Some unsolved problems in ship hydrodynamics

DI011ARSI IliSillill

S H IP H U ¡ I O I H H E SE A REH i H Sill UT E

lA

R EH

Technische1

19oI

Deth

Some Unsolved Problems ¡n

Ship HydrodyNamÌcs

e

by

Dr. F. H. Todd

Superintendent, Ship Division, National Physical Laboratory, Teddington, England

Paper to be presented at the Symposium on the

Towing Tank Facilities, Instrumentation and

Measuring Technique

Zagreb 22-25 September 1959.

by

Dr. F. H. Todd

We are gathered.together in Zagreb this week to celebrate the opening

of another major facility for ship hydrodynamic research. _{My first}

words must therefore be to congratulate our Jugoslav colleagues on the

wonderful new laboratory which they have built here. On behalf of the

staff of Ship Division I would alzo like to offer them our very best

wishes for success in the work which lies ahead for them. _{That there}

is no lack of subjects for research can be well illustrated by the amazing

growth which bas occurred in the number of towing tanks, cavitation tunnels

and aziliary facilities throughout the world during the years since the

war.

We in England regard William Froude as the pioneer worker in our

country in the field of ship model research, and it is therefore with the

greatest of pleasure that we find his naine not unknown in your country

also, ahi indeed honoured by the naming of the approach road. to this new

laboratory as Froude Road.

When I was invited to give a paper on this auspicious occasion and

began to look around for a subject, it seemed that a review of some of the

problems

which

are still unsolved in our field might be timely. Despite the advances made in recent years, naval architecture still remains a blend

of art and science, The designer who has to meet so many conflicting demands

must always, I believe, remain something of the artist if the ship is to be

both technically and aesthetically satisfying. The scientist engaged on

bydrodynamic research must endeavour to.provide the designer with the basic

-1-data he requires and. ensure that he is kept up to date in all those matters

which have reached the stage of' practical application. In some cases this

may mean many years of basic research before such a stage is reached, and

this is a period during which the designer must give his encouragement and

help to the scientist to ensure that he is given adecjuate financial and moral

support. Only by such cooperation will the unsolved problems of today be

translated into the accepted practice of tomorrow. Fortunately we are

living today at a time when such cooperation is being fostered all over the

world by research associations and such bodies as the International Towing

Tank Conferemce, in which research workers, designers and ship owners can

discuss the needs of industry and the potentiality of the research facilities

to answer them.

We can look forward, therefore, to an increasing rate of progress in

research and to the solving of many current problems. However, this does

not mean that we can afford to relax our efforts. Rather the reverse, for

technological progress is moving so fast today that the solution of one

problem seems only to reveal two or three more which call for still further

effort.

The research facilities are being augmented everywhere - what we must

ensüre is that there is a corresponding increase in the number of first

o]Äss people willing to engage in our field of research. Otherwise the

finest tanks and tunnels will not produce the answers which our industries

demand, for the quality of the staff is the mainspring of amy research

establishment.

In considering the problems facing us today, it will be convenient to

divide them into those concerned with resistance, propulsion, cavitation and

other fields, although there is of course a great number of them which fall

into more than one of these categories, and, no definite boundaries can be drawn between them.

Ship resistance

The estimation of the resistance of a ship,arxi therefore the power

which must be installed, is one of the basic problems in design, affecting'

as it does the weight of machinery, ani so the displacement, the space

necessary for ita installati'on axi. for the stowage of the required fuel.

Today such estimates are usually based upon the results of model experiments, either specially made for the design in question or obtained from methodical

series tests carried out as general research. _{The prediction of the ship}

power from the measurement of model resistance is thus one of the primary

problems in our subject, and a very difficult one. Ship resistance is made

up of mrr components, such as frictional, wave-making, form and eddy

resistance, and the fact that these do not conform to a single physical law

means that the "scale" effect between model and ship is different for different

components. _{It is impossible therefore to scale the model resistance to the}

ship by a single process, and the model resiatanoe has to be broken down into

components which have to be re-united at the ship scale to give the predicted.

power. _{Herein lies a formidable number of our unsolved problems today. The}

different resistance components interfere with one another - the wave

formation changes with speed and.with it the wetted surface and so also the

frictional resistance, whilst the latter, through the formation of the

frictions,], wake, virtually modifies the hull form and so the wave resistance.

In the past model experimenters have followed Froude in dividing model

resistance into two components - a frictional resistance equal to that of

an equivalent "plank" and the remainder, called by Froude "residuary"

resistance. The latter is assumed to be proportional to displacement at

corresponding speeds (equal values of, ) in accordance with Froude's law.

For the extrapolation of the frictional component from model to ship, two

methods have been commonly used - the Froude frictional coefficients and

'b

a curve of frictional coefficients varying with Reynolds number. There

have been a number of these latter curves, perhaps the beat known being

the American Towing Tank Conference (A. T. T. C.) I 94.7 line, originally due

to Schoenherr. When used to predict ship resistance, this line gave

values lower than Fronde, and it is usual to add. an arbitrary "roughness"

allowance tC = + O.000L,, which results in ship powers very close to those

given by the use of the Froude coefficients.

