Turning and manoeuvring trials

(3)

(5)

Sy-zoosium ;Ship Trials and service

erfoLLaance

Newcastle/Tyne

11 april 1,760

ARCH1EF

Lab.

V.

_{Scheepsbouwkunde}

Ted:--.:!!sche Hogeschooi

TURNING AND MANCEUVRING TRIALS

Veiff

By R. N. NEWTON, R.C.N.C.

Superintendent, Admiralty Experiment Works, Haslar

SYNOPSIS.This paper outlines and distinguishes between the qualities which

together constitute the manceuvrability of a ship, but which are to some extent

conflicting. The technique employed by the Admiralty Experiment Works for

conducting turning trials to determine the geometry of the turning path of a ship at different speeds of approach and rudder angle is described in somedetail.

A brief description is given of the principles of spiral and zig-zag manceuvres, by

which the course-keeping qualities can be assessed, together with a note on the order of accuracy of ship-model correlation to be expected.

Introduction

THE

objectives of turning and manceuvring trials can be manifold

and it would, therefore, not be out of place to enumerate the

important data, some or all of which determine the nature of the

model or ship trials required in a specific case. These are

(1)

Steady state information such as turning diameter, transfer,

advance, drift angle, heel and speed on the turn for a range of

approach speeds and rudder angles.

Character of the directional stability, which partly determines

the course-keeping qualities of the ship.

Response of the ship to her rudders, causing the ship to leave an initial straight course or checking an initial turning rate ; usually referred to as controllability, and which also influences course-keeping qualities.

(4) Information on acceleration and deceleration at different powers

ahead and astern, which is of special importance when berthing, or in collision emergency.

Ability to turn in confined waters.

Manceuvrability is the term usually applied to the combination of the qualities embracing (1),(2) and (3), or, more strictly, a compromise between them since they

are to some extent conflicting. As Davidson and Schiff pointed out,' "turning ability" has a well recognized index in the numerical ratio MIL, between the

minimum turning diameter with maximum rudder angle and the length of ship, as determined from the results of trials under paragraph (1) above. In this excellent

paper he developed the general principles of "ease of steering from

both the practical and theoretical aspects and discussed the course-keeping qualities of different types of ship. The conflicting nature of the components of manceuvrability was brought out clearly and has since been proved by model experiment and ship trials.

For instance, increasing the cut-up area will generally reduce the turning diameter, but will also reduce directional stability. Furthermore, provided

that the ship is still directionally stable, the increase in cut-up area will improve 1Numerals refer to list of References at the end of the paper.

Analysis

-TURNING AND MANCEUVRING TRIALS

control, or response to rudder. On the other hand, an increase in rudderarea is favourable to both stability and control and also results in a smaller diameter

of turn. _{The ability of the main machinery and propellers to decelerate or}

accelerate the ship, determined by what are usually known as stopping, starting

and reversing trials is not linked to directional stability or control. On the

other hand, ability to manoeuvre in confined waters while obviously _{related to} these qualities is very much a function of the depth of water and width of channel.

Presentation of "Turning and Course Keeping Qualities" byDavidson and Schiff' was closely followed by the publication of a collection oftechnical circulars and test reports of the investigations by Dieudonne at the Paris Basin,' containing

Dieudonnes original proposals for the conduct and evaluation of spiral

manoeuvres. The present-day interest in the course-keeping qualities _{of ships}

stems from the original work of these two authorities and is doubtless stimulated

by the great improvements made in recent years in instrumentation _techniques

and the acquirement of facilities more suited to the task of model investigations.

The primary object of this present paper is to put on record details of the

technique developed at the Admiralty Experiment Works for conducting_turning

trials and to outline the principles of spiral and zig zag_{manoeuvres which latter}

have recently been included in the first of class trials programmes of H.M. ships.

