Seagoing qualities and manoeuvring of ships

LABORATORIUM VOOR

SCHEEPSBOUWKUNDE

TECHNISCHE HOGESCHOOL DELFT

SEAGOING QUALITIES AND MANOEUVRING

OF SHIPS.

by

Prof. Ir. J. Gerritsma.

Course "Koninklijk Instituut van Ingenieurs"

L

n

SEAGOING QUALITIES AND MANOEUVRING OF SHIPS.

by

Prof.Ir. J. Gerritsma.

Course "Koninklijic Instituut van Ingenieurs", May 1969.

The rapid evolution of naval architecture which takes place at present, has caused a situation in which a number of technical problems cannot always be solved optimally by extrapolation from earlier obtained empirical knowledge.

Unti], recently, the naval architect could help himself very well with the empirical methods because characteristic quantities of ships, like displace-ment and speed changed only very gradually. That is why in general there was little need for a thorough investigation of the hydrodynamic and

structural strength problems of the ship. A good example of this situation is the treatment of the steering and manoeuvring properties of a ship in the design-stage. Demands regarding these properties are not specified for a proposed ship. At most, certain model tests and manoeuvring tests with the

ship are required, but in most cases no explicit quantities are stated by the customer. The classification companies demand a certain minimum angular

speed of the rudder; next only specifications are given about the rudder stock diameter the steering engine and possibly some other parts of the steering year.

Indeed the conditions have been simple with surface ships: complaints about steering and manoeuvring properties did not occur often and they usually could be overcome easily. A small amount of course instability has hardly been a difficulty.

The lack of good specifications regarding the steering characteristics of ships has ultimately been the cause of the slow evolution of the theoretical and experimental research in this field.

A first attempt to define specifications based on statistical data, has been presented by Gertler at the first Symposium on Manoeuvring in Washington D.C. in 1960

fi)

Society of Naval Architects and Marine Engineers which has published a proposalL2).

-2-2

The empiricaicharacterof this attempt is reason to dissuade extrapolation

to larger ship sizes than has been used for the statistical analysis.For

very large ships, the recommendations by the S.N.A.M.E. are useless. A

comparable situation occurred with the development of the nuclear sub-marines: The required speed was at least double that which has been used previously. The manoeuvring properties at this high speed could not be derived from earlier experience. For safety reasons it was evidend that a

conscientious investigation was needed; the ships move at high speed in a layer of water of which the thickness is hardly more than twice or thrice their length. Inadequate manoeuvring properties may cause rapid exceeding of the safe depth, with the possible result of disaster.

Theoretical and experimental research soon resulted in the development of methods with which the steering properties can be estimated in detail. An extremely important aspect of this development is the understanding of the mechanism of manoeuvring and steering.

It roved to be possible to give pertinent specifications in digital form. Steering and manoeuvring properties were simulated by means of analogue computers and the results of the model experiments. "Crash stops" which are so important as regards submarine$ could be simulated and it is possible to exercise ships crews with such "link trainers". Also it is

possible to optimize auto pilot tuning beforehand in order to save the time consuming adjusting at sea. The result is good, presuming that the hydrodynamic qualities of the ship are known from model tests.

In the model investigation needed for this procedure, oscillation techniques are used which have proved their usefulness earlier in the study of ship motions in waves. The fundamental character of these tests and the

application of modern computer techniques have made this development possible. Later, the oscillator technique will be described in some more detail.

The very large size of tankers such as there are sailing today, their small length to breadth ratio and their large fullness have led to remarkable changes in the steering characteristics.

A very readable and skillful article on the subject has been published by Professor Nonio6o who is a well known author in this field (3].

This kind of ships has a relatively small yaw damping . Supertankers there-fore have relativeLy small turning circles but the transitory effects

between two stationary conditions can take very much time. Such ¿hips react

very lazily at the helm and they have a very large overshoot especially

at low speeds.

-3-The natural course stability is nearly always negative. -3-The steering properties of supertankers therefore require great care while manoeuvring

in narrow channels. In this respect Nomoto discusses the size of the rudder area which may be small from the point of view of turning properties. The large rudder areas such as are used at present must take care of the course stability to some extent, but it seems to be more logical to try to achieve this with vertical fins and to choose the rudder area in relation to the desired turning capacity. The application of stern shapes which

reduce the local effective lateral plane must be strongly discouraged. The negative course stability and an insufficient compensation by the auto pilot can cause in such large ships, a relatively badly damped yawing motion when sailirg at a straight course. This produces an additional resistance because the centrifugal force has a resistance component. Nomoto in this case mentions a possible resistance increase of from 5 through 10 or even 20 percent.