Over the years there has been a growing realisation of the inadequacies

of these uiethod8 of extrapolation. The experiments upon which the Froude

and A.T.T.C. frictional coefficients were based were made with planks of

different aspect ratios and in some cases serious surface effects - they

are therefore in

no way

representative of the resistance of a smooth plane of infinite aspect ratio in 2 dimensional turbulent flow. The frictional

resistance of the model cannot be expected to be equal to that of the

"equivalent" plank, for it has both longitudinal and transverse curvature,

both of which will increase the model resistance above that of the plank

-an increase often referred to as "form effect". The wave-making resistance

- the major component of Froudes "residuary" resistance

at high speeds

-varies basically as V6 , and. therefore may be expected to be vanishingly

small at low values of V/'JL. On the other hand, the residuary resistance as measured at these low values of, is certainly not zero, but amounts

to some 12% of the "plank" resistance for very fine models and up to

40 or 50% for full ones. This must be made up partly of additional skin

friotion drag due to the curvature effects mentioned above, to the effects

of viscosity on the pressure distribution around the hull and in full

forms to eddymaking. Each of these componeit s is of a different basic

character, and will scale either with Reynolds or Froude number, but as we

cannot separate them at present, this is not of great help in solving the

problem.

The limiting alternatives are to scale the whole of this form

resistance with Froude number, classing it with wave-4naking resiatanoe, as

is done in using the Froude or A.T.T.C. coeffioients, or with Reynolds number,

including it in the frictional resistance, as is proposed by Hughes.

Here then is the first and greatest of our unsolved problems - how to

correctly estimate ship resistance from that of a model.

We know that

there are unexplained differences when we attempt to predict the resistance

of a large model from the resulta of experiments on a maaller one, and this

problem is obviously such greater in the extrapalation from even a large

model to the ship, where the ratio of Reynolds numbers may be of the order

of 200 or 300.When using the Froude coefficients, it has been the practice

to use an allowance of 10% above the "mnooth" ship predicted power to allow

for full scale roughness and other effects.

Today with very long all-welded

ships, it is sometimes neoessaxy to use a ship predicted power some 10% to

15% below Froude in order to account for ship measured tria], results.

This

brings us to a further problem - the allowances necessary to reconcile the

actual ship powers with those predicted for a "smooth" ship direotly rrom

the model resulta.

 great deal of work has been done in this field by

obtaining first of ail full scale thrust or torque values on the ship and

then running the equivalent ndel tests and so obtaining the necessary

ship - model correlation faotor (frequently referred to by the symbol t

_{0p ).}

This is by no means a straightforward process.

Obtaining the ship

resistance from the meared thrust or torque is dependent upon some very

doubtful assuxnptions, as we shall see later, and also the final value of

itself depends upon the method of extrapolation used between model and

ship.

_{Obviously negative values of 0p , as sometimes obtained. using the}

Froud.e or A.T.T.C. coefficients are unrealistio, and. this fact together with

-5-an apparent need for a steeper correlation line to reconcile the results

of different scale modela led in 1957 to the adoption at Madrid of the

I.T.T.Q. model-ship correlation line as an interim measure0 Although the

use of this line in published work was agreed to at that time, it has not

yet been implemented because of the lack of knowledge of appropriate values

ofC .

Much research has since been done on this aspect of the probln, and some agreenent on suitable values may be reached at the next meeting

of the I.TOT.C. in Paria in 1960. _{The line is undoubtedly a compromise}

-a working engineering solution it h-as been c-alled - -and we still h-ave much

to learñ in this whole field..

Apart from the questions involved in the model-ship correlation problem,

there are also a number of others concerned with the measurement of model

resistance itself. Repeated tests on the same model

in

the saine tank

sometimes show unexplained differences _{These have been attributed to}

changes ini model surface, in the state of the water itself to currents in

the tank and so on. _{An attenpt to explain these differences, or at least}

to take account of then, by the running of a "Standard" mode]. has long been

practiced at the Admiralty Tank at Haslar. In 1958 four British Tanks

engaged

in

commercia], testing each obtained an identical model in glass-reinforced plastic, from the sane mould, which have been run at frequent

intervals to trace any changes in resistance with time. At present it is

too early to draw any conclusions from this work, but a number of other

European tanks have since obtained similar models for their own use. The

question of turbulence stimulation will also be raised by these tests.

Currents or tides in the tank water are another source of trouble, and

their measurement by instruments is a very difficult probln. At NPL this

has been tackled, with considerable success, by having "curtains" across the

tank at frequent intervals along its length. These are raised between

successive runs and 80 help to kill the water movement set up by the model. When comparisons are made between models run in different tanks, an

additional factor is involved in the ratio of size of model to size of tank,

the so-called blockage effect. Data on this can be obtained by geosm

tests, but we are not yet in the position of being able to make a definite

correction on this account.

