The development of the pitching, rolling and stabilising instrumentation & technique in a medium sized ship model basin

Pełen tekst

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ARCHIEF

ll!DOARSI ISTJ1IIÎ

SIPBVILDI6 RESEARCII INSIIIUT[

ZAE8

¡he Development of the Pitching, Rolling and Stahulising

Instrumentation & Technique in a Medium Sized Ship-model Basin

by

Hans Volpich, B. Sc., M. I. N. A., M. I. E. S.

Superintendent of the Denny Experiment Tank, Dumbarton

Lab.

y. Scheepsbouwkjnde

Technische Hogeschool

rt t

PAPER No 7

Ueitt

Paper io be presented at the Symposium on the

Towing Tar k Facilities, Instrumentation and

Measuring Technique

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i

THE PITCHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE by H. Volpich

A. Introduction

Frani the early days of history, when man ventured out to sea, the problem of ship behaviour at sea has been with him. Even Noah in his ark must have been confronted with pitching and. rolling, whether in regular or irregular

seas. Probably our forefathers approached the problem in some crude fashion by comparing the behaviour and perfor-mance at sea of full sized ships. However, the advent of model experimenting opened a new way of studying and im-proving the seakeeping characteristics of ships. Already the Fraude family had embarked on certain model experiments especially with regard to rolling; there are several tests at Denny's Tank on record as far back as the latter end of the last century and no doubt other establishments of

this kind will have ventured at one time or other into

si-niilar model tests.

There has been a noticeable upsurge in interest in sea-keeping qualities of ships over the last decade and. proof of this is the establishment of several so called seakeeping basins as at Wageningen, Washington, Haslar and Feltham, where models can be tested in al sea conditions and

all

headings to waves. But quite apart from a large initial capital expenditure for buildings and equipment the actual running costs of such establishments are outwith the avai-lable means of Experiment Tanks run by private

concerns

and. hence the latter have to rely on more simplified yet

modern methods and equipment. This paper is therefore in-tended to show, how in a medium sized ship model basin seakeeping tests within certain limits can be carried. out with modern equipment.

Before going into the details of these experiments and their relevant equipment a brief description of Denny's

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THE PITCHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

Tank must be given:

The overall length of the towing basin including a dock at one end. is 330'-O", the breadth 22.4u and the normal water depth about 8'-O" to 8'-6". The present carriage

/See Fig. 1/ electrically driven from an overhead power pick up and built in 1940 is of the narrow span type of

only 1+ ton weight, built of light metal construction

running on solid rubber tyres.

]?ig. 1. Dynaiaometer Carriage in Denny Experiment Tank

All normal ship work up to 16 ft./sec. speed can be done without m-iy assisted acceleration, thereafter a catapult arrangement of falling weight type is employed, whereby the carriage plus two operators on board can reach a top speed of about 32 ft./sec. in 30 feet and then maintain this speed to the end of the run. This catapult system also installed

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-2--THE PITCHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

in

l9t40

permits therefore high speed work to be done too

in a relatively short model basin and for this reason it can be regarded as quite unique.

B. Pitching Tests.

There ars sorne records in existence of pitching tests having been carried out in the. Denny Tank before the turn

of the century, but the first real recorded tests still in

existence go back to 1910, when 10' models of oil flats were tested both in calm water and. in waves not only in deep water but also in shallow water of varying depth, a false bottom being employed in those days. The tests con-sisted of simple resistance experiments with the models in the normal manner under the old carriage. The waves were generated by simple man operated flaps, the period

of the waves being controlled by a pendulum of varying

lengths. Since the waves were relatively small the movements of the models were therefore also small and could be recor-ded on fore and aft trim cylinders attached to the carriage. The man operated flaps for wave generation were still being used in the years following the first world war, when

qualitative comparison tests were carried out with larger paraffin wax models of normal and the early Maierforrn

designs. The only advance in technique was the substitution of the pendulum by a metronome. Moreover since the movements of the models were becoming larger the pitcfring records on the trim drums had to be suitably reduced through a lever arrangement. In those days self propulsion experiments were still in their infancy and hence only the resistance increase in waves was measured.

A significant advance was made in l94 when a plunger type wavemaker similar in shape to the one installed in the then large Hamburg Tank was designed and manufactured at Dumbarton. Since the Denny Tank for obvious reasons of its size does not employ models longer than 20 feet the wave-maker dimensions and. capacity were confined to waves up to

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THE PITCHtNG, ROLLING AND STABLLISING INSTRUMENTATION

AND TECHNIQUE

O feet in length and about 12" to 15" in height. Thereas the original Hamburg Tank Wavemaker could be moved on

wheels to any desired position along the Tank the Dumbarton one was permanently build in at the South end. The drive is through a 10 HP electric motor, so that by varying the speed of the motor the wavelengths can be varied. The va-riation of wave height is accomplished by changing the

stroke on different radial crank positions. The plunger itself has a sinoid front and vertical back. The waves generated if not 100 % perfect throughout are regular enough not to affect adversely any pitching tests. Never-theless a further improvement is contemplated. To this purpose a wooden Tank of a 1/lo scale of the existing Tank /See Fig.2/ has been constructed, in which at present

experiments are being carried out in order to perfect the plunger itself and also devise a completely satisfactory beach arrangement.