Before attempting to do this, it is worth noting that whatever the type of trial,

care is necessary with the selection of the trials area which should not be subject to fast tides or exposed to the weather. _{Depth of water is important from two} aspects ; it should be as uniform as possible to avoid extraneous effects from the bottom and if it is known that the ship will operate for lengthy periods in shallow

water then an area with the appropriate depth should be chosen. Turning Trials

Two methods are in current use for recording turning data of H.M. _ships, both employing a central recording position and using similar equipment. _In the first method the instrumentation is carried on board the ship which_{is made}

to turn around a freely-floating buoy and in the second case, which incidentally

derives from the method used for model experiments, the instrumentation _is fixed ashore at specially prepared bases and used to plot the path of the ship. This second method is not so accurate as the first and is used only for_{ships of} small size which cannot accommodate the recording gear, or in which the gear

would be washed down when on the turn as in the case of a submarine.

Method IShip-based Instrumentation

Bearing recorders of the type illustrated in Fig. 6 are fitted in the ship,

one forward and one aft, on level bases with their axes parallel to the

centre-line. An operator at each position keeps the recorder sighted _{on a}

freely-floating buoy, the bearing of which is autographically recorded by _{a pen on an}

arm attached to the spindle of the sight. _{The term "autographic record" is}

here meant to imply a continuous line record on paper with a suitable time base record included.

In the original version of the apparatus the pen was at a fixed radius on the

arm and registered on a moving roll of paper. _{The system now preferred is with}

the pen moving along the arm, with a reversing switch at each end, the paper being stationary and renewed for each run. _{The bearings are also recorded at} a central recording position by means of a magslip transmitter and receiver. The former is attached to the spindle of the sight and the_{receivers from each} recorder position are brought to a common box and photographed. _Between

runs each new sheet of paper is marked with the zero and 90 degrees bearings and the zero values are checked in the magslip box. _{There is always the possibility}

of human error in aligning the sight with the buoy and, in _{some cases, a small}

angular error in the bearing can make an appreciable difference _{to the apparent}

position of the buoy. To eliminate these errors a camera is mounted on the sight and this automatically photographs the buoy's position relative to the

sight line.

TURNING AND MANCEUVRING TRIALS s81

The angle of heel is measured by photographing the jack-staff against the horizon, but it may be preferred to use a heel gyro producing an autographic

record. The rudder angle is recorded autographically by a linkage system

attached to the rudder stock. Shaft revolutions are also recorded autographically and used to determine the ship's approach speed from measured-mile data. This

speed is also checked from bearing records taken during the approach. The officer in charge of the trial is stationed on the bridge and at intervals during the approach and during the turn he depresses a morse key which fires

each camera simultaneously and also makes a mark on each autographic record. The key is depressed at certain changes of heading relative to the initial course but, again to avoid the possibility of human error, at each depression of the key

a camera is fired which photographs a gyro course recorder and stop-watch. The appropriate run number appears in every frame of film record.

In addition to these precise records, entries are made in a "bridge book" of draughts at the beginning of each day, run number, nominal approach

speed, rudder angle and the time to reach specified changes of heading. Wind and sea states and directions are noted together with any other comments which

may affect the results. The importance of making fully documented records

of pertinent events cannot be over-emphasized.

The number and layout of recorders in the ship varies with the type of ship. To give a clear view to both sides it is often necessary to fit two recorders aft

and sometimes two forward. A typical arrangement is shown in Fig. 1 which is drawn for a ship fitted with three bearing recorders.

Long experience has shown the need for several precautions in the conduct of turning trials. Trials are not carried out if the wind is too high depending on

the size of the ship and the windage area she presents, or if the sea is too rough.

A large aircraft carrier for instance can be tested in water which would be too

rough for a small minesweeper. Probably the best guide as to whether conditions

are suitable is the response of the ship in pitch and roll. Approach runs are made directly up or down wind and a minimum approach run of three minutes

is stipulated after the ship is on her desired course. Between runs rudder angle is limited to 15 degrees.