Another unsolved and important problem in the resistance field is that

of scale effect on bossing resistance. For many years it was the practice

at N.P.L. to run the model naked and then with bossings (or open shafts and

brackets), drawing two separate© curves. The difference in© value due to

the appendages was then halved before applying it to the ship, this reduction

being based upon some early experiments by Froude. In the "Lucy Aahton"

experiments carried out by BSRA, in which the resistance was measured with

arid without bossings, this procedure seemed to be justified. _{However, it}

was realised that the bossings fitted to the "Lucy Ashton" were not in the

line of flow, and subsequent geosiin tests carried out at NFL on models with

"in-line" bossings suggested that rio such reduction was necessary. Since

that time N.P.L. practice has been to halve the increase in © value for

open shafts arid brackets, but to make no such reduction in the case of full

bossings. _{The two principal factors influencing the choice of these two}

alternative stern arrangements are their relative effects upon

propeller-excited hull vibration and their relative resistance. Since the additional

resistance of either type may amount to as much as 12 per cent on fine models,

the question of whether we halve one of these arid not the other in _{going to the}

ship may have a deciding influence on the choice - and, in our present state

of knowledge we may well make the wrong choice

A similar scale effect problem exists with other appendages also, such

as rudders, st&oili8ing fins, etc., which equa].iy needs methodical investigation.

A comparison between the prediction for a "smooth" model and the actual

tria], results gives an overall correlation factor

_*CF.

_{This is influenced}

by msny things, an important

_{one being the roughness of the ship s surface.}

It is usual to consider this

_{as being due to three causes - structural}

roughness, paint roughness and fouling.

_{Structural roughness is due to the}

method of building the ship, and

_{covers such items as rivet pointa, welds,}

plate seams and butts, waviness between

_{frames and, the initia], character of}

the plate surface. Corrosion of the latter will in time increase the effects

of structural roughness.

_{The type of paint used and the method of application}

may decrease the overaJ]. roughness by covering

_{up some of the lead structural}

characteristics, or increase it by having itself

_{a very rough surface. Lastly,}

in service the resistance is increased

_{by fouling due to weeds or marine}

animals.

_{This can be controlled to some extent by}

_{the use of special}

anti-fouling paints but also depends upon the trade routes involved and above all

on the proportion of time the ship spends at

_{sea -. the greater this is, the}

less the foling.

The control of structural roughness

_{and the development of}

paints is not the province of the

_{research worker in hydrodynamics, but he}

is vitally interested in their

effects upon ship power.

_{A great deal of work}

has been done by experiments

_{on planks and on ships such as the "Lucy Ashton"}

to evaluate the effects of different

_{its of roughness, but we still lack}

_a

reliable means of oorrelating the

_{geometry of the roughness with the adal}

tLon&l

resistance it causes and

_{are also not yet able to predict the effects}

_{of fu].].}

scale roughness from experiments on sinai]. ones.

In the effort to break down the

_{resistance of a nde]. into its}

_components

to obtain an insight into the mechanism of resistance and. so improve

our

correlation practice, such effort has been

_{expended on mathematical methods}

fo r calculating a mode].'

_{s wave-system and wave-making resi stance.}

This work.

has paid handsome dividends in

_{explaining how wave-making resistance}

_arises

and particularly in throwing light on the interference effects which give

rise to the humps and hollows in the () curve. However, its achievements

have been mainly qualitative and we are still not in a position to calculate

the actual wavemaking resistance of an actual ship fous. This is

due in part to the inherent difficulties of the mathematics, which liait

the application to "thin" ships of variishingly small beam, and. to the fact

that it has been virtually impossible to allow for the effects of viscosity

when dealing with a real fluid except on an empirical basis. Further

development of the mathematical theory may now be possible, because of the

advent of the highspeed computer, which has taken much of the labour out

of such work, and at N.P.L. an attempt is being made to apply relaxation

methods to this problem. Even ìf absolute answers cannot be obtained in

this way, guidance as to the relative merits of various possible hu].].

shapes would be of great help to the designer,

Work on viscous and pressure drags can also assist in the extrapolation

problem, and methods are now in existence for carrying out wake traverses

behind a model and pressure plotting over its surface and. in the surrounding

flow. These kinds of experiments are extremely tirne-consuming when run

on the ordinary towing tank, because of the long time involved in returning

down the tank and awaiting the settling of the waves, and. also because

with the large number of instruments involved it is often necessary to

repeat runs at given conditions a number ol times. All these experiments

can be carried out much more expeditiously in a circulating water channel,

where the model is at rest and. the water mo'ving, and it is believed that

such a facility is an essential piece of equipment in. any hydrodynamics

research laboratory, It also has many other uses - too many tc give here

in detail. Today when we mify a hull form we do so on the basis of

previous experience

and

a statistical knowledge about earlier models. If our change in shape results in a decrease in resi stance we only know

of this from the overall measurement of resistance and have no knowledge

of the cho.nge in flow around the hull which has been caused by the change

in shape and is the reason for the improvement. Equally if our modification

proves to be a bad one

and

increases the resistance, we still do not know wby A circulating water channel, by enabling us to see the flow and study

it at leisure, would in time add enormously to our knowledge of ship

resistance. Sudi visualisation of flow is also of the greatest use in

propeller design problems, including that of propeller-excited vibration,

since it enables ar peculiar flow conditions ahead of the propeller to be

detected before the stern design is finalised.

