank
This smaller of the two tanks, which was originally installed by R.E. Froude at Haslar from 1885 to 1887 had been in constant use more or less in its original form until 1956 when it was decided to lengthen and. modernise it, mtkng its new dimensions
538-ft. x 2o-ft. x 9-ft, depth. It had become evident some time previous to this that the original carriage was beginning to feel its age. The design of this carriage which was executed
in hollow box girders of yellow pine at a total weight of only 800-lbs. was a tribute to the engineering genius of the Froudes. Despite its light weight it had successfully coped with instru-ments and observers to a maximum weight of 3o-cwt. Propulsion was by the simple means of an endless wire hawser secured to a winch barrel ashore. Its disadvantages with advancing age were,
lack of consistency of speed owing to rear on the rail joints; a maximum carriage speed which could not cope with modern
requirements and a low acceleration at the higher speed settings which resulted in an unacceptably small length of run at con-stant speed. Also, the proximity of the carriage to the water made it difficult to pass models with modern inner drive
pro-that at the time of its construction the largest ship in the Royal Navy was H.M.S. TRAFALGAR, a vessel 65-ft. long, dis-placing 12,000 tons and with a maximum speed of 10 knots.
The design of the new carriage was developed, over a period prior to the extension of the tank, to achieve a maximum
con-stant speed of 25-ft. per second over a distance of not less than 200-ft., with an accelerating run of 126-ft., 88-ft. for decelerating and 65-ft. for braking and. arresting.
Propulsion is by four electric motors each of 16 horsepowe-i, one at each corner of the carriage, controlled from a central driving position on the carriage. A speed variation on the run of not more than one-eighth of one per cent and a variation of set speed of not more than one-quarter of one per cent were specified and have been achieved in the final design. The carriage which is illustrated in Figure 1 has overall plan dimensions of 1-ft. x 2L1._ft. with extensions to carry the logs and photographic equipment. In principle it consists of two main transverse girders carrying the driving motors and wheel assemblies connected by two vertical longitudinal apparatus girders which provide a working space of dimensions 22-ft. x 6-ft. for conducting experiments and. placing dynamometers. Simi-lar but lighter girders at the sides of the carriage are cormec-ted to the in.ner apparatus girders by light pin-joincormec-ted bracing to form hollow-braced girders.
This design of carriage provides walking ways with easy access from side to side and fore and aft and has ample space
for observers and instruments. The measuring bay in the centre of the carriage is free from any crossbracing above and below, allowing dynamometers to be lifted in from under or above the carriage quite easily. The separate driving assemblies in each corner on "silentbioc" mountings transmit the minimum vibration from the wheels. The driving motors are electrically linked, and the supply voltage from a Ward-Leonard set ashore is stabi-used by a small electrohydraulic speed control unit which will be described later. The finished weight of the carriage with all the equipment and the maximum number of observers is eight
No.
ISHIP TANK CARRIAGE.
This
considerable increase on the original Froude carriage
weight required the installation of a new track of conventio-nal section mounted on cast steel sleepers as shewn on Figure2. The main driving wheels are unflanged. and the carriage is maintained on a fore and aft line by guide rollers on the starboard side of the carriage which bear on the vertical sides of the upper flange of the track. The rails are in 28-ft. lengths connected by machined fish plates recessed into the flanges, the butts of the rails being scarphed at an angle of 75 degrees. Both the rails and the cast steel sleepers were machined and set up in the workshop before erection in the tank. It was clear that careful setting up and alignment of the rails and sleepers in the tank was essential if the specified speed accuracy was to be attained and a brief
de-scription of the method adopted may be of interest to others contemplating a new
installation.-Initial setting up was achieved by a single horizontal phosphor-bronze datum wire set up on one side p! and parallel
to the centre line of the tank and kept at constant tension. This wire, supported at 50-ft. intervals along the length of the tank on slotted screws fitted to wooden floats, ¡Figure 3/, enabled the sleepers to be set up to an accuracy of ± 20-th thousandths of an inch in alignment. A light alloy width gauge positioned the sleepers on the opposite side of the tank and the securing bolts were then grouted in. The sleepers were then set to the correct height within ± 10 thousandths of an inch accuracy by setting up from a datum at the mid-length
of the
tank, using a Cassella gaugeas shewn in Figure
The track when laid was aligned to within
2 thousandths
of ati inch accuracy
bymeasuring from the datum line using
a
telescopic sight mounted on a block clamped to the side of the quide rail. The height of the track was adjusted in the same way as the sleepers and a final check on the height, achieved by adjustable height hook gauges mounted on eachside of the carriage
and. measuring from the water level in the"T
CT.
A.AT IOTQ., .i.
O tt%Sb4 T*pQa
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!R ttjj.4 jOt.
CARRIAGE TRACK AND
SLEEPERS
SECTiON AND ELEVATION.
ARRANGEMENT FOR AUGNING TRACK.
USE OF CASELLA GAUGE
FOR CHECKING HEIGHT
MEASUREMENT OF HEIGHT OF TRACK
BY HOOK GAUGE.
the track as laid shewed. it to be correct to within ± 2 thou-sandths of an inch in level and
± 3
thousandths of an inch in alignment, which was considered acceptable.Speed Control
The specified requirements of speed variation of not more than one-eighth. of one per cent and not more than a quarter
of one per cent in set speed are particularly exacting but have been achieved by an interesting form of e].ectro-hydraulic
governor, designed by the Harland Drive Co., which is directly coupled to the motor of the starboard after driving wheel of the carriage. Figure 6 shews a diagranmatic line drawing of its internal arrangement.