Little systematic research has been devoted to the manoeuvring qualities of large full ships. This is curious because of the vast issues which are at stake, also for the Netherlands. The analytic system and the experimental possibilities are now available just as the know-how which has been yielded by the fundamental research in this field.

Some other aspect draws the attention in connection with the entrance of harbours and the traffic in narrow waterways like the Channel. Large ships have large time constants due to their size and mass. This requires early decisions from the ships staff. The adaptation of man as a pilot of objects

dth large time constants has been an interesting problem especially in

the early days of supertankers. Nearly immediately the question arose how the crew could be trained to steer the ships. The exercise with sailing scale models as it occurs abroad, makes little sense due to the time scale; the issue is the adaptation of the human reaction in relation to the

slowly reacting ship. This does not mean that it is senseless to test models to study definite manoeuvres. Such tests may be valuable to distinguish possible manoeuvres from those which may endanger the ship. It is clear that apart from the user,, above all the harbours designer is interested

in this kind of experiments. o

The training of ships pilots appears to be well possible with the aid of a steering simulator analogous to what becomes more and more common with

analogue computer with the use of hydrodynamic properties of the ship

which have been measured on model scale, An additional complication is the substantial influence of shallow water depth on the hydrodynaic properties. The increasing size of the ships causes that ever more water-ways must be considered shallow.

A different subject in which the dynamic behaviour plays a role is the

motion of a ship or of a floating drilling platform in waves. In many

cases it has proved to be possible to describe the movements with a linear system of eq,uations of motion of which the coefficients are dependent of frequency. In some cases these coefficients can be calculated as for heave and pitch of a ship but in other cases they can be measured in oscillation tests.

The principle of these tests is as follows.

The ship or the floating body is being subjected to a forced harmonic

excitation with a known amplitude and frequency. Measurement of the amplitude and phase of the first harmonic of. the force which is required to support the continuous motion, gives sufficient information to derive the hydrody-namic inertia and damping of the oscillating system in water. With these data the coefficients of the equations of motion are known.

The wave loads can also be measured, experimentally by force measurement on the fixed model in simple harmonic progressing waves. Model tests have

shown that in many cases a practical description of the dynamic behaviour is possible by assuming linearity. Also it has been confirmed experimentally that superposition of results is allowed; this means that the results of

experiments in simple waves can be used to determine the behaviour in

confused seas. This holds also for rather complicated movements in which coupling effects play a role, like pitch and heave of a ship or like roll, sway and yaw. This last group of motions has become important in recent times in relation with the shipment of goods in containers. This has led to the application of roll-damping devices of the passive as well as of the active type. As regards both types it must be stressed that in order to get best results, fundamental knowledge of the roll characteristics is important.

Also in relation with the safety of small vessels like fishing cutters, a

better understanding of the stability in waves is required. The present

regulations have been based partly on the static stability curves in still

water because of a lack of 3nowledge of the roll characteristics in waves.

-5-5

The problems which occur as regards behaviour and the dynamic positioning

of oil drilling floats in ocean waves can be handled in much the same way.

By oscillation tests with a model the coefficients of the equations of motion can be determined and the exciting forces can be measured on the fixed

model in waves. Drilling platforms are designed with very large natural periods in order to prevent the occurrence of resonance phenomena. The

experimental determination of the hydrodynamic quantities at low frequencies makes high demands upon the stability and the accuracy of the measurement

apparatus. The experimental information can be used for the optimal design

of a dynamic positioning control, for instance by means of an analogue computer. Aleo the influence of different anchoring systems can be

tracked.

Some aspects of the three subjects which have been presented, namely the

steering, manoeuvring and. motions in waves of ships and drilling floats, will

be treated in more detail in the three following lectures.

Literature.

(i) M. GERTLER and S,C, GOVER.

Handling Quality Criteria for Surface Ships.

First Symposium on Ship Manoeuvrability. DTMB report ilL61

1960.

Notes on Ship Controllability.

Technical Research Bulletin no. 1-27,

Society of Naval Architects and Marine Engineers.

K. NOMOTO.

A study oi Steering Qualities of Large Oil Tankers of Present Day.

Seakeeping and Manoevring Qualities

PROF. IR. J. GERRITSMA

Intröduction

The rapid evoii.tion of naval architecture which, takes place at

preseit, has caused a situation in which a number of.techniai problems cannot always be solved optimally by extrapolation from

earlier obtained 'empirical knowledge.