Much of the increase in knowledge in the field of ship resistance has

been due to systematic work on series of models in whioh the design features

have been varied methodically. There still remain a number of areas in

this field where further knowledge is needed, calling for further experiments

-these include ocean-going ships of greater fullness, such as modern

super-tankers, the proper use of a

bulbous bow,

the merits of transom sterna and the use of simplified

stre.ight-line

forms to reduce building costs.

To summarise, some of the principal unsolved problema in the field

of ship resistance

are:-a correct method for predicting ship resistare:-ance from model results,

including greater knowledge of the various components of resistanoe

and how they interact with one another.

further knowledge required -on wave-making resistance, pressure drag

ansi viscous drag, in order to assist the problem stated in a),

and

-the development of improved ma-thematical analytic methods.

o) a correct scaling method for resistance due to bosainga, A brackets,

stabiliser fins and. other appendages;

a) improvement of experiment techniques, _{by use of standard models to}

detect day to day variation in resistance, methods to determine or

eliminate currents in the tank, blockage effects, etc., turbulence

stimulation, etc.

extension of standari series work to certain other cla8ses of ships;

study of flow conditions around models to explain the effects of

various changea in hull form upon resistance and so improve our

ability to make the correct

_changes

in any given case; a codification of various types of hull roughness and their

corresponding effects upon resistance, including scaling laws

and h) greater knowledge of the correlation factors between the predicted

resistance of a "smooth" ship and the measured resistance of the

real ship.

Ship propulsion

Having obtained the resistance and EHP for a new design, the next

problem is to devise the most efficient propulsion system.

At the present time, the only practica]. means of developing the necessary

thrust is the marine propeller, althoug.be

_higher

_{speeds which nay} come in the future other devices will become important. _{In designing a}

propeller, we have to ensure that the propeller itself is an efficient

instrument for turning torque into thrust, and that it is properly matched

to the hull. _{The former is referred to as its "open water" performance,}

in which there is nothing ahead of it and the inflow is uniform over the

disc. _{For the selection of the correct diameter and pitch ratio, in any}

disc, after which propeller design charts may be used. These represent

the results of methodical series experiments carried out in many tanks,

and aÌ available to cover most normal merchant ship types.

In order to ensure proper matching between hull and propeller, a

detailed design study should be made, and for this purpose the wake

distribution over the propefler disc must be known. This can be found

by pitot tube or Vane wheel surveys, and the average wake determined

circumferentiall,y around successive radii. The pitch distribution along

the blade can then be chosen to give optimum performance at each radius,

although of course conditions during one revolution will change because

the circumferential wake can only be an average. The determination of

the wake pattern can be a rather long affair, although the analysis

of the records can now be considerably speeded-up by the use of a

high-speed computer. At the present time, it is not uncommon to use constant

pitch propellers in twin screw ships, where the wake variation is small,

and propellers with a constant pitch over the outer half and reduced

towards the root in single screw ships. However, for all propellers of

the heavily-loaded type, where cavitation may occur, a wake survey should

be made and a detailed analysis carried out. For other ships, it would

be of great help if tF wake distribution could be approximated to from

wake distribution experiments made on standard series models such as

those run by the British Shipbuilding Research Association at N.P.L. or

Series 60 at Taylor Model Basin. Since these models are afl related one

to another, it should oe possible to interpolate the wake values for

other designs with some certainty.

Having obtained the wake distribution, there exists a number of

design methods, all based on a strip-type analysis, and involving a number

rf

correction factors. These correction factors have been devised by

-various workers in the field, and at the present time these do not agree

with one another, so that propellers designed by these different methods

to meet the same design conditions would not be identical. This is a

field for further research, both theoretical and experimental. Pethape

some of the moat important unsolved problems exist in the field of scale

effect in propulsion. The whole process of design depends upon model

teats in one way or another, and although we know that some scale effects

must exist, hitherto it has not been possible to attempt ary allowance

for these as we do in the case of resistance. When a propeller is run in

open water, the drag on the blades is partly frictional, and therefore we

might expect that the ful], scale propeller would be more efficient. On the

other hand, there is some experimental evidence that in open water laminar

flow may exist over part of the model blade, and this would have the

opposite effect. Whether such laminar flow would exist in the wake behiz

a model or ship is another question. Experiments on blade sections typical

of marine propellers have also shown scale effects on both lift and drag, and

these will affect both the accuracy of chart data and the detailed methód

of design calculations. It may be necessary to induce turbulence on model

propellers, as is done on the hull by studs or trip wire but no satisfactory

method has yet been devised. This experimental work, both on complete

propellers and on blade sections,must be continued.