Analysis of the results starts with development of the films of the compass

readings. Plots are made of the ship's heading against time using the gyro

compass data and of the bearing from each recorder against time. The bearings

are corrected for human error using the film from the sight cameras. From these plots the bearing of the buoy from each recorder position at 5, 10, 15, 30,

45 and 60 degrees change of heading, and every 30 degrees change of heading thereafter, are found and used to prepare a drawing similar to Fig. 2. From a

central point representing the buoy, radial lines are drawn at the angles represent-ing the changes of headrepresent-ing quoted. To one side, a representation of the ship is drawn with the recorders marked in their correct relative positions. By setting

out the appropriate angles from the bearing recorder positions the locus of the

buoy relative to the ship is obtained as shown. From each point on this locus a

perpendicular is dropped on the centre-line of the ship, giving the point P

Using the perpendicular distance B of the buoy from P and the distance A of P from the amidships point of the ship, the position of the ship relative to the buoy

can be plotted on the first diagram. A curve joining the successive positions of the amidships point is referred to as the path of ship. The angle between the tangent to this path and the ship's centre-line at any point is called the drift angle. The tactical diameter, advance, transfer and drift angle read off from this curve are then cross-plotted against the rudder angle used and approach speed. The corresponding angles of heel during the turn can also be plotted.

Transfer and advance usually relate to 90 degrees change of heading, but

are also tabulated, in reports summarizing the trials, for other changes of heading,

Method IIShore-based Instrumentation

With this method one can choose between using two low-level land bases or

a single base at high level from which to take bearings of the ship. In the former case the bearing is taken of an easily discernable object in the ship, for

instance a point on the mainmast, and a correction has to be made for its position relative to amidships. The records taken on board comprise shaft revolutions,

rudder angle, heel and ship's heading. The records taken ashore are

synch-ronized with the changes of heading of the ship by means of a visual signal from

the ship as it turns through certain angles.

One of the main objections to using a land base is that the results, when analysed, give a distorted path of the ship due to the effect of wind and tide. Also, owing to small delays in the ship to shore signals, drift angles may be

less accurate.

In the case of the ship-based trial the buoy, as well as the ship, drifts with wind

and tide and the results are usually very good. When using land bases, there-fore, it is the practice to place a buoy in the trial area, plot its movement during

the trial and so make a correction to the turning path. A typical arrangement of land bases is shown in Fig. 3.

When using a single elevated land base, the two recorders are replaced by

either a camera or a theodolite.This results in a considerable saving in both staff

and equipment deployed for the trial.

When a theodolite is used, the path of the ship is determined from the angles

of depression and horizontal bearing of the ship measured by the theodolite.

The height of the theodolite above the water surface is thus important and

allow-ance must be made in the analysis for changes in tide level. The path of the

freely-floating buoy can be plotted from sightings between runs and corrections made as before.

Using a camera, the method adopted is to take a multiple exposure on a

single plate or film. In some cases it is necessary to use several frames of films in succession and these must then be fitted together in the subsequent analysis,

using any convenient land marks which appear in the photographs to facilitate

the process. The technique of analysing the photographs is precisely similar to,

and in fact derived from, that developed many years ago at A.E.W. for model

turning experiments.

Knowing the height of the camera lens above the datum plane, which is the

plane through the sighting point chosen in the ship, the depression of the camera

line of sight, which is fixed, and the focal length of the camera lens, a " perspective " grid is constructed of transparent material, taking the form shown in Fig. 7. Applying this grid to the photograph the coordinates of the sighting point with respect to the camera position can be read off directly and the path of the ship plotted. If it is desired to determine the drift angle, two

sighting points are used and plotted in the same manner to give the ship's heading

relative to her path.

Spiral Manwuvres

It is not possible within the space allocated for this paper to treat the subject

of directional instability adequately or even to summarize the extremely interest-ing original work of Dieudonne, which has inspired modern investigations into the subject. The reader can only be referred to reference 2 which contains the

test reports issued by the Institution of Naval Construction, Paris, and an

attempt will be made only to emphasize the physical or practical aspects which

confront the navigators and helmsmen of ships that possess the quality of

directional instability.