The governor head consists of a rotating ball valve through which a small quantity of oil, supplied by an electrically driven pump, is made to flow continuously. Since the centri-fugal force on the ball is proportional to the square of the speed of rotation, the oil pressure contained by the valve is directly related to the carriage speed. This pressure is applied
to the end of a four-way pilot valve and is balanced by a
control spring acting on the other end. Friction is eliminated by rotating the pilot valve through its control spring.
A variation in speed causes a displacement of the pilot valve from its central position, which by a servo-mechanism applies pressure on a carbon pile rheostat and so changes the voltage of the electrical generator of the Ward-Leonard set ashore.
There is also a fly-wheel mounted on the governor head but capable of a limited radial movement with reference to it, which during acceleration exerts an additional force on the ball, so that the oil pressure is a function of both speed
and acceleration. The acceleration component dies down as the steady pre-set speed is reached, but its presence is necessary
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AHYDRAULIC SPEED
GOVERNOR
FOR No.1 SHIP TANK
CARRIAGE.
Adjustment of' the anchored end of the control spring alters the force balancing the governor head pressure and provides
control of the governor head speed from 1,850 to 1,200 R.P.M. in loo graduations, each of which represents about a half per cent speed change.
A double train of change speed gears provides for changes in the speed ratio between the governor head and the driving motor in order to cover the governed speed range of 0.9 to 25 feet per second. By a combination of coarse and fine
adjustment, this provides nine ratios in geometric progression ranging from 1/1 to 18.75/1. A calibration chart of carriage speed against dial position in the 1/1 gear was made on the site. To preset the speed in any other gear ratio it is only necessary to multiply the required speed by the gear ratio and read off the required dial setting from the card.
A reversing train of gears permits the governor head to rotate in the same direction during the return run.
Speeds below 0.9 feet per second can be obtained by operation of a special trimming rheostat. Figure 7 shews a typical record of ti variation of speed on the run at settings of 600-ft/min. and 9oo-ft./min.
Acceleration Control
Up to speeds of 20-ft./sec., a constant acceleration period of 5.7 seconds results in accelerations varying up to a maximum of 3.5-ft./sec.2, depending on the speed setting. Between speed settings of 20-ft./sec. and 25-ft./sec. the acceleration re-mains constant at 3.5-ft./sec.2 and the period of acceleration is allowed to increase to a maximum of 7.1 seconds. By adjust-ment of the limit lever, however, it is possible to control both the acceleration and deceleration rates independently.
VPJA11ONS CF EWERI4ENT CAIPGE SPEED CN THE RUN.
CAIAGE SPEED - 600 FEET PEP MINUTE.
TYPICAL RECORDS OF SPEED.
Measurement of Speed
Since the constancy of speed is established by the hydrau-lic governor, it is sufficient to i4me the carriage over a
set distance of 166-2/3rds feet in order to measure the speed, viz., speed in feet per minute = inverse of time in seconds x lOk. An electronic timing device known as the 1Cintel" deci-mal counter chronometer records to five significant figures, the units being either 100th seconds, milli-seconds or ten micro-seconds.
The starting and stopping of the timer is electronically controlled by vanes secured to the tank roof which cut rays of light impinging on photo-electric cells on the carriage, separate systems being fitted for starting and stopping.
The most frequently used time-base is in units of milli-seconds using a tuning fork controlled oscillator operating at 1,000 c.p.s.
A second time-base is in units of ten micro-seconds using a master Oscillator, crystal-controlled, working at a frequen-cy of 100 Kcs. per second and a third in units of 1/100th of a second by using twice the mains frequency. The latter is normally used only for testing purposes.
A specified accuracy of recording to 1/10th of one per cent can be met quite readily with this system.
Carriage Vibration
The design calculations shewed a critical vibration frequency at 675 c.p.m. As a check on these calculations, a survey of the vibration characteristics by vibration generator was under-taken and established a critical frequency at 725 c.p.m. with
Self-BalancinR Thrust and Torque Dynamometers and. Revolution Counters
A desirable objective in propulsion experiment technique is to devise instrumentation which requires the minimum number of experimenters consistent with reliable records, which can be used directly without further tedious analysis. The inner drive dynamometër for surface ship models used in No. 1 Ship Tan.k was developed with these requirements in mind
and the result is a self-balancing dyn.ainometer which will cope with single or twin shaft arrangements. The dyiiamometer records values of thrust, torque and R.P.M. with an accuracy more than sufficient for normal plotting. Illustrations are shewn in Figures 8 to 10.
The essence of the design is that about ninety per cent of the thrust and torque values are balanced by weights in scale pans, the remaining ten per cent being balanced by a spring, the tension of which is adjusted automatically.
The driving motor is mounted in a casing supported in ball races from brackets below the base plate. The spring, which balances about ten per cent of the reaction torque of the motor, is connected at one end. to the casing and at the other to a rack and pinion which adjusts the spring tension. Movement of the motor casing between two limit stops from the null
position is detected by a coincidence transmitter whose signal is amplified and fed to an induction motor driving the pinion on the rack. Coupled to the induction motor is an induction
generator which stabilises the motion and prevents the mechanism hunting by a feed-back circuit. The pinion is also geared to
a synchronous
link
which indicates the torque on a dial. The movement of the pointer represents the amount of rack movement and thus the tension in the spring.On either side of the link operating the coincidence trans-mitter is a micro-switch. If the force required from the spring
SELF- BALANCING THRUST
AND TORQUE
I'NAMOMETER.