The wake behind a hull has three principal components, due respectively

to potential flow around the hull, orbital velocities in the wave system

and the skin friction. The first two may be expected to scale directly

from model to ship, but due to the lower specific frictional resistance of

the ship, the third component, and so the whole wake, would be lower on the

ship than on the model. This reduction of wake with increasing size has

-been f outh on geosim series, such as those on models of the "Victory" ship

run at Wageningen. _{It would only apply to a "smooth ship - as soon as}

she begins to get rou.gji through corrosion, bad. paint _{or fouling, the wake}

wil]. increase. _{Comparison of ship and model figures for new, clean ships}

show that for the same pitch of propeller, the revolutions per minute for

the ship are 1 or 2 per cent higher than predicted from the model, suggesting that the ship wake is somewhat less than that found on the model, but this

is baaed on the model open water results and. the ship self-propulsion _results,

and so is bedevil].ed by the scale effects mentioned earlier. _{Also, it is}

likely to disappear very ciuiok],y in service as the ship gets rougher.

The "Victory" ship geosiin tests also revealed a serious scale effect

on thrust deduction fraction, t, which was found to increase with increasing

size of model, and this has been confirmed by ful]. scale and. model _{tests on}

the U.S. submarine Albacore. _{This combination of decreasing wake and}

increasing t results in a very large scale effect on hull efficiency, which

may be as much as 25% in going from a 19 ft model to the 445 ft Victory ship.

These figures are very disconcerting, and taken at their face value would

suggest a very large reduction in propulsive efficiency with increasing size,

Unfortunately the propulsive efficiencies for the Victory geosima have not

yet been published, but general experience in the correlation of model and

ship results does not seem to leave room for a correction of this magnitude.

Nevertheless, there are so many unknown factors involved that we cannot be

content with the present state of our knowledge, and research imist be continued

with a view to clearing up the whole problem of scale effect on propulsion

factors, This will entail not only geosim model testa, but also the

corresponding rll scale ship trials, in which it will be necessary to measure

the resistance of the ahip and the thrust and torque characteristics of the

propeller both behind the ship and in open water - a formidable undertaking,

but one which must be undertaken if we are to reach a real understanding of

-the whole propulsion problem.

The question of model-ship correlation has been discussed under.

resistance, and it is equaliy important in the propulsion field.. In

deducing the correlation factor Op we have to compare the predicted

resistance of the "smooth" ship with that of the actual ship. This

latter quantity is never measured on ship trials, and we have to deduce

it from the thrust or power measured on trial - usually the latter. In

doing

so, we virtually assume

that there is no scale effect on the self-propulsion factors and that the model values can be applied directly to

the ship to convert the Dli? into ENP and so to resistance. It is thus

clear that the quantity is indeed an all-ebre.cing correlation factor,

covering a multitude of unknowns. It must be the aim of research to attack

these one by one until we finally come to a complete understanding of them all.

The principal areas in which we require further knowledge about the

bydrodynamics of ship propulsion, therefore, must incide the following:

the wake distribution behind a number of series models, so that the

distribution behind other designs can be found with reasonable

accuracy;

developments in the theory of propeller action, particularly with

regard to the various correction factors and their determination

by theory ad experiment;

e) scale effect on propellers in open water, inciwiing the effect

of laminar flow;

d.) scale effect on blade sections;

e) scale effect on wake, thrust deduction ad propulsive efficiency

ad t) full-scale and geosia model tests to obtain a full uMerat"4ing

-of the model-ship correlation problem and the assessment -of the various components which make _{up the total LCF value.}

Cavitation

Cavitation, whether it occurs on propeller blades or on the hull and

its appendages, is a phenomenon we wish to avoid at all costs. It causes

vibration, erosion, noise and in its most advanced form loss of propeller

thrust and, so of ship speed.

The usual way to study it is by n.mning models in a cavitation tunnel,

where correct scale pressures can be obtained at will and the onset of

cavitation and its subsequent development studied by using stroboscopic

lighting. _{Comparison between model and. ship results in the past have}

shown that cavitation occurs much earlier on the ship than on the model,

at perhaps only half the speed predicted from the model _{tests. This seems}

to be due to two principal causes - the uniform inflow in the tunnel as

compared with the very non-uniform inflow behind a hull, and the effect of

the air-content of the water.

The first of these problems has been attacked in a variety of ways - by

running at less than the correct scale pressure, by varying the water

velocity in the tunnel so as to simulate the correct inflow velocity at

different points of the blade in succession, by inserting appendages _ahead

of the propeller or by simulating the wake field (previously measured on a

model in the towing tank) by means of grids, rings or an intricate system

of valves. _{Further work is required in this field to erable us to predict}

with greater assurance the speed at which cavitation will manifest_itself.

When a propeller is running in a tunnel at reduced pressure and cavitating,

the air in solution in the water appears as bubbles in the water in the low

pressure area behind the propeller and this air is then removed as the water

-circulates. Thus in a short time the tunnel water becomes de-aerated and

is no longer typical of orthnazy sea water. One way of overcoming this

is to have a deep section of the tunnel, of large cross-section, in which

the water moves slowly under high pressure until the air bubbles are all

re-absorbed into solution. In this way it should be possible to maintain

any desired air-content at will and so stu&y the effects of varying air

content on the inception and. development of cavitation, and. so improve

model-full scale correlation still further.