A directionally unstable ship responds "capriciously" to application of rudder

if the angle of rudder applied is less than a certain value. According to the amount of rudder already applied and hence the direction of the turn, to port or starboard, the extra rudder may cause the ship to increase or decrease her

-TURNING AND MANCEUVRING TRIALS s83 rate of turning, or rate of change of heading. The actual rate of turn depends,

for a given rudder angle, upon the initial conditions before the change of rudder

is applied. For instance, suppose a directionally unstable ship, momentarily on a straight course with zero rudder, turns to starboard when, say, 4 degrees of starboard rudder is applied, but turns to port if only 3 degrees of starboard rudder is applied this implies that the critical angle of rudder for positive

control is more than 3 degrees. Consequently, the ship is difficult, if not

impossible, to maintain on a steady course if rudder movements less than this

critical angle are used. Not only are rudder movements continuous and

numerous, but the physical and mental strain imposed on the helmsman is severe and the steering system undergoes continuous wear and tear.

The phenomenon is readily recognized by carrying out a spiral manceuvre. Starting with a steady rudder angle, say 15 degrees starboard, this is held until

the rate of change of heading becomes steady when the rudder is reduced to, say, 10 degrees starboard which is again held until a steady rate of change of heading

is reachedand so on, reducing the rudder angle in equal decrements passing through zero to 15 degrees port and returning to 15 degrees starboard. The

r.p.m. of the propeller(s) are maintained throughout the trial.

A plotting of the steady rates of change of heading against rudder angle

exhibits a loop ", as shown in Fig. 4(a), within which the unstable conditions apply. Considering this diagrammatic figure, suppose the ship is turning to starboard steadily, with a rudder angle OP and steady rate of turn OL, and that

the rudder angle is then reduced to 0 Q, the steady rate of turn, or rate of change of heading, changes to OM, and so on, reducing the rudder angle in steps when a steady state is reached, following the arrows, through zero rudder, into a port

turn and back into a starboard turn again, until the rudder angle, OQ to

star-board, is reached again. At this stage, when the spiral is being performed with

increments, instead of decrements, of rudder, the steady turning rate is OM,'

less than OM. If now the rudder is increased to OP the steady rate of turn becomes OL, as before. In fact, beyond points R and S the ship is effectively stable and

only one rate of change of heading applies, whatever the rudder angle and regardless of which way the ship is swinging initially. If rudder angles less

than OT to starboard and OUto port are used, the ship will respond capriciously, changing her heading to either side indiscriminately and making course keeping very difficult if not impracticable.

In carrying out a spiral manceuvre, the only information required is the

initial approach speed, which can be deduced from records of shaft revolutions

the rudder angle and the rate of change of heading with time, which can be obtained either by direct observation of the compass or, more precisely, by

photographing a compass and stop-watch. The principle of the analysis of the results is inherent in previous remarks.

Thus the character of the directional stability of a ship or a model is easily determined and if a loop in the curve of steady turning rate is found corrective measures can be taken to provide positive stability or at least to reduce the

degree of instability. As the general form of the ship, being designed to meet many requirements, cannot be altered to any large degree, recourse must be

made to increasing the lateral surface area aft by introducing a skeg, or skegs, or by increasing the rudder area. The latter case usually results in an improvement

in the response of the ship to rudder, i.e. in controllability, as well as reducing

instability.

Zig-Zag Manauvres

As previously noted, not only directional stability but also response to rudder movements affects the course-keeping qualities of the ship. Response to rudder is itself a function of several parameters, namely, the personal reflexes or reactions of the helmsman, the time which the steering system takes to bring the rudder to

TURNING AND MANCEUVRING TRIALS

it is moving over and thereafter until she reaches a steady rate of turning. The

same parameters are involved when the ship is being brought back from a

steady turn.

Zig-zag mantruvres are conducted to provide a measure of the ship's response to rudder movements. Starting with the ship on a straight course, the rudder is ordered to, say, 20 degrees starboard, held until the ship's head is, say, 20 degrees to starboard of the original heading, then reversed to 20 degrees port and held until the ship's head is 20 degrees to port of the original heading.