SE LE- BALANCING THRUST
AND TORQUE DYNAMOMETER.
CARRIAGE INDICATiON OF THRUST.
TORQUE AND RPM.
will operate and switch off the current. In the same circuit are indicator lights shewing where the lack of balance exists. As the motor R.P.M. are changed the lights will shew green or red depending on whether the force is greater or less than the added weight and hence whether the torque is within the control range. Small adjustments can be made to the R.P.M. to bring it within the range an.i the unit will then self-balance.
Tbrust is measured in a similar fashion by taking off the force through a swinging frame from a thrust ring on the after side of a four-jaw coupling between the shaft and motor.
The driving motor shafts are fitted with ten-toothed sprocket wheels running in close proximity to a small pole piece to form a phonic indicator. The swltchwhich starts the counting of the passing teeth also starts an electronic chronometer to provide the experimenter with an accurate indication of the number of revolutions over a given time to complete the records.
Submarine Inner Drive Dynamometer
In the early stages of development of the submarine, when greater emphasis was placed on surface performance, there was no great need to measure the hull efficiency elements submer-ged and existing apparatus was adequate. The advent of higher submerged speeds led to the use of inner drive equipment and the first experiments were lengthy and tedious because of the need to bring the model to the surface frequently for adjust-ment of the dynamometers. The need for remote operation in order to be able to conduct the experiment in a reasonable time led to the development of a submarine inner drive dynamometer which could be accommodated in the small space available. The apparatus was simplified by measuring both thrust and torque as a summation of port and starboard values. The system of measurement adopted was to
counterbalance the thrust and torque forces mainly by added balance weights and the residue on a
manometer tube using a head of water.
The d.ynamometer is purely mechanical in operation, the thrust of the propellers being transmitted to thrust races in a bell crank frame, connected by linkage to the scale pan
and a "hydroflex" bellows. The latter transmits water pres-sure from a manometer tube above the model on the tank carriage.
The system for measuring torque is similar to that for thrust except that the added weight and. bellows unit connects
to an epicyclic torque reaction box.
The torque and thrust manometers are arranged side-by-side on a back board and can be adjusted to a height suitable for reading during the experiment..
A system of motorised cams in the model raises or lowers the required weights on the thrust and. torque levers and these cams are remotely controlled from a switchboard on the carriage. Coloured lights incorporated in the switchboard indicate to the experimenter the value of the weight on the levers. Figure 11 shews a photograph of the equipment.
The unit in its present form will measure a total maximum thrust of 40-lbs. and. a total maximum torque of 6.0-lbs.ft.
As the model is pressurised, it is necessary to introduce an equal pressure to the ends of the manometer tubes. A sili-cone anti-wetting agent applied to the manometer tubes improves the accuracy of recording and. ensures that no globules of
SUBMARINE IN NER DRIVE DYNAMOM ETER.
FACILITIES AND SHIP-MODEL NSTRUMENTATION AT THE ADMIRALTY
EXPERIMENT WORKS HASLAR
Inductive Thrust and. Torque Inner Drive Dynamometers
A number of electronic inner drive dynamometers have been developed employing the differential transformer principle to
ma]e measurements of thrust
and. tordue. The windings of thetransformer are coaxial, dust-cored
solenoids which donot
rotate with the propeller shaft and thereby eliminate theneed for slipring transmission. The dynamometers are compact
and lightweight, ideally suited. to running
in models of high
speed coastal force boats. An illustration of the dynamometer in an unassembled state appears in Figure 12.The key to this illustration is as follows:
Thrust core. D. Dust yoke for coil unit.
Torsion core. E. Coil unit assembly.
Coil unit-windings. F. Outer casing.
Thrust and. torque and propeller R.P.M. are recorded on a potentiometric pen recorder.
A similar method. of measuring is employed in the inner
drive now installed in the large Cavitation Tunnel and. referred.
to later.
Two standard. sizes of dynamometer have been designed. to cope with values of thrust from 0-50-lbs. and O-700-lbs.,and of torque from 0-10-lbs. ft. and O-60-lbs.ft. Limits of
accuracy of this type of
dynamometerare
one-half per cent.
Three and Six-Component Balances
Special balances have been developed, using the same diffe-rential transformer principle, to measure the forces and mo-ments acting on models on the rotating arm in the Manoeuvring Tank. These are to be used in experiments on directional and dynamic stability of surface and submerged models and
comprise:-FIGURE ¡2.
/a/ A three-component balance for surface models to record
the resistce, side force and yawing moment, the
mo-del being free to heel, trim and heave.
¡bi A six-component balanöe for upright submerged models for investigations into directional stability and. manoeuvring performance of submerged bodies.
/c/ A six-component balance for submerged models on their side for dynamic stability investigations in the ver-tical plane.
The design of the balances is illustrated in Figure 13, which shews the three-component balance. Essentially, the ba-lance is inserted into the model support system and all the forces and moments are measured by virtue of the relative movements of the model and earth frames. These movements are restricted by spring units and are measured by the differen-tial transformer units. In the design of the balance great care has been taken to eliminate cross-coupling effects and to reduce the physical movements to a minimum. In the case of the submerged model, the balance is within the model itself and. thus the recorded forces and moments do not include com-ponents for the supporting struts. The records have, therefore, only to be corrected for the strut interference effects and not for strut id.1es".
Use of Strain Gauges in Instrumentation
Many tanks make use of electric strain gauge technique in the development of instrumentation and, at A.E.W., much use is made of the Saunders-Roe printed foil type. These gauges have the important property of being able to pass comparati-vely large currents which obviate the need for amplification before recording. The example shewn in Figure 1k and 15 shews their use in instrumentation developed for measuring the lifts and torques of the control surfaces of high speed submarine modéls. In this example, the gauges are mounted on the spindle of the control surfaces.