Fig. 2. Motor & Drive for plunger experiments in 1/10 scale reproduction of Denny Tank.

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-4-THE PITCHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

In line with recent experiments in other Tanks the inten-tion is to concentrate on a straight wedge plunger or one having two straight tapers. The present beach arrangement consists of' an angled board with horizontal slats above the still waterline, the beach stretching right across the entire width of' the Tank. This beach has to be lowered into the Tank prior to wave experiments being taken. In order to overcome this rather cumbersome procedure it is intended to hinge a proposed new beach permanently at the North end of the Tank with a special arrangement for raising or lowering it, when not in use. Experiments on a small

scale with a curved beach incorporating horizontal slats /See Fig. 3./ have already shown that with an efficient beach arrangement the actual waiting time between each run during rough water experiments could be substantially reduced thus increasing the working capacity of the esta-blishment.

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TFIE PITCHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

With the introduction of the plunger type wavemaker in

l95

the method of taking experiments and the recording had to be entirely revised. The models were then already

self propelled and were guided from the carriage by spe-cial balanced scissor guiders which allowed a large am-plitude of freedom for pitching and heaving /See Fig. 4/.

The models also had freedom in the direction of motion of about plus and minus 4 inches from a mean position. The models were self propelled beneath the carriage and the revolutions of the model propeller were controlled in such a maimer that the longitudinal motiQn of the model relative to the carriage did not reach the limit of plus and minus 4 inches referred to above. If this limit was exceeded the test was cancelled and repeated. A trim and heave indicator /See Fig.4./ consisting of two large pulleys combined with a smaller one and a vertical rod was attached to the carriage, thin wire leads running

from the pulleys down astride of the centre of gravity of

-thie model. The movement of the vertical rod up and down

indicated the heave, while the centre pulley rotated giving the pitch angle in either direction. Although this system worked quite well, visual readings had. to be taken

through-out the run and. then meaned through-out, which method relied too much on the human element. The wavelengths could be

ob-tained quite accurately from the R.P.M. of the wavemaker

calibration curves and the heights were taken by a conti- I'

L6

nuous graphical record on a stationary float. Along with the movement of the vessel the increase in resistance, thrust, r.p.m. and torque could be measured and compared with the flat calm analysis. This system was extensively used for tests with models of trawlers and other fishing vessels. No attempt was made in those days to correct the ballast position for a definite radius of gyration, although the models were ballasted for a correct transverse G-M.

The participation of Denny's Tank in 1954 in the first International comparison tests in waves with a model of the 0.60 block coefft. Todd-Forest series necessitated

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THE PITCHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

another complete revision of technique and adoption of different apparatus.

NO TMM GEAM OVER LC

ST S

VERTICAL MOVEMENT 0E PLLLEV

I([OROING A(AVE ROTATIONAL

MECOROING TRIM ANGLE

ALL PURPOSÇ CAMMIA

SKETCIl 5I4OINÇA APRANCEMENT OF EARLY HEA[

AND TRIM GEAR FOR PITCHING lISTS

FI.4,

At the same time emphasis was laid upon having the model dynamically correct, which meant the accurate determination of the radius of gyration by the bifilar suspension method. The models were complete with gear and ballast and then suspended horizontally by two wires. Knowing the complete period of oscillation "Ta of the model, the length li of

the wires and. their distance udii apart the radius of

gyration was obtained by the formula

K=

Td

Ly

l/

This method of assessing the radius of gyration is adhered to to the present day. Of course the ballast had to be

adjusted also vertically to give the correct rolling period. Since considerable difficulties were encountered in

those early days of self propulsion, when taking experiments in waves, all efforts were concent±ated this time in obtai-ning the optimum resistance records along with the movements

0RWA RO SEISE OR

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THE PITCHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

of the vessel and therefore the instrumentation was accordingly.

The entire apparatus was mounted on an open carriage towed behind the dynainometer carriage /See Fig.

5/.

In this arrangement the simple gravity dynamonieter method was adopted, again with the model completely free to pitch, heave and surge.

Fig.

5.

Dynamometer carriage and all purpose carriage coupled together for rough water tests - l95Ll..