Fundamental work on the inception of cavitation is also necessary.

Water of a high degree of purity has been shown to be capable of withstanding

large tensions without breakdown, and the fonnation of cavities in water

is believed to originate on minute nuclei present in ordinary water. The

behaviour of such bubbles on formation and collapse is also of interest in

regard to the noise caused by propeller cavitation. With the continual

increase in ship speeds it is becoming more and more difficult to avoid

cavitation. The blade area on a propeller cannot be increased indefinitely,

because excessively wide blades interfere seriously with one another, and

the diameter is limited by the draft of the ship or by manufacturing and

transport facilities. In such cases extreme care is necessary in the design,

and we become interested in the behaviour of blade sections in the cavitation

zone so as to learn how to delay its inception as long as possible. _{Most of}

the blade section data used in marine propeller desi today have been

obtained in wind tunnel tests, and so throw no light on cavitation qualities.

Such data are urgently needed, and hydrofoil dynamometers have been designed

and are in operation in more than one water tunnel today.

Such information has, a course, many other applications which make it

of use to naval architects, such as the design of rudiers, stabilising fins

and lyürofoil craft, and. we can expect the emphasi8 on this type of work

-to increase.

For heavily loaded propellers, it may be of advantage to depart from the

norma]. marine propeller and adopt some form of ducted. propeller, in which the

water velocity can be controlled to minimise the pressure reduction at the disc

and the propeller tips kept close to the walls of the duct so as to delay the

onset of tip vortex cavitation. Such Installations require much careful thought

and design, for they carry with them a serious penalty in the form of the

increased drag of the ducting. Experimental data in the application of auch

ideas to the marine field are today practically non-existent.

One of the moet troublesome manifestations of cavitation is the erosion it

causes on propeller blades and. on appendages, especially those situated in the

propeller race. Materials differ quite markedly in their mechanical resistance

to such erosion, and a great deal of work has been done on this subject in the

past. It needs to be coctinued to evaluate the qualities of new propeller-blade

materials as these are developed.

The following, then, seems to list the more important research problema

In the cavitation fie:kI ¡

improvement in model-ship correlation regarding the inception and

subsequent development of cavitation, particularly as regarda t

effects of uneven wake distribution and air oontent of the water;

the role played by nuclei in the water on the inception of cavitation;

o) the generation of lydrodynamic noise by the growth

and

collapse of bubbles;

the experimental and theoretical determination of the characteristics

of Idrofoil sections in cavitating oonditiona;

investigations into the design of o.ucted propellers and similar proposals

which *y delay the inception of cavitation

and

f) the resistant qualities of new materials to cavitation erosion.

-Ship vibration

Vibration of a ship' s hull may be generated in a number of ways, such

as out-of-balance forces in main or auxiliary machinery, propeller action or slamming in rough seas. _{The response of the shipt s struoture to such}

forces will depend. on a number of factors, such as the frequency of the impressed force in relation to the _{many natural hull frequencies, the}

clamping and virtual mass effects of the surrounding water and, the internal

damping in the structure.

Some of these are not hydrodynamic in character, but the virtual mass is most important, amounting as it does to more than the displacement of

the ship, and so having a very large effeot upon the natural hull frequencies. Much work has been done on the calculation of thia virtual mass, particularly that for vertical motion nonual to the free surface of the water. Such

calculations have been done on a strip-analysis basis, _{using coefficients}

for each section shape appropriate to infinitely long prismatic bara of that

shape, The correction to the virtual mass _{so calculated in orier to allow}

for three d iniensional effects has been done in a somewhat arbitrary fashion,

usually by using a factor dependent on the length-beam _{ratio. Different}

authorities have given quite different values for these correction factors,

arid, further work should be done to clarify the situation. _{For horizontal}

vibration still less is known of virtual mass, since the free surface is then

a much more complicated factor in the problem, and the same may be said

for torsional vibration.

There are so many natural hull frequencies - vertical, horizontal and

torsional - and a ship runs at so many different conditions _{of loading and}

displacement, that with the present defects in the methods of calculating

these frequencies it is not possible to design machinery and propellers to

4

-avoid

these resonant

speeds

except

perhaps

for the lowest 2

and 3

node types. Emphasis has thus become concentrated rather on reducing the disturbing

forces to a minimum and being able to calculate the response of the ship

to known forces. The latter is still extremely difficult, if not impossible,

because of our lack of knowledge of internal damping in the material, virtual

mass and the complicated type of structure met with in a ship. Nevertheless

it remains as a target, for if the naval architect had a standard of

acceptable vibration which he had to meet, and a means of calculating the

response of the hull to known forces, he would be in a much happier position

to please the shipuwrier

The reduction of out-of-balance machinery forces is the province of the

marine engineer. In the hydrodynamic field the principal cause of vibration

is the propeller which, working in an uneven wake, gives rise to time-varying

forces on the hull, transmitted to it either througi the shaft bearings or

through the water to the plating. This is generally known as

propeller-excited vibration, and

has

become of greater significance as the power per shaft has increased which is particularly the case in single-screw ships in

recent years, which also have the most uneven wake distribution.