It is found in such a manceuvre that the maximum change of heading of the

ship would be, say, 30 degrees, i.e. the ship is said to" overshoot "by 10 degrees, which in itself is quite significant. The times between successive rudder orders

and when the ship reaches her maximum change of heading made up of the components noted above, are also significant. Provided that the ship is

directionally stable, the smaller the overshoot angle and the shorter this time interval, then the better is the response of the ship, or 'controllability ".

The manceuvre can be repeated for a number of speeds and for different rudder angles and changes of heading. The records taken are similar to those for the

spiral manceuvre and are usually plotted in the form shown in Fig. 4(b).

Here again ways to improve response are fairly obvious but not so easy to apply. A more powerful steering gear should reduce the time of rudder

move-ment and permit a larger rudder to reduce sluggishness of the ship, but space to

accommodate it, and expense, are not always available. Similarly, the intro-duction of automatic steering control may improve upon the performance of a

human helmsmen, but introduces a maintenance as well as a space problem. Acceleration-Deceleration Trials

In this case the technique used is very similar to that for turning trials, except that the ship runs on a straight course past a freely-floating buoy. It is important

accurately to record the instant at which the engines start or stop and shaft

revolution records are taken. As it is difficult to judge precisely when the ship is either at steady speed or at rest, recording is carried on for sufficiently long to enable these times to be determined from the analysis. It is important to keep

a careful record of the ship's heading as at low or zero speed she may pivot giving a change in the apparent bearings of the buoy.

Tidal effects can be more influential in these trials and, for this reason, ship-based instrumentation and a freely-floating buoy are preferred to land bases. In

the latter case correction for the tide is essential for accuracy of results. Typically, the acceleration and deceleration runs vary between six and ten ship lengths for

warships. For acceleration runs greater consistency of results is generally

obtained if the times taken to reach, say, 95 per cent of the speed corresponding to the ordered shaft revolutions are quoted. This is because of the difficulty of determining precisely when the ship has reached her full speed.

The trials are repeated for a range of initial and final shaft revolutions.

Manteuvring in Confined Waters

Ability to manceuvre in confined waters is not a general requirement of H.M.

ships and consequently only a small amount of experimental work, limited to

special requirements in specific cases, has been carried out at Haslar, the results

of which are not of immediate interest nor applicable to merchant ships whose

service can demand such ability. However, a few general notes would not be

out of place.

The need for ability to manceuvre in shallow water at slow speed when coming

to or leaving a berth, and to negotiate safely artificial or natural channels has

become somewhat acute with the rapid increase in the size of ships, particularly tankers. Different circumstances apply to different types of ship and their

From Dieudonne's investigations it seems clear that the directional stability,

undergoes a distinct change when a ship passes from deep to shallow water ; indeed the effect may be to change the condition from one of instability to

stability. At the same time, however, response to rudder movements, or

controllability may be much reduced.

This effect is presumably still present when negotiating a canal but may be far less important than the interaction effects due to the sides or banks of the canal, which can greatly influence control. Rudder movements which in open water cause the ship to react normally have precisely the opposite effect if the canal is

sufficiently narrow in relation to the size and form of the ship. Whereas this

can be accounted for in a directionally stable ship by good nagivation, it must introduce a note of danger and hence the need for highly experienced pilotage in the case of an unstable one.

Manceuvring through narrow harbour entrances where complex tidal condi-dons exist adds to the difficulties of the situation as demonstrated by the case

of H.M.S. Nelson, described by Gawn.3

There can be little question of the value of model experiments, on as large a

scale as practicable to eliminate large-scale effects, during the early stages of a new design intended for operation under any such conditions as these. It would be interesting to learn from contributors to the discussion on this paper whether any such model experiments have been conducted and correlated, and with what

degree of success, to full-scale tests.