The pitch and heave recorder was of a modified British

Admiralty pattern, where pitch and. heave were simultaneously recorded on a revolving drum /against distance and tinie/ through a very light ut rigid system of aluminium levers /See Fig. 6/. The model fore and aft drift for a given towing weight was transferred to this drum through a simple pulley arrangement, so that all records could be

synchro-ni sed.

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THE PITCHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

Fig. 6. Close up view of

1954

pitching gear.

The model was guided in a fore and aft direction by means of two pairs of vertical guiderods at bow and stern, which engaged in a tautly stretched wire, but still giving the model ample freedom of movement /See Fig.

7/.

A spring loaded accelerator bar was fitted to the open bay carriage in order to provide a rigid connection bet-vïeen model and carriage during the accelerating portion of the test run.

For a given fixed waveheight an average of five test runs were taken for each wavelength and speed. For each of these runs the weight on the gravity dynamoraeter was

varied causing the model to drift either forward or aft

relative to the carriage. This drift was automatically recorded as described in the foregoing paragraph.

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SKETCH OF APPAR/\TUS

FOQ PEcopNNe PEROMANCE

o MOEL5 IN WAVE5

1954 TYPE

® CAIA&E CAINC APPARATUS.

H

4J

,P

® WOOb BAM5 ON CARRIAGE

P.ME C) --() ALUMINIUM RAMEWOK. _ ALUMINIUM STU VO rOWING MObL.

® MULTI-PURO5EPLATTO5UppOTF. ® 5PfING LOAbE.I

CELERlTOU.

® F'ULLEYS Çe

RSI5TANCE wE.I.4-rs e eLMS

® QESISTANCE WLI.WT5

® i-ivs.

PITCH MEASURING GEp.

® RObS LINI,Nc NObEL TO

,

Q GUIE POAS ON MODEL. ® RODS SUpPoRrINo WIQE GUIDE

OR MODEL.

® PECOQDING GEAR WITH PENS. ® MODEL UÑDEQGOING TEST. © TOWIIsIG BRAC4T INSIbE. HObEL.

PEIIMENT TANK

FUMBAQTON

Fig.

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THE PI I'CHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

Along with the drift record a complete record of pitching angles and heave over the length of the test run was again graphically obtained. As the wavelength for each run was known from the wavemaker motor revolutions the period of encounter was calculated through the formula

X

Te

YA 2.26v

where ) = wavelength and VA = speed of carriage in ft./sec.

The calculated periods of encounter were also checked with a simple electric probe fitted in front of the model and were always found to be in close agreement. The

wave-heights were obtained by photographing a train of waves for each experiment against a long board graduated both verti-cally and. horizontally0 The waveheights were then measured off the photograph /See Fig. 8/.

Fig. 8 Typical wave in l95 rough water test showing graduated board for wave height assessment.

In the evaluation of the re'ults the actual drift of the model in ft./min. graphically recorded for any given towing weight had first to be corrected for any small error in carriage speed. The towing weights were plotted to a base of corrected drift, a mean line was drawn

through them and the resistrnce picked off at point of zero drift..This was repeated for each speed at every

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THE PITCHENG, ROLLING AND STABILISING INSTRUMENTATION AND TECHNIQUE

wave length. For each individual pitch and uieave record

a mean line was drawn through the peaks thus giving a mean value for the test run.

For practical comparative purposes the final

presenta-tion of results was referred to a LlOO' ship and the three dimensional method of plotting was adopted for percentage resistance increase /See Fig.

9/,

pitch angle ¡See Fig.lO/ and. heave /See Pig.11/, in other words all results for a constnt waveheight were collected in three separate

diagrams. As the flat calm resistance curve is always avai-lable in such experiments, the speed loss for any given

wave length and. specified waveheight could be readily

ob-tained by crossplotting.

EWERItNTS

CUfvS OF Y. HCAS H E. H.P AI VALOUS SPEWS

FOR A 400 SHIP IN OIFFE4T LD61IIS OC WAVE

O SP1ACC)d1 1100 0w1 Lk.

IAOIUS OP 4V!*il0s e;L

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THE PITCHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

EXPERIMENTS IN JAVES

CURVES or PITCHING ANCLE Al RRIOUS WELDS TOR A 400' S441P

OIrFLRENr LENC'THS OT WAVE.

ßCEPLN'r '7600 ?ONS L.. ADWS 0f yA?IDN L$. 4.

-

13

-Wed! HCI41' S 33 TW1O4JÇf OUI

Although the last method described was in itself quite successful apart from the fact that it was solely designed and adopted for resistance experiments in waves, the time factor was its only drawback, as at least five test runs had. to be taken for each speed and wavelength in. order to determine the point of zero drift.