DI FFERENTIAL TRANSFORMER.
A + B.
A - B.
C. OUTPUT PROPORTIONAL TO. YAW FORCE. YAW MOMENT. DRAG.RECORDING EQUIPMENT USED
WITH STING DYNAMOMETER.
The same technique has been used. on work which is probably unfamiliar to other tank authorities, i.e., full-scale investi-gations into the strength of anchors and propellers. Anchor development at Haslar has been proceeding for some years and has resulted in a considerable improvement in the critical
ratio of holding pull/weight of anchor for both ship and mooring anchors. Tests are initially on the model scale in a specially designed tank and subsequently on the full scale at represen-tative holding grounds to check that the model tests scale up. In conjunction with the full scale tests, a strength investi-gation is carried out whereby the distribution of stress in the anchor is examined. To put this into effect, it was necessary to develop a technique for waterproofing the electrical resis-tance strain gauges.
A Ll-2-cwt. steel mooring anchor strain gauged and ready for dragging trials is shewn in Figure 16. Measuring positions were selected on the back of the fluke and along the outside of the shank of the anchor where the risk of damage to strain gauges by the abrasive action of the sand was considered to be least. The total number of active gauges was 32.
As shewn in Figure 17 small channels chipped in the fluke and the shank connect the strain gauges, which are 1-in, gauge
length, 5 ohas. resistance, to a junction box on the upper surface of the shank. At each of the selected gauge positions, the anchor surface was ground down to bright metal and polished leaving a slightly saucer-shaped area and the gauge was stuck at the centre of the area using strain gauge cement comprised of an araldite resin with slate content. Waterproofing of gauges and leads was accomplished by applying successive layers of strain gauge cement, the surface of the anchor being restored to a good finish. In the watertight junction box are mounted dummy gauges for temperature compensation. A 150-core screened
cable, made buoyant by means of floats, was led to a mobile laboratory which housed the batteries, galvanometers and film type recorders.
STRAIN GAUGED MOORING ANCHOR.
STRAIN GAUGES ON THE FLUKE
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... , . -. .; fr J , f. f'WATERPROOFED GAUGES AND CHANNELS
Each active gauge was wired into an indivictual Wheatstone bridge circuit. To allow for the resistance of the cable, four cores were utilised for each bridge, two each from the active and dummy gauge s.
Immediately before dragging commenced, all 32 bridges were balanced and datum traces recorded on the films. Each strain
gauge .bridge was calibrated 'by changing the resistance of both active and dummy arms by 0.03 ohms. using a helical potentio-meter. This corresponded to a strain of
5.05
x l0 being equivalent to a change in the active gauge resistance of 0.06 obms.The electrical resistance of waterproofed strain gauges has been found to remain satisfactory after several weeks of
immer-sion provided that care is taken to exclude dampness and dust from the cement during the curing periods between the applica-tion of successive layers.
In the case of the strength investigation of a destroyer propeller whilst in service, a particular obstacle to be over-come was the transmission of measurements from the gauges on a rotating propeller to a fixed position on board. To avoid a large number of sliprings on the propeller shaft, the recor-ding equipment was mounted on a disc which rotated with the shaft. The absence of amplifiers was an important advantage in this application.
Wavemakers
Before the arrangements for the new Manoeuvring Tank could be finalised, a great deal of development work was undertaken on the system of waveinakers and beaches in association with Cambridge University. Much of this will be included in a paper to be presented to the I.N.A. so that a brief reference only will be made here.
Both the Ship Tanks at Haslar are equipped with paddle-type wavemakers, electrically driven. In the case of the original Froude No. i Ship. Tank, it is of interest to note that the paddle which is of wooden construction, is much the same as
when originally introduced in 1905. When the tank was lengthened, it was foand to be in an excellent state of preservation and, with the original wooden carriage which has now been discarded,
is a tribute to the lasting qualities of the material and
wor1anship of those days.
Certain improvements incorporated during the resiting ope-ration resulted in an improvement in the form of wave produced. These consisted of closing the small gap at the lower hinge between the paddle and the tank bottom by a flexible rubber strip and reducing the clearance between the ends of the paddle and the side walls to the minimum practicable.
In the larger Ship Tank, the wavemaker Is of steel construc-tion and was installed during the last war to carry out some urgent investigations. It was therefore kept as simple as possible. An illustration appears in the late Doctor Gawn's
I.N.A. Paper of 1954. After the war, a fly-wheel wasadded to the drive with the object of improving the wave form and a pneumatic clutch was introduced at the same time, so that waves of the correct size can be generated much more quickly by
avoiding a long build up period.
To enable complex wave forms to be generated in the Manoeu-vring Tank, two sets of wavemakers are employed, disposed at right-angles to one another in plan. These two sets are capable of generating regular trains of waves of maximum dimensions 40-ft. by 2-ft. height and 20-ft. by 1-ft. respectively. Each set of wavemakers, which are of the plunger type, is made up of five independent units, 40 feet long which, by operating out of phase, frequency and stroke, will provide such complex systems as may be found necessary for the representation of confused seas. Model tests on a large number of plungers of
differing sections, /Figure 18/, were conducted on a 1/8th scale in the Ship Tank and to 1/22nd scale in a model of the new facilities. Figure 19 shews the final selection.