It is obvious that the more even the 'wake the less will be the resultant

varying forces on t}e propeller, so that one important line of research should

be to find, the hull forni, particularly at the stern, which

will give

such a

wake. We have also seen that this is an important item in propeller design

and should be investigated systematically. We know, for example, that U

shaped stern sections give a more horizontal and. uniform wake t1n V shaped

ones, and other stern shapes might be developed, perhaps of a bulb type,

which would carry this effect still further. For certain types of ships,

particularly those

of

large beam to draft ratio, hull f orms designed to give predominantly buttock-line flow may be advantageous, and

to

obtain higher

-propulsive coefficients in twin screw ships, twin skeg sterns have been

proposed. For all experiments of this kind, a circulating water channel

is a most valuable adjunct to the towing tank and cavitation tunnel.

The other principal factor affecting propeller excited vibration is the

clearance between the propeller blades arid the surrounding structure. As

the pressure field associated with each blade passes near to the hull or

appendages, the latter experience a fluctuating force of blade frequency,

which may excite local vibration, forced vibration or even resonant vibraLion

of the whole hull.

It is extreme],y difficult to investigate this phenomenon using a complete

model, but some success has been achieved by detaching the stern and measuring

the forces experienced by this portion when the propeller is running. What

is really lacking is fundamental information on the amount and extent of the

pressure fields around. a propeller both in open waterand in the vicinity

- of plane and curved surfaces. This problem is being investigated both

theoretically and experimentally. Once the techniques are established, it

will be necessary to apply them to the determination of the effects of

different clearances between propeller and hull, rudder, bossings, A brackets

and other appendages and under conditions simulating the boundary layers for

different lengths of ships, so that the designer may be given adequate and

scientific guidance as to the amount of clearance necessary to keep the

exciting forces within acceptable limits.

There is obviously much work to be done in the field of ship vibration,

melding such items as:

the virtual mass associated with ship hulls in vertical and.

horizontal vibrations of different models, with special emphasis

on three-dimensional effects;

the calculation of the ship response in vibration to forces of

-different magnitudes and frequencies;

e) flow experiments on a variety of hull forms to obtain the most

uniform inflow to the propellers

and d) exploration of the pressure field. around a propeller, both

in open water and near the hull, to obtain reliable guidance

as to the proper clearance and the optimum number of blades in

order to avoid excessive vibration.

Seagoing qualities of ships

During and since the wa has been a stea. increase in the

speeds demanded of new ships, so that today it is not uncomnn to find

tankers capable of 17 and 18 knots and cargo liners with speeds of

22 knots. This has had its origin

in

a number of factors, incinding defence considerations, the need to make full use of an exceedingly

costly investment, the decreasing importance of the fuel bill in view

of the high costs of building and operation and the long delays in port,

of nuclear propulsion plants will only be justified. by their proper

and. efficient use in service.

Now it is of little use providing the extra power to give these

higher speeds in smooth water unless the ship can maintain proportionally

higher speeds in average weather at sea also, and so the demand

for

higher speeds has inevitably led to a study of the seagoing qualities

of ships.

Most model experiments and full scale ship trials in the past have

been confined to smooth water performance, and little systematic data

are available upon the effects of

rough seas.

In

consequence most ships running today have been designed for optimum smooth water

performance, and not for day-to-day behaviour at sea. The owner is

22

largely interested in his ship maintaining a good speed at sea under adverse

weather, and therefore one of the principal problems in this subjoet is to

design ships which will fulfil this requirement. It is generally .agreed that

in service it is not lack of reserve power which causes a ship to slow, but

rather excessive motion, and so the problem becomes one of how to design ships

which will pitch and roll less, and not ship green seas or be wet with spray.

Research in this field in the past has been limited because adequate

model facilities and instrumentation were not available, the numerical work

involved in any theoretical approach was prohibitive and we knew very little

about the shape of the sea surface or how to describe it. _{This picture is now}

entirely changing. _{Oceanographers have shown us how to measure and describe}

the sea, the high speed electronic computer has taken the _{labour out of the}

theoretical approach, instruments to measure pitch and roll at sea, both

absolutely and statistically, are now available, arid new model facilities

are in use or building in many countries, in which meaningful _{seagoing tests}

can be carried out on free-running, radio-controlled models.

The method of describing the state of the sea by enerr spectra is now well known. _{It is probable that this will become the basis for carrying out}

comparative tests on different models, arid we therefore need to choose two

or three spectra for each ocean, typical of its different _{moods, which can be}

used as appropriate in model testai _{This means accumulating data about the}

seas of the world, and, this calls for a cooperative effort from all seafaring

nations. A beginning has been made, arid sea-state meters _{have been fitted}

to a number of vessels, inclnding warships, cargo ships, oceanographic research ships, light vessels and weather ships. _{The data so gathered are supplemented}

by the relevant weather data, and in many cases by simultaneous measurements of ship motion.