With the general introduction of electric measuring devices in ship model tecbnique it was felt that the time had come to redesign the entire gear and make it adaptable both for resistance and self propulsion tests with

auto-I6

'3 '4 , IO IO

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THE PITCHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

XPRIMEWTS W4 WAV$

CURVES OÇ HEAVE AI P$S4$ AI

V.jgious 5PCED 0R A 400 SNiP

IN DIrPUEN? LCNITI1S WAVI

03PIJCMEÎ 8O0 TøIt5 LK.

ÇYRA?1G £5L.

- l

-I. *0

FI ii.

inatic synchronised recording. It was entirely designed and made at the Dux«barton Tank in 1957/58 and has already been successfully in use /See Pig. 12/.

The model is again placed under the open bay carriage with fixed fore and aft guide pillars engaging into

rollers on the gunwale of the model. On the centre of gravity of the model a solid horizontal baseplate carries a pivoted vertical tube, which slides up and down in roller bearings inside a larger tube attached to a small carriage. The latter moving on roller bearings inside guidechannels fixed to the carriage has a free fore and aft movement of around 18"O A potentiometer near the

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THE PITCHING ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

2

SKETCH OF I58 PITCH AND HEAVE SEAR WITH ELECTIC MEASL1IN DEVICES FII.

bottom of the pivot transmits the angular pitching move-ment, while a second potentiometer attached rigidly to

the outer sleeve and. connected by a strong thread to the top and bottom of the vertical column transmits the heave. In order to obtain linear calibrations a cathode follower had to be introduced into both recording systems.

For resistance experiments the small carriage is connected

via a ¿1 i ratio pulley to a belicrank dynarnometer with spring and weights. In the case of self propulsion expe-riments this dynamometer can still be kept in use, but normally it is disconnected, as most of the tests are

then being carried out at the model self propulsion point.

In connection with the new gear an improved electronic wave height gauge was designed and made by Messrs .Ituirhead

15

I PITCH POT 7IOqET

a lOUER OUWERS

HEAVE POTENTIOMETER

4 SURGE CARIAIC

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THE PITCHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

of Beckenham /See

Fig0

13/, who are long associates ol' Messrs. Denny's in the field. of stabiliser control gears.

Fig. 13. Electronic wave height gauge coupled to Hartmann-Braun "Lumiscript" recorder.

The equipment is designed for use on the moving carri-age in a model ship tank to record the wave height and contour of the waves encountered by the model under test. It consists of two units, the probe and an electronic unit. The probe consists of a thin copper strip 18?! long sand-wiched between two strips of Perspex and shaped into a stream line section. An earth electrode also suitably shaped and held a few inches behind the trailing edge of the electrode is clamped to a small platform, on top of which is mounted. an electronic unit

5+1!

long l4- diameter,

This is connected by a flexible cable to the main electronic unit approximately

9" x 7j-"

x 8" into which the

50 c/s

A.0. supply is connected and which has also a dìsplay meter

to inaicate the wave height.

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-THE PITCHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

The equipment depends for its action upon the use of a capacity bridge, one arm of which is unbalanced by the height of water surrounding the probe. The readings indi-cate the variations in height and three scales are provided covering ranges of = ± l', ± 2 u a + 511 A facility is provided for the operation of a recorder so that conti-nuous records of the change of wave height can be taken /See Fig. 13/.

All records, that is in case of resistance experiments heave, pitch, resistance and complete wave profile from the probe are brought together and synchronised in a Hartmann & Braun 'Lumiscript" phototrace recorder.

In this high speed recorder a light source, in this case a super high pressure mercury vapour lamp and special photographic paper produce a recording which is immediately visible without developing, fixing or any chemical treat-ment. The measuring systems /galvanometers/ have a capacity,

where 1 rn/in light point deflection on the chart corresponds

to about 24 millivolt deflection or since the chart paper

is 60 rn/rn wide the full scale of the chart corresponds to

about 1 -- volts full scale deflection.

In the analysis work it is now common practice to present the motion results to a dimensionless base parameter of

wave length

combined with dimensionless parameters of pitch ship length

and heave, i.e.

Total pitch amplitude

Maxirnim wave slope

and

Total pitch angle in degrees x wave--length in ft.

Heave in ft. Wave height in ft.

If in addition the experiments are taken under.self propel-led conditions, a composite diagram can be obtained, which while embodying most of the test results gives the practical

15 x wave height in inches

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-THE PITCHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

mariner a better idea of the ship's behaviour under

dif-ferent sea conditions. Such a diagram is arrived at by crossplotting and crossfairing all the results and finally relating them to the ship's available full continuous machinery power. The application of such diagram could best be illustrated by an example of a

400 ft. cargoliner with a service speed of 15

4

knots and a continuous machinery power of 5800 B.H.P. /See Fig.14/.