Model-Ship Correlation

In the case of turning trials, the results obtained with self-propelled models at

Haslar correlate fairly consistently with those of the full-scale ship. A typical comparison between a large and small model and the ship is shown in Fig. 5. The curves are typical of the correlation obtained although there is some variation with speed and rudder angle. The models were run in a condition corresponding to that of the ship on trial except that no attempt was made to reproduce the longitudinal moment of inertia of the ship in the models. This

should not affect the steady turning conditions but could lead to small dis-crepancies during the initial part of the turn.

For experiments aimed at determining the character of the directional stability it is, of course, highly important to arrange for the inertia coefficients to

corres-pond to those of the ship. A technique for conducting spiral and zig-zag manceuvres with models is at present being developed at A.E.W. and when complete it is hoped to obtain as good correlation between model and ship as

with steady turning tests. The new manceuvring tank will be a valuable asset

in this work since it will allow the most part of a spiral manceuvre to be

completed. It may be found necessary to do part of the spiral, at very small rudder angles, separately in one of the long ship tanks, i.e. apply the same

technique which it is understood is now being applied, for all rudder angles, by

some other ship tanks.

The alternative approach to the general problem of directional stability is to determine the hydrodynamic coefficients to serve into the equations of motion and to do this it is necessary to run a model of the vessel on a curved path by means of a rotating arm and measure the forces and moments imposed on it at different speeds, attitudes, and settings of the control surfaces. Correlation with the results of full-scale tests, made comparatively easy by the use of

com-puters, should be as reliable, if not more so, than that obtained with previous methods which were necessarily of an extempore nature in the absence of rotating-arm facilities. Up to the present the number of these desirable facilities has been small but is now on the increase as illustrated by Wright.4 This approach is also currently receiving attention at Haslar now that the rotating arm has been brought into use.

1,4414

TURNING AND MANCEUVRENG TRIALS s85

Conclusion

Broadly speaking, in the case of ships which operate mostly in open water the requirement is to obtain an acceptable compromise between directional stability

and controllability. In effect, the problem is to achieve "just positive" directional stability and so obtain the maximum controllability, or response to rudder, within the practical limitations imposed by way of space and weight

available for the steering gear and its control systems.

On the other hand, designs of ships intended for self-operation in confined waters require special consideration, each case on its merits according to type

and size.

Brief as these notes are, it is considered that they are adequate to indicate the

value and importance of conducting model experiments and ship trials to

produce data upon which the manceuvring qualities of new designs can be based with confidence.

The importance of the subject is reflected in the number of new facilities being

brought into use to study it, by the forthcoming Symposium, in May 1960, on Manceuvring of Ships sponsored by the S.N.A.M.E. and by the proposal to

establish a separate international committee to deal with it, so far as test technique

is concerned, at the 9th International Towing Tank Conference in September

1960.

The author is hopeful that contribution to the discussion on this brief note on the subject will provide at least an outline of the progress with research in the field by other ship tank authorities.

Acknowledgments

The author is indebted to the Director of Naval Construction, Mr. J. H. B. Chapman, C.B., R.C.N.C., for permission to use the information presented in this paper and is grateful for the assistance rendered in its preparation by Mr. E. C. Tupper, B.Sc., R.C.N.C., Mr. R. H. Torrington, and other members of the staff concerned at the Admiralty Experiment Works, Haslar.

TURNING AND MANCEUVRING TRIALS s87

REFERENCES

DAVIDSON, K. S. M. and SCHIFF, L. I., "Turning and Course-Keeping Qualities," S.N.A.M.E. 1946, 53.

DIEUDONNE, J., "Collected French Papers on the Stability of Route of Ships at Sea," Institute of Research in Naval Construction, Paris, 1949.

GAWK, R. W. L.," Steering and Propulsion of H.M.S. Nelson in a Restricted Channel," I.N.A. 1950.

WRIGHT, E. A., "Some International Aspects of Ship Model Research,"

S.N.A.M.E., February 1958. 4.

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TURNING AND MANCEUVRING TRIALS

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