Until recently, the naval architect ould, help himself very well with the empirical methods because characteristic quantities of

ships, like displacement and speed changed only very gradually. That is why iñ general there vas little need for a thorough

investigation of the hydrod.yna.mic and structural strength problems

of the ship. A good examie of this situation is the treatment of the steering and manoeuvring properties of a ship in the design

stage. Demands regarding these properties are not specified for a

proposed ship. At most,, certain model tests and manoeuvring tests with the ship are' required, hut' in most cases no explicit

quantities are state4 by the customer. The classification companies demand a certain minimum angular speed of the rudder; next only specifications are given about the rudder stock diameter the steering engine and possibly some other parts of the steering

gear.

Indeed the conditions have been simple with surface ships: complaints about steering and manoeuvring properties did not occur often and they usually could be overcome easily. A small

¡V Seakeeping and Manoevring Qualities 254

a2nount of course instability has hardly been a difficulty. The

lack of good specifications regarding the steering

characteris-tics o ships has ultimately been the cause of the slow evolution of the theoretical and experimental research in this field.

A first attempt to define specifications based on statistical data, has been presented by Gertler at the first Symposium on Manoeuvring in Washington D.C. in 1960 (i). Most of Gertiers recommendations have been adopted by the Society of Naval

Architects and Marine Engineers which has published a proposal (2). The empirical character of this attempt is reason to dissuade

extrapolation to Ïarger ship sizes than has been sed for the

statistical analysis. For very large ships, the recommendations by the S.N.A.M.E. are useless. A comparable situation occurred with the development öf the nuclear submarines: The required

speed was at least double. that which has been used previously.

The manoeuvring properties at this high speed could not be derived from earlier experience. For safety reasons it was

evidend. that. a conscientious investigation was needed,; the ships

move at high speed in a layer of water of vhich.the thickness is hardly more.than twice or thrice their length. Inadequate

manoeuvring properties may cause rapid exceèding of the safe depth,

with the. possible result of disaster..

Theoretical and experimental research soon resulted in the development of methods with which the steering properties can be

estimated in detail. An extremely important aspect of. this

development is the understanding of the mechanism.of manoeuvring

and steering. .

It proved to be possible to give pertinent specifications in digital form. Steering and manoeuvring properties were simulated

255 PROF. IR. J. GERRITSMA

by means of analogue computers and the results of the model

experiments.. "Crash stops" which are so important. as regards

submarines, could be simulated and it is possible to exercise ships

crews with such "link trainers". Also it is pos sible to optimize

auto pilot tuning beforehand in order to save the time consuming adjusting at sea. The result is gôod, presuming that the

hydrody-namic qualities of the ship are known from model tests..

In the model investigation needed for this procedure, oscillation

). . techniques are used which have proved their usefulness earlier in

the study of ship motions in waves. T1e. fundamental character of

these tests and the application of moderncomputer techñiques have made this deielopment possible. Later, the oscillator technique

will be desc.ribed in some more detail.

The very large size of tankers such as there are sailing today, their small length to breadth ratio and their large fullness have led to remarkable changes in thé steering characterisitcs.

A very readable and. skillful article on the subject has been published by Professor Nomoto, who is a well known .author in this field (3) .

h, This kind öf ships has a relatively small yaw damping. Super-tankers therefore have relatively small turning circles but the transitory effects between two stationary conditions can take very much time. Such ships react very lazily at the helm and they have

a. very large overshoot especially at low speeds. .

The natural course stability is nearly always négative.. .The

steering properties of supertankers therefore require great care while manoeuvring in narrow channels. in this respect Nomoto

dis-cusses the size of the rudder area which may be small from the point of view of turning properties. The large nidder areas such

IV _{Seakeeping and Manoevring Qualities 256}

some extent, but it seems to be more, logical to try to achieve

this with vertical fina and to choose the rudder area in relation to the desired turning capacity. The applicatioii of stern shapes

which reduce the local effective lateral plane must be strongly

discouraged. The négative course stability and _{an insufficient} compensation by the auto pilot can cause in such large ships, a relatively badly damped yawing motion when' sailing at a straight course.. This produces .an additional resistance because the

centrifugal force has a resistance component. Nomotoin this _case mentions a possible resistance increase of from 5 through 10 or even 20 percent.