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18

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u

iI.pøTL

HEAV:____

SI Q

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THE PITCHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

The ship isproceeding in a pretty regular sea or swell, when by stop watch the pitching period or period of

encounter is found to be 64- seconds for the complete cycle, the wave height from trough to crest is estimated by eye to be say 7 ft. The

ship

speed with 5800 B.H.P from the diagram would then be roughly 13 knots or

2 knots less than the service speed. Passing to the bottom diagram the resulting heave should amount to

7 -

ft.

and

from the top diagram the total pitch angle

5 f°

or the

total bow movement equal to 26 ft.., assuming the pitching centre to be approximately

-f

from -the ship's sterne The

ship is obviously pitching in synchronism. If the vessel could now be turned more into the waves to reduce the pitching period to say

5-f-

secs., the heave would reduce

to

3f

ft.

or by more than 100 % and the pitch angle to

under 3 or by almost 100 %. Incidentally the same could be done by lengthening the period of encounter depending on the direction of the swell or sea to the ship's course.

The advent of experiments in seakeeping basins will not destroy the value of rough water experiments in the

orthodox iriaaner. Actually some useful indi cation could

be obtained by running models in the normal manner in head seas

and

then transferring them into the seakeeping basin, so that in the long run some broad deductions could be made from the normal method tests to the behaviour of a vessel in a sea coming from a different angle.

C. Rolling

and

Stabilising Tests:

Although rolling investigations had been carried out in a perhaps perfunctory manner many years ago, e.g. in the investigation of the effectiveness of Frahm's anti-rolling tanks, the period of serious investigation began with the development of the Denny-Brown ship stabilisers, when the first model tests were taken at Dumbarton in 1933.

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-THE PITCHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

Since then a proportion of the year's work usually inclu-des rolling and stabilising tests plus stabiliser fin in-vestigations, the Latter both in the ship-model basin and in a cavitation tunnel. For the purpose of this paper the section on rolling and. stabilising must be taken to-gether both for instrumentation and. tecimique.

When one looks back on these first tests 26 years ago they now appear rather crude, but one ought to appreciate the terrific stDides forward instrumentation alone has made especially in the last decade. Nevertheless useful

information was accumulated even with the rather primitive instrumentation then available, but traight from the start large models of 16 to 18 ft. length were used, since rol-ling experiments were normally followed by stabiliser tests with activated fins, where even on this scale the fin units on the model were necessarily small. The first step was to produce a mechanism which could effect a rolling movement similar to a regular beam sea. The basic design of

this apparatus is still unaltered from 26 years ago, al-though various minor modifications and improvements have been made to it. An electric motor causes through gearing

a light horizontal arm to oscillate to and fro about the centre-line plane of the model, and at a height just above the gunwale /See Fig. 15/. As this mechanism represents a major part of the model's ballast, its vertical position is conditioned by the final determination of the correct metacentric height through an inclining experiment. The oscillating arm is pivoted at one end on the centre line, and supported at the other end by a small wheel running along a track shaped in a circular arc in plan view. The arm is made to perform an approximately simple harmonic

motion on this track, the period being controlled by varying the current supply to the electric motor. The rolling

energy is controlled by varying the mass attached to the arm, or varying the radius at which a particular mass is attached to the arm.

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-THE PITCHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

Fig. 15. Rolling machine fitted into model.

Before proper roll recorders came on to the market repe-ated. attempts had been made to record the roll amplitude through a lever system; however, unless the model was running perfectly straight, a certain amount of yaw was included in the roll amplitude. The first commercial re-corder

was a

French M.A.R. /Braibant/ recorder of the pen-dulum type, which was not very satisfactory either. The introduction of the British Admiralty type /A.R.L./ gyro roll r corder represented. a great step forward, although the instrument as fitted into the model weighed

at

least 50 pounds. Nevertheless this type of instrument was used successfully for years despite the disadvantage of having to reach down into the model to recover the records. The

obvìous next advance was to find the means of bringing the

recording up to the carriage instead. o± inside the model. This has been done in tle last two years by introducing

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-THE PITCHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

a potentiometer. The pendulous gyro was removed from the

old.. A.R.L. recorder, mounted on its own baseplate and coupled to a potentiometer pick-off, so that the amplitude records can be transmitted nowadays like in the pitching tests to a Hartmann-Braun TLumiscriptt recorder /See Fig.

16/.