During the investigations on the l/22od scale model of the Manoeuvring Tank, the phenomena of cross waves was encountered. See Figure 20. At certain strokes above certain frequencies, a large standing wave was produced along the face of the wave-makers. With the wavemaker shape then being considered, a deep 15 degrees wedge, cross waves would have occurred at 10-ft. wave length and below i± the phenomena scaled up. Theoretical investigations shewed that this was likely to occur and cross waves were produced at full scale at N.P.L. and in No.1 Ship Tank at Haslar. At N.P.L., the cross waves reached a height of 6-ft., and in the case of No.1 Ship Tank at a nominal wave
length of about 18-ins., but both instances occnrred well outside the normal working range of the waveniaker. Experiments indicated that the critical period at which cross waves occurred could be reduced if the plunger stroke could be reduced for the same wave height. Widening the water line of the plunger was helpful in this respect, and after further experiments a ttdouble wedget1 plunger was evolved which gave a good wave shape over the
whole length and for which the period of cross waves occurred after the main waves had broken and outside the normal working range.
Wave Suppression by Beaches
The requirement for efficient beaches is that they must absorb the waves and prevent reflection, and bring the water surface to rest quickly when waveniaking ceases-. Even though the wavemakerprocìuoes good waves, the wave system in the tank will not be of good shape or regularity if efficient wave suppression and absorbing arrangements are lacking. Even if ninety-nine per cent of the wave enerr is absorbed, the one per cent reflected will result in a reflected wave of ten per cent of the incident wave height.
r
TYPICAL SELECTION
OF WVEMAKER
PLUNGER SECTIONS.
/11111)11
/
2'
/1//fl f 11f / i/I
FINAL WAVEMAKER PLUNGER
SECTION.
CROSS WAVE JUST COMMENCING
SMALL STRIATIONS ACROSS MAIN WAVE
SMALL CROSS WAVES APPARENT IN PERCO OF MAIN WAVE
SOME SPLASHING BEHIND PLUNGER
SMALL CROSS WAVES BUILDING UP
:
---e...-CROSS WAVES NOW CHANGING PERIOD :
CROSS WAVES HAVE NOW BUILT UP INTO LARGE WAVES OF PERIOD TWICE ThAT OF MAIN WAVE
Experiments on the detailed shape of the beaches for the Manoeuvring Tank were conducted in conjunction with the wave-maker development. Typical examples of the various types of beaches tried are illustrated in the diagram Figure 21, and the final form in Figure 22. This is in the form of double layers of slats, the slope relative to the horizontal of the top layer being 8 degrees, and the bottom layer 15 degrees, with a trap wall at the back to forni a re-circulation channel behind and under the beach. The top layer of slats is extended back over the traps above water and. improves the quietening of
the water and results in less splashing. The slope of the top layer of slats and the position of the water line relative to this layer are critical for quick quietening of the water.
A half-scale model of the beach was subsequently fitted in No.1 Ship Tank.
Wave Height Recordinp
A most important aspect of seaworthiness experiments is the accurate measurement of wave forni. The method of measurement found to be satisfactory uses the change of capacitance of a
nickel plated 3/16th-in, diameter brass rod contained in an airtight glass tube, partly suspended in the water at the end of a rigid arm secured. to the tank wall. See Figure 23. Wave motion alters the water level around the tube changìng the capacitance of the system. This capacitance probe is connected in one arm of a special radio-frequency bridge, whose output is amplified and passed to a recorder from which the wave height can be quickly determined.
The overall calibration may be finally checked by photograp-hing a wave running along a thin aluminium sheet arranged
parallel to the length of the tank and in close proximity to the stationary wire. The system has been found to be extremly stable and calibration does not vary over periods up to a week.
FIGURE 21
/
/
/
/
/
/
24'x 2SLATS SPACED 2'APART
SLOPE 6°
L
L,
T RAP WA LL
FIGURE 22.
4'x2SLATS SPACED 2"APART
/ / / / / /
WAVE HEIGHT RECORDING EQUIPMENT
Since it is more important to know the wave height encoun-tered by a model, the apparatus has been modified to accommo-date a wave probe on the moving carriage where the wave
frequency is so much greater. The rod and tube have been replaced by P.T.F.E. coated, standard tinned copper wire of outside diameter 30-thousandths of an inch, and. the records are taken on a high speed quick response recorder.
In the Manoeuvring Tank it is intended to carry out a survey of the complex wave patterns that will be possible and a
rnul-tiple probe system, using up to 48 P.V.C. coated probes siinul-taneously, will be employed. Depending on the results of such a survey and the progress made in recognising a confused sea, it is hoped to be able to use a much smaller number of probes.
The seaworthiness experiments in the tank will use free,
radio controlled models and here it is important, in conjunction with the recorded motions of the model, to have a continuous record of the waves encountered. The present intention is to use an accelerometer to correct the signal from a wave probe fixed to the bow of the ship model.
Radius of Gyration of Seaworthiness Models
When producing ship models for seaworthiness experiments, it is essential to represent accurately the ellipsoid of inertia of the ship, since any discrepancy will affect the maximum motion especially in the critical period of encounter.
The method employed at A.E.W. is to oscillate the model about its centre of gravity on a swinging frame shewn diagram-matically in Figure 24 and to equate its period of oscillation with that of a simple solid of known moment of inertia. This apparatus takes account of the inertia of the guiders, towing rods and recording gear, which, in the case of large models, can be quite substantial.
APRATUS FOR DETERMINING
RADIUS OF GYRATiON.