Little systematic research has been devoted to the rnanoeuvring

qualities of large full ships. This is curious because of the.

vast issues which are at stake, also for the Netherlands. The analytic system and the éxperimental possibilities _{are now}

available just as the. know-how which has 'been yielded by the fuidamental research in this' field. .

Some other aspect draws the attention in connection with t'he

entrance of harbours and the traffic in narrow waterways like the. Channel. Large ships have large time constants due to their size and mass. This requires early decisions from the ship staff. The adaptation o.f man as a pilot of objects with large time' constants

has been an interesting problem especially in the early days of

supertankers. Nearly immediately the. question arose how the crew could be trained' t'o steer the ships. The exercise with sailing scale models as' it occurs abroad, makes. little sense due to the time scale; the is'sue is the adaptation 'of the human reaction in relation to the slowly reacting ship. This does not mean tha't it

is senseless to test models to study definite manoeuvres. Such tests may be valuable to distinguish possible manoeuvres from

257 PROF. IR. J. GERRITSMA

those which may endanger the ship. It is clear that apart from the user, above all the harbours designer is interested in this kind

of experiments.

The training of ships pilots appears to be well possible with the aid of a steering simulator analogous to what becomes more and.

more common with respect to submarines., The time constants can be

adjusted by means of an analogue computer with the use of

hydrodynamic properties.of the ship which have been measured on model scale. An additional complication is the sustantial

inflüence of shallow water depth on the hydrodynamic properties. The increasing size of the ships causes that ever more waterways must be considered shallow.

A different subject in which the dynamic behaviour plays a role is the motion of a ship or of a floating drilling platform in waves. In many cases it has proved to be possible to describe the movements with a linear system of equations of motion of which the coefficients are dependent of frequency. In some cases

these coefficients can be calculated as for heave and pitch of a

ship, but in other cases they can .be measured in oscillation tests.

The principle of these tests is as follows.

The ship or the floating body is being subjected to a forced harmonic excitation with a known amplitude and frequency.

Measurement of the amplitude and phase of the first harmonic of

the force whiçh is required to support the. continuous motion,

gives sufficiént information to derive the hydrodynamic inertia and damping of the oscillating system in water. With these data the coefficients of the equations of motion are known.

The wave loads can also be measured experimentally by force measurement on the fixed model in.simpie harmonic progressing

waves. Model tests have shown that in many cases _{a practical} description on the dynamic behaviour is possible by assuming

linearity. Also it has been confirmed experimentally that

super-position of results is allowed; this means that the results of experiments in simple waves, can be used to determine the

behaviour in confused seas. This holds also for rather complicated

movements in which coupling effects play a rolé, like pitch and

heave of a ship or like roll, sway and yaw. This last group of motions has become important in recent times in _{relation with the}

shipment of goods in containers. This has led to the application

of roll-damping devices of the passive as well as of the active

type. As regards both types it must be stressed that in order to

get best results, fundamental knowledge of the roll charateristics

is important.

Also in relation with 'the. safety of small vessels like fishing

cutters, a better understanding of the stability in waves is reqùired. The present regulations. have been based partly on the

static stability curves in still water because of a lack of knowledge of the roll characteristics in waves.

The problems which occur as regards behaviour and the dynamic positioning of oil drilling floats in _{ocean waves can ce handled} in much the same way. By oscillation tests with _{a model the}

coefficients of the equations of motion can be determined and the exciting forces can be measured on the fixed model in waves.

Drilling platforms are designed ith very large natural periods

in order to prevent the occurrence of' _{resonance phenomena. The} 'experimental determination of the hydrodynamic quantities at low

frequencies makes high demands upon the stability and the _accuracy of the meásurement apparatüs. The experimental information can be used 'for the. optimal design' of a dynamic positioning control,, for instance by means of' an analogùe computer. Also the influence IV _{Seakeeping and Manoevring Qualities}

259. PROE. IR. i. GER'RI1SMA

of different anchoring systems can be tracked.

Some aspect's of the three subjects which. have been presented,.

namely the, steering, manoeuvring and motions in waves of shipa

and drilling floats, will be treated in more 'detail i the three

following lectures,.

Literature.

(i) M. GERTLER and S.C. GOVER.

"Handling uaiity Criteria for Surface Ships".

First Symposium on Ship Manoeuvrability. DTMB report i16i;

19.60.

( "Notes on 'Ship Controllability".

Technical. Research Bulletin no. l-27.

Society of Naval Architeçts and Marine Engineers.

(3) K. NOMOTO.

"A study on Steering Qualit:ies of Large Oil Tankers' of

Present Day".