22

-Fig. 16. Layout of model rolling & stabilising gear in 16 feet long paraffin wax model showing from front to back pendulous gyro with potentiometer for recording roll

amplitude, hydraulic relay unit for fin operation /potentiometer for fin movement

recording not visible/ and rolling

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THE PITCHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

Model tests with the Denny Brown flap type fins in-troduced the problem of operating the same either for

forced rolling the vessel with the rolling mechanism locked in its central position or stabilising the model against the action of the rolling machine by means of the fins. Up to 1958 this was achieved by hand control through Bowden cable connections, so arranged that no appreciable force was applied to the model by the operation of the cables. The fins were so controlled, that, when one went up, the other went down. An operator sitting astride a

bridge on top of the carriage moved the operating levers watching at the same time a mast fitted near the bow of the model. The angle of tilt of the fins could be varied by suitable stops on the finquad.rants inside the model. The period of roll was obtained from the gyro recorder, the period of the oscillating weights and of the fins through electric contacts. This method, in use for many years, was quite successful. Forced rolling tests /very often carried out to the beat of a metronome/ caused no difficulties. However, when stabilising the human element entered into the picture, as the operator with the model almost stabilised, was apt to lose the place and impose a roll on the model, when getting out of step.

In 1958 a Muirhead B-882 gyro controlled hydraulic unit was purchased in order to cut out the fin operation by manual control and make it fully autoniatic as on the full

size ship /See Fig. 16, 17 & 18/. This hydraulic unit /See Fig. 19/ representing a self contained control system responds to the velocity of rolling. A feature incorpora-ted in the system provides a measure of anticipation of the roll velocity, when the residual roll is sinusoidal in character. It is actually a miniature version o± the by-draulic unit used in the normal Denny-Brown-Muirhead ship stabiliser control except that it has only one stage of' hydraulic amplification. Three sizes of power pistons are

available for the hydraulic unit thus making the response and output torque to be pre-arranged.

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-THE PITCHING, ROLLING AN D STABILISING INSTRUMENTATION AND TECHNIQUE

Fig. 17. Model fin arrangement in large twin fin installation.

It has been

found

by experience that for the normal range of model stabilising work the smallest cylinder with an output torque of 12 lbs. ins, and a response time for

a 150 movement of 0.075 sec. is sufficient.

Theibasic control movement is derived from a gyro whose precessional movement is proportional to roll

velo-city. The gyro itself consists of a small motor with a flywheel mounted on its spindle, the spindle axis being horizontal and atbwartships. The velocity of any rolling motion imposed on the model causes the gyro to precess about a vertical axis and move against the tension of

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-THE PITCHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

centralizing springs. The movement of the rro is taken,

by links and levers, to the pilot valve of the hydraulic unit; with the unit having only one stage of amplifica-tion the pilot valve also funcamplifica-tions as a control valve.

Fig. 18. Layout of model rolling & stabilising gear in a ten feet long wooden model. Fins removed.

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-TILE PITCHING, ROLLING AND STABILISING INSTRUMENTATION AND TECHNIQUE

26

-Fig. 19. Gyro-controlled hydraulic

unit, type B-882

for automatic

model

fin

operation,

designed

and. manufactured by Messrs. Muirhead. Top cover removed.

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THE PITCHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

The hydraulic unit comprises a motor-driven gear pump

supplying oil at 100 rn/in"2 pressure. A pair 0±' power

cylinders and associated pistons actuate arms attached to the output shaft. The movement 0±' the output shaft is proportional to the movement of the control valve but has greater amplitude and force; a mechanical linkage frona the output shaft resets the control valve.

Since the normal technique in this type of model work consists of both forced rolling and. stabilising a switch is embodied in the unit, which reverses the direc-tion f the gyro motor rotation, but is so situated that it cannot be interfered with during one set of experiments.

In such tests the models are always towed by a long

f line ahead of the bow fitted to a spring /See Fig.20,21 & 22/ and by a similar line at the stern passing over a double pulley and an associated. tension weight.

Big. 20. 16 feet long paraffin wax model at speed /self propelled/ rolling under action of the rolling machine. Fins removed.

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-4

THE PITCHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

Fig. 21. 10 feet long wooden model at speed rolling under action of the rolling machine. Fins removed.

Normally the models are just towed, although some have been self propelled, which only increases the difficulty

of accommodating all the gear including self propulsion motor, etc. without any difference in the results.

However, whether the model is self propelled or just towed, the vertical setting of the tow wires is quite important, because there may be introduced a yawing motion to the model advancing along the Wank.

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-THE PITCHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

Fig. 22. 10 feet long wooden model at speed being forced rolled by action of stabiliser fins. Many experiments have been taken to find the perfect posi-tion, which appears to be very close to the waterline.Other forms of fore and aft control such as guiders in double roller slots have been tried, but all have been discarded in favour of the long guide wire method, where, even if a small amount of yaw should take place, no large interfering forces would be brought into play. Many experiments have also been carried out to determine, whether there would be any effect on the rolling performance of the model by varying the back tension weight. It was found, that so long as the tension weight was sufficient to keep the model under con-trol, any variation in the tension weight had no effect on the rolling performance of the model.

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TIlE PITCHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

It is the normal practice to tests the models with full appendages, as, especially in hull forms having a cut-up stern, large rudders and. other stern appendages may have an appreciable influence on the rolling

quali-ties.