The apparatus consists of a swinging beam with counterbalance weights supported in ball races in two brackets connected to
the base plate and restrained by a spring at each end. The
height of the bean' can be adjusted so that the model oscillates about its centre of gravity, and its period of oscillation is equated with that of a solid of the required inertia made up of a planed rectangular wooden block of uniform section with adjustable weights on screwed metal extensions.
For purposes of adjustment, the model on its frame is placed in the dock at the end of the tank under the experiment carriage.
All the recording apparatus, guiders, etc., are connected to the model and the ballast adjusted, without altering the posi-tion of the longitudinal centre of gravity, until the period of oscillation is correct.
Cavitation Tunnels
Two tunnels are installed at A.E.W., the smaller of these being of conventional design and size with a working section
of area 2-ft. by 2-ft. This was installed in 1940 and has been employed almost the whole of its time on the testing of 12-ins. diameter model propellers. Arrangements are incorporated for carrying out experiments with the model shaft at angles of inclination up to 22-f degrees to simulate ship conditions as
far as inclination of water flow is concerned.
The difficulties in trying to relate ship results with experimental results in a conventional tunnel, even with the
added advantage of an inclined shaft arrangement, are well known and it is clearly desirable to be able to represent ship conditions more accurately. This can be achieved in one way by tunnels equipped with means of regulating the water flow to represent the wake but it was felt that this fell short of tests with hull-propeller combinations; these are undertaken in the large Cavitation Tunnel, the installation of which was completed in
1958.
This tunnel is unique in that it has anextremely large working section of dimensions 7.9-ft. by 3.9-ft. cross section. A view of the tannel is shewn in Figure 25. Its characteristicsare large easy bends, relatively few guide vanes, a bifurcated expansion section aft of the model propeller and rectangular sections except in way of the impeller. Apart from its large size, the design of the tunnel follows fairly conventional lines.
The main dimensionse as follows:
Overall height 39-ft. 9-ins.
Overall length 63-ft. O-in.
Total weight full of water 360 tons.
Weight of water 190 tons.
Size of measuring section Height 3-ft.
hf-ins.
Width 7-ft. li - ins. Length 17-ft.6-f
-ins. Maximum water speed throughthe measuring section 26.O-ft./sec.
Most of the normal working operations are carried out from a comprehensive control panel which includes a "Mimic" diagram shewing the state of all the valves, auxiliaries and so on, see Figure 26.
A portable wheeled console is provided for the experimenter atad, permits fine control of the model propeller, recording of results and operation of the stroboscopic lighting.
The tunnel is provided with an additional inner drive unit which enables ship models to be powered internally with the shaft lines installed at the correct angle to the water flow. Contra-rotating propeller systems can also be investigated. This inner drive unit is mounted from the hath cover on the top of the measuring section and is driven from a 28 H.P. motor fitted outside the tunnel. A variable angle gearbox will cope with shaft inclinations from O to 15 degrees to the horizontal and. a range of speeds from 500 to 2,500 R.P.M.
VIEW OF No.2 CAVITATION TUNNEL.
"MIMIC" DLRAM
No. 2 CAVITATiON TUNNEL.
The thrust and torque are measured by inductive thrust and torque dynamometers previously described. These will measure torque up to 60-lbs.ft. and thrust up to 700-lbs.
Measuring Apparatus
Considerable attention has been paid. to the measuring appa-ratus so that it is convenient to use. Screw thrust, torque, revolutions and water speed measurements are displayed on the experimenter's console.Water speed is indicated by differential pressure across the 3 to i contraction section. Pressures are measured on a differential manometer operating on the
self-balancing principle, originally develed for wind tunnel work,
and registers on a dial.
Propeller Thrust is measured from a thrust bearing on the shaft which records on a springless weighing machine of standard design. A system of added weights extends the range from the
o
to 500-lbs. dial reading by steps of 500-lbs. up to a maximum of 2,300-lbs. Remote indication of the thrust registered on the dial is provided by a servo mechanism at the experimenter's console. Thrust can be measured to an accuracy of + of one per cent.Proe1ler Tor,ue. The angular twist on a reduced diameter
of the propeller shaft is measured by a contact arm moving round a toroidally wound resistor, the change in resistance being transmitted from the torsionmeter by a slipring assembly. The permissible angle of twist in the shaft is 25 degrees and this large amount of twist entails the use of slender
torque bars, five of which cover the range of O to 5,280-lbs.ins. Remote recording of torque at the console is obtained by simi-lar methods to those used for thrust. Torque can be measured to an accuracy of of one per cent.
Shaft R.P.M. are measured on an electronic tachometer counting the pulses from a phonic wheel fitted to the model propeller shaft. The count is made for a pre-selected period
of one, two or four seconds and. is displayed on four Dekatron tubes for half-second after which the cycle is repeated.
Tunnel pressure is measured by a baro-vacuum gauge having an extended scale allowing absolute pressures from O to LIl_ins. of mercury to be measured. The vacuum pressure system is de-signed to maintain a constant pressure above the water level in the coaming. Pumping is by a single cylinder water-cooled pump with a capacity of 800 cubic feet of free air per minute. A large pressure drain tan] of 800 cubic feet capacity provides a reservoir cushioning the system against fluctuations of
pressure. Control of pressure is by fine and coarse throttlé valves that allow air to pass in or out of the system, depen-ding on the coalning being evacuated or pressurised, the valves being remotely controlled from the control panel.
The limits of absolute pressure in the tunnel are from 1 to
14-6-ins. of mercury.
De- aerator
It has been the practice at A.E.W. to conduct experiments in the cavitation tunnel with the water
at a standard air
con-tent. In the smaller tunnel, the usual practice was to de-aerate the water to an air content ratio of 0.2 compared with thesaturated air content, by thrashing the water at low pressure in the tunnel by running the model screw in reverse against the water stream. Such a process is lengthy and would not be acceptable in the larger tunnel whose volume is eight times that of the smaller. A de-aerator plant has been provided which, by cross connections, will deal with both large and small
tunnels.
Propeller Photography
purpose an ultra-high speed camera, working up to a rate of 200,000 exposures per second has been acquired, see Figure 27. Probably the biggest problem in setting this to work is the provision of a high enerr flash for a short duration and it
is hoped this will be met by the use of a special electronic flash tube rated at 16,000 joules maximum. The ener is pro-vided from a bank of condensers connected to give a capacity of 2,200 micro-farads and which can be charged up to 6-Kv. The time of duration of the flash will be adjusted by varying the capacitance and voltage and refinements in the circuitry are under development to give a sharp cut-off to the flash which normally decays exponentially.
Digital Computation
Then the Manoeuvring Tank installation was commenced in
1953,
it was foreseen that the analysis of records from therotating arm and. seaworthiness basin would be a prodigious task which would have placed an onerous burden on the relati-vely small staff and. also would have resulted in unacceptably long delays between experiments. It was therefore decided to install a digital computer as part of the initial equipment, with the primary function pf experiment data reduction. As has happened in many other establishments, the computer, once set to work, is always fully occupied on the many problems it can solve quite apart from analysis work. Its use not only saves a considerable amount of effort on normal routine calculations such as, for example, reduction of measured mile standardisa-tion trial data, cavitastandardisa-tion tunnel and experiment tank analysis, but also can be brought to bear on calculations which could not previously have been attempted. Such problems include the evaluation of energy spectra from ship motion wave height records and calculations connected with the dynamic stability of submarines. The latter, if carried out by computing staff working with hand calculating machines, takes such a long time that the chance of an error in one of the many stages becomes so high as to prejudice the practical completion of the
As an example ot the time saved, the manual computation for a specific submarine problem which would take one operator working a seven-hour day about 18 months to complete, can be solved by a digital computer in half-an-hour. Of course the initial programme requires some time to devise /about three weeks/ but it is still a very small proportion of the time taken in the earlier method, and once set up can be used again and again.
The computer used at Haslar, a Stantec Zebra, /Figure 28/ is a relatively new production of modest price / under £ 20,000/ but has a performance approaching that of some of the very much more expensive machines. The main store has a capacity of 8,192 words with an access time of five milli-seconds. The most useful feature of the computer is the provision of a simple
code whereby, at the sacrifice of a number of storage locations which are used for special interpretation routines, programming is made very simple so that a course of five one-hour lectures will equip a trained engineer for compiling such programmes. It is only necessary to go on to the more complex "normal' code for programmes of a considerable length that have been used repeatedly. Sorne further details of the machine's performance are as follows:
Addition and subtraction -
312
micro-seconds.Multiplication - 11 niilli-seconds.
Division -
35
rnilli-seconds.Analogue Computers
Two analogue computers are in use at Haslar, one of which is a fairly small machine by Saunders-Roe Ltd. mainly used at present for the solution of vibration problems. Apart from the more simple problems where the differential equation of motion is represented directly on the computer in the normal way, a method of estimating the response and natural frequencies of
'ZEBR
DIGITAL COMPUTER.
Where a very large analogue computer is available, the normal practice has been to employ a separate amplifier and potentio-meter unit in each of a number of sections into which the ship is divided for mass, flexural stiffness, rotary inertia and associated damping. This requires 80 amplifiers for a ship divided into 20 sections.
In the arrangements set up at Haslar, there is a large panel in which these quantities are set up on a number of potentio-meters which are then switched in turn into the computing cir-cuit by means of a uni-selector switch, see Figure 29.
For the solution 01 more complex equations particularly on submarine behaviour, a very much larger computer by Elliotts has been acquired. Although many such equations can be solved with equal facility on the digital computer, the analogue computer becomes particularly valuable when step-by-step
methods are impracticable owing to the desirability.of reprodu-cing the behaviour of the submarine in real time or when a number of the components include non-linear functions such
as, for example, the lift of a submarine hydroplane in relation to angle of attack, or the backlash and limited speed of operation of the associated hydraulics.
Ship Motion Recording
This subject covers the recording of ship motion both in model and full scale, it being essential to correlate the
model work in the tank with ships at sea. In the past, a number of trials have been carried out in which ship motions have
been recorded in amplitude forni on photographic film or with pen and ink recorders. The difficulty here is that, unless the analysis is confined to measurement of peak and average ampli-tudes, the time required is prohibitive.
There are three possible approaches along the lines of auto-matic analysis which will provide speedier results:
'SARO' ANALOGUE COMPUTER SET UP
FOR VIBRATION ANALYSIS.
/a/ - Analogue Recording where the record is produced in a form which can be played back, thereby giving a voltage which can be analysed by an electronic frequency analyser or by an analogue computer in order to obtain, for example, ener
spectra or heave displacement from acceleration at a given point and angle of pitch. The disadvantage of this method lies in the difficulty of obtaining a sufficiently accurate analogue record without employing very heavy and complex equipment.
/b/ - Diital Recording can be employed using either digiti-sers connected at the back of pen recorders or, alternatively, electronic analogue-digital converters. Results are presented in the form of punched tape at suitable time intervals. This method has the advantage that the results can be passed all together in conjunction with a suitable programme directly into the digital computer for analysis. The disadvantage is again the exireme complexity and expense of the equipment and the difficulty that, unless a very short time interval for the punch is chosen, the possibility exists of insufficient data being available for analysis. In any case, there is no direct read-out of peak amplitudes.