The usual testing procedure is divided into three sections, i.e. free rolling, forced rolling with fins and stabilising. In the last section half the run is taken free rolling by action of the rolling machine and the latter half stabilising with the fins, whereby in the first half the fins act as bilge keels /See Fig.

23/.

-

30

-FI 3.

TYPICAL. MODEL T05T CCOO FACt ROLLINI ANO STABILISING

ì extending and retracting mechanism for the model fins

would be too complicated and in any case the stabilising tests are taken with the same roll energy /rolling weight

in same position/ as for the free rolling tests. In the latter at a given speed with the model at the correct GM and with the ballast disposed for the correct period cur-ves of rolling amplitude on either side of synchronism are built up by using various weights at different posi-tions of the pivoting arm of the rolling machine /See Fig.

24. /

r,5 NA CTIV C FINS ACTIVE

STABILISES BELL VALE NOII.

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TI-lE PITCHING, ROLLING AND STABILISING INSTRUMENTA1'ION

AND TECHNIQUE

The maximum possible rolling amplitude of the full size ship has always to be covered.

TYPCAL CiPVES 0F ROLL AIjTUO TO EAS OF ROLLINO PERI AS PRO.0 ) AN IB' MOOEL

Y MBANS OF TI.4C ROLLINO MACHINt,

FIG. 24.

These experiments are preceued by stationary declining tests and also at speed. All these axperiments are repe-ated over a range of speeds, range of GM and sometimes also over a range of displacements, these representing the main parameters for fin design.

The forced rolling tests are carried out at synchronism in pre-arranged steps of fin angle up to the maximum /See

Fig.

25/.

The stabilising tests are often started at low

speed with a fairly substantial residual roll down to full stabilisation at the fin design speed. The model is made to roll under the action of the predetermined size of weight with the hydraulic relay unit taking over during

the second part of the run in quenching that roll /See

Fig.

23/.

The significant importance of the various steps in the experimental procedure is borne out by the final analysis.

31 POIL WCWT 1UN5 P *1051 SPEll) IIPIC PERI 23SO YDS 5* 33., '1% Pl PEES

--u.-NOØEL I4IO Ttl&L YRM

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THE PITCHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

The declining experiments permit the assessment stationary and at speed of the a" and tib?? constants in Froude's

decremental equation - aG + and their comparison

with different hull forms. They are also used for deter-mining damping correction factors in the assessment of

finheel at speed. on full scale stabiliser trials.

FORCED RCLLIM WIT14 SAILISEI ÇWÇ II/. IN0.r5 34.3 CT PER

u

ACTUAL 041P O RUALT rNtT4LE

c. MEEA MAIN FIN,

DIt. 10 MAIN DItI, ¡0'YAIL FIN -EQATAL FIN 13 TAIl. FIN -EQ/OR 10/13 30 30 34 3E S 4-3 4E .4

0_ ROLLIAÇ. PERIOD SECO2S

TY°EAL CLRVEO OF FOEO RLEO AMPLTUOES ACTION OF M000L FINS ON IB FEET MOl OF P. & O. LINER CE.SAN' AND COMPNOS WITI OCTUN 3UP TRIAL AT %AU FIN A.LE.

From the free rolling tests, where normally four dif-ferent rolling moments produce a range of roll amplitudes, the wave slope at synchronism can be calculated as shown in the Appendix to this paper. The relationship of ampli-tude to wave slope gives the amplification factor. As the wave slope to be combatted forms the basis of fin design this amplification factor permits the assessment of forced roll to be expected or finally the amount of roll quenching.

The forced roll tests with the fins serve the purpose of building up ship/model correlation factors, although in most cases in spite of the expected scale effect close correlation between ship and model could be achieved so

far /See Fig. 25/. In case of superliners like the ttQueen

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-TITE PITCHING, ROLLING AND STABILISENG INSTRUMENTATION AND TECHNIQUE

Elizabeth" predictions made from model results were within

20 out to out, while on small vessels full scale trials

corroborated the model tests within i %.

Since the same rolling moments are being used for both the free rolling tests in section one and the stabilising tests in section three, which means the saine rolling

energy is being put in in both cases, the so called sta-bilisation factors

- Stabilised. roll can be produced from

the combined analysis of both. If more than one fin area is used for the tests, a diagram can be drawn showing the stabilisation factors in terms of speed for any given fin area /See Fig. 26/.

SThSILISAfl0F FACTORS l TERMS 3F SPEED DR TWO

CM VLUS, .

'-'

. 4-O"

2G.