/c/ - Automatic Analysis of Film Records. This is the method we have selected for initial use in the seaworthiness work in the Manoeuvring Tank. In a self-propelled model in the seaworth-iness basin, there wìll.not be the space and weight available for any more sophisticated recording method and neither the effort nor money is available to embark on the very expensive telemetry required for passing the results ashore. It is in-tended to record in the model roll, pitch, heave, acceleration, shaft revolutions, course and wave height, all on a miniature ten-channel film recorder type s'E" which has been specially developed by the Admiralty Research Laboratory, Teddington Since there will be equally as many model records as full scale ship trial records, it is considered most desirable that simi-lar methods should be employed in each case and, to this end, an analyser has been developed which will automatically convert the film recorder trace into digital information without the continuous attendance of human operators.
Machines d.igitising film records using a human operator and a cursor attached to a digitiser have been considered, but it was thought that the work would be most tedious and possibly, owing to operator fatigue, insufficiently accurate. The employment of this simple method of recording on board ship using the minimum of electronics and. capable of being rigged in a very short time and occupying little space is of considerable advantage. Ship motion trials may last for several weeks whilst a considerable range of weather conditions is investigated and it is an obvi-ously desirable feature of such equipment that there is no
re-quirement for skilled electronic engineers. Moreover, its
reliability would seem to be very much greater than more complex equipment and require fewer staff to operate it.
Pick-up Units for Ship Motion Recording
For model work, sensing units have been developed for produ-cing signals from roll, pitch and linear accelerations using aircraft-type gyroscopes on which are mounted sub-miniature accelerometers. The gyroscopes are fitted with suitable poten-tiometers for roll and pitch signals and the records are taken on the miniature A.R.L. Type E ten-channel film recorders pre-viously referred to, see Figure 30.
In ship work, a slightly larger and. more robust gyroscope is used. and the somewhat larger A.R.L. recorder Type A.
Turning Trial Recording by Photographic Methods
A system of photographic recording of turning experiments was developed some years ago for model tests on a large open lake in Portsmouth Harbour. The method has proved so successful, both as regards the manpower required during the experiment and analysis time, that it was subsequently adapted for small turning models in the large Ship Tank at A.E.W., and has been
PICKUP UNIT FOR SHIP MOTION RECORDING.
applied in the new Manoeuvring Tank. It has also been used successfully on ship turning trials, using a camera position
on a hillside overlooking the trials area.
Briefly, the method is to mount lights at the bow and stern of a self-propelled model at such heights that, allowing for the trim and sinkage of the model when underway, they lie in a horizontal plane. The model is allowed to run up to speed and the rudder is put over to the required angle, either by radio control or by mechanical means, the path of the model
during the run up and. during the turn being recorded photograph!-cally by a multiple exposure on a single plate. The single came-ra used looks down on the turning area so that the resulting
circle shews the position of the bow and stern lights at various points run but introduces a perspective effect, that is to say, the circle has an elliptic appearance.
The perspective effect is analysed out by using a specially constrùcted grid which is placed over the photograph to enable the position of the bow and. stern lights to be read off in their correct cartesian coordinates. From the resulting plot, all the relevant turning characteristics - tactical diameter, transfer, advarce and drift angle - can be deduced. A typical photograph and grid are shewn in Figure 31.
As originally applied in Portsmouth Harbour, the models used were about 2--f t. long. In the large Ship Tank the models used are only 6-ft. long, but it has been found these small models provide a sufficiently accurate result to justify their
use at an early stage of the design, the tactical diameter predicted compared with the ship trials is within ten per cent/. The larger models are run when the design is in its final form. In the Manoeuvring Tank it is intended to use models about 16-ft. long, this being the most suitable size to use for experiments on the rotating arm also.
Dti
h:,PHOTOGRAPHIC RECORDING
OF MODEL TURNING
FIGURE 31. : 70 60 4 40Vibration Recording
In common with other fields of research and development, investigations into ship vibration depend largely upon the facility and accuracy with which it can be measured.
Broadly, there are two requirements. To support the more theoretical aspects of propeller excitation, measurements of the vibration of the hull girder are needed. They must be sufficiently accurate to detect small incremental changes resulting from modification in propeller geometry, provide information on the phase lag of the vibration relative to the propeller balde position at any particular frequency and. in-clude the facility of presenting tbe results directly in a convenient maimer. Secondly, to meet the considerable effort that has been devoted to vibration surveys inttFirst of Class" units of the Fleet, the need is primarily for small and por-table equipment to assess overall levels of vibration on a multi-channel basis and to study problems concerning the
vib-ration of ships' equipment that occur from time to time.
To some extent these requirements are conflicting and it was decided to develop two separate sets of instrumentation.
The first set is based on the use of a phase-conscious volt-meter of the type used to test servo-mechanisms. Briefly, this
voltmeter operates as a wattmeter and indicates on a pair of dials the product and relative phase angle of two voltages at identical frequencies. Applied to the measurement of ship vibration, it is arranged that one of these v'oltages is deri-ved from a synchro-gear driven from the propeller shaft. The second voltage is the output of the vibration sensing unit.
Thus, the component of vibration occurring at the same
fm-quency as the synchro is displayed. directly in terms of its in-phase and quadrature ordinates. The equipment is illustra-ted in Figure 32, in which:
A - a pair of syncliros;
B - a counter for frequency determination; C - amplifier and power supplies;