If this is repeated for more than one GM value a noinogram

/See Fig. 27/ can be constructed for the particular vessel, which for a predetermined free rolling amplitude shows at what speed full stabilisation could be achieved for a given fin area and metacentric height or vice versa what fin area would be required for a given speed and

metacen--

33

-22S ri514 ¡42tJU SL?!. i4 io

JI!1II

TIITI'J

HH

il

¡/IjjI

ÇN I I i I J e

ta

L i

Il/I

It

if

1/

I!

tI)I//

'J

/ I ¡ / i f /

if

I I ¡ /

//

I «I / /

/

¡ i

/

I I / li / / I t

/

,

'/1/

/7

/ / / /

//

/;

'

/// //

2

-; -- -;-_

sîai.issisc i O

- -w-wo o IS i iNOT3 24 5 I 54. se

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THE PITCHENG, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

tric height to achieve full stabilisation. At the same

OMXR* tr ATOff' PM AREA, iD

PR A STA&!SAT CT 40 FREE

RL JT TU JT RE1JOED TO ( JT ID JT.

Fl1. 7,

time with the alci of the previous diagram the performance of the required fins at other than the design speed can.

be readily determined.

D. Conclusion:

This paper serves to show in broad outlines how instru-mentation and technique in these special types of model tests have been developed and how the results can be use-fully employed to predict the pitching and rolling per-formance of full size ships even with the limited means

at disposal of a medium sized ship-model basin. It is certainly true, that for too long a time ships have been designed mostly to satisfy certain measured mile conditi-ons and. rough water performance was only of secondary importance. With the emphasis shifting more towards good

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-TI-LE PITCHING, ROLLING AND STABILISING INSTRUMENTATION AND TECHNIQUE

seakeeping qualities some useful contribution to this problem can still be made by the ordinary ship-model basin, where facilities are not unlimited, so long as sufficient enthusiasm and ingenuity are available.

E. Acknowledgments:

The development of the instrumentation and technique described in this paper refers to that in the Denny Experiment Tank, which forms integral part of the Leven Shipyard belonging to Messrs. Win. Denny & Bros. This

paper is published by permission of the Board of Directors,. of whom Mr. W.P. Walker, Director of Design &. Research, took special interest in its preparation, as he at one time formed part of the Experiment Tank team actively engaged in all the development.

The author desires also to acknowledge the assistance rendered by Mr. John Bell, M.Sc., M.I.E.E., Chief Research Engineer of Messrs. Muirhead, in supplying photographs and detailed descriptions of the "Muirhead' instruments in use. In addition the author wishes to thank Mr. R. Cameron, Naval Architect of Messrs. Harland & Wolff, for permission to publish a composite pitching diagram evolved in connection with certain experiments carried out to their account at the Denny Experiment Tank.

The valuable assistance of the entire Denny Tank staff in the preparation of this paper is also gratefully

acknowledged.

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-THE PITCHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

F. Biblioraphy:

[i] - J.F. Allan: "The stabilisation of ships by

activated fins", T.ION.A. 1945.

- H. Volpich: "International Comparison tests in waves with 10 ft. model of 0.60 Todd-Forest Series", Denny Tank Report No. 3156.

-

J. Bell: 'Hydraulic Relay" - Technique, Journal

of Instrument Engineering, Vol. 10, No. 1.

- H. Volpich: "Rough water tests with nodern fast Cargo liner", Denny Tank Report No.

3771.

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-THE PITCHING, ROLLING AND STABILISING INSTRUMENTATION

AND TECHNIQUE

APPENDIX

Calculation of wave slope from action of rolling mechanism.

Acceleration of weight across model = sin q'

;

Force due to horizontal motion of weight

x -- sin

2 t

hw y2 2Jrt.

Moment of above force

= g X

-r

sin

T

Moment due to weight of rolling mass = wd. sin

-

3?

-IJnladen weight of rolling arm of mechanism = 2.11 lbs.

CG of arm from pivot

= 9.38w

w = Weight + arm moving in

simple harmonic motion across model.

ci. = Maximum transverse

dis-placement of w.

h = Height of CG of moving weight above CG of model. T = Complete period of

rol-ling weight in secs.

2rt

T

W = Displacement of model

in lbs.

M = GM of model in ft.

max = Maximum effective wa-ve slope.

2rt

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THE PITCHING, ROLLING AND STABILISING INSTRUMENTATION AND TECHNIQUE

Total capsizing moment at time t = wd sin 2 í t (i hv

)

c/i

42h

) = wd. sin

gT

Total capsizing moment due to wave with slope e = G'W1

wd / h 180 in degrees; Whence e max -Ç1 + 2 / Numerical example: w = 10011 lbs; d = 0.76 ft; h = 0.716 ft; W = 356 lbs; M = 0.229 ft.; T = 1.70 sec.;

Maxiii. effective wave slope =

10.11 x 0.76 (i 2 x 0,716 180 6.86°.

- 356 x- 0.229 32,2 X

l704

=

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