So far as the naval architect is concerned, his _{aim is to produce a more}

seaworthy ship, by which we mean one which will pitch and

roll

_{as little as}

-possible, ship few or no green seas, be reasonably free from spray and

maintain a high sustained speed in bad weather. _{At the moment there are}

no absolute standards to which he can design, and most of the work will be

comparative in nature. _{From the model testing point of view, this means}

the ability to test two alternative models successively in an exactly

similar sea as defined by a specific sea state spectrum. _{The second point}

on which researoh is needed, therefore, is how to reproduce in the model

tank those sea states chosen as typical of the required ocean routes. This

is being attempted now in a number of "models" of the seakeeping tanks now

coming into commission, and electronic controls are being developed to feed

in to the wavemakers the desired wave programmes. _{There is also the problem}

of measuring the model sea state, and for a three-dimensional sea this is

by no means easy.

Having reached the stage of being able to produce, and reproduce, at

will any desired sea state, we are faced with many problems in model

technique. Some people favour entirely free models, with 'ro-controlled

rudders to keep them on the desired course regardless of the irregular seas.

Others are arranging bridges or wires across the tank, to guide the models

and to form a datum from which to measure their motions. Both methods have

merits and dc-merits, and time will tell which will prove the better. It

is more than likely that all tanks will wish at some time to run entirely

free models, and this means carrying a great deal of equipment, such as

batteries, motors, pitch and roll aros, recording gear and enough ballast

for trim purposes. The models will have to be strong but light, and we will

have to perfect the tediniques of making them in reinforoed plastic and of

miniaturising the electronic gear. It will also be necessary to plot the

course of the free running model, and various schemes for doing this are being

tried out at NFL, including ultrasonic and photo-electric methods.

Before the designer can be expected to have confidence in the relative

-merits of two designs tested in the ways described, it will be necessary to

show him that, in general, the results of model tests are borne out by the

behaviour of the ship at sea. In other words, we again have a correlation

problem. While the new model facilities have been building, a number of

seagoing trials have been and are being conducted in order to have the

necessary ship data at hand. The first testa in the new seakeeping tanks

will then be of the models of these ships to see if acceptable agreement is

obtained. The U.S. Navy and Maritime Commission have carried out service

trials on two fully instrumented Liberty ships in the North Atlantic, the

National Institute of Oceanograplr (N.I.O.) in Great Britain has conducted

similar trials on the research ship "Discovery", and the British Shipbuilding

Research Association, Admiralty, N.I.O. and. N.P.L. have in hand trials on

a weather ship ani. a cargo ship in the North Atlantic. _{An important point}

in such trials is that the results should be recorded in digital form if at

all possible. _{This saves an enormous amount of analysis time, since the}

data can be fed directly to a computer.

Having chosen representative sea states, we now wish to estimate the

behaviour of a ship in them. If we assume that an irregular sea is made up

of a series of regular components and that the response of a ship to regular

waves of different heights is linear, then to a first approximation the motion

in an irregular sea will be the sum of the motions due to the different

componeit s. If these motions in regular seas can be calculated or measured,

then the motion in irregular seas could be found.

The equations of motion for the ship contain a number coefficients

which must be known before any numerical calculations can be made. These

include such items as the value of the entrained water and the damping

coefficients, and the possible dependence of these on frequency. Some

-J

attempts have been made to calculate these coefficients, but in the present

state of our knowledge they are probably best determined by experiments with

modela forced to pitch and heave in calm water. The forces on the model,

which depend on her shape, on the amplitude of the waves, the phase

relationship between wave and ship and interference effects due to the

fship s pre8ence, also must be evaluated for the different regular components.

For progress in this field and a better understanding of the _fundamental

factors involved, such theoretical analysis is very necessary, though it is

in any actual case a long task. _{Model experiments are necessary to determine}

the above coefficients for a methodical series of forms, so as to assess the

effects of specific changes in different parameters and enable the designer

who wishes to use these methods to approximate to their values in any

particular case.

As an alternative to such calculations, a model may be run successively

in regular waves of different freauencies and measurements made of pitch and

heave. _{From this a curve of response operators may be plotted which in}

association with the spectrum of the sea, will enable a ship response spectrum

to be determined, _{This shows that model experiments in regular waves still}

have an important role in seakeeping research.

Lastly, the behaviour of the ship may be predicted directly from that

of a model run in the desired irregular sea.

However the research into ship motion is _{carried out, we need to know}

the causes of excessive motions and means to reduce them. _{One of the most}

fruitful lines of research will undoubtedly be the use of methodical series

tests in which the effects of fullness and proportions upon sea behaviour

can be investigated. _{In this case, we are concerned with the above water}

form as well as that belaw water, and any methodical research must include

the effects of freeboard, especially at the stem, and flare as well as fine

-entrance lines arid bulbous bows. _{Apart from the effects of change in}

hull form on ship motions, they can also be reduced by the use of special

stabilising and damping devices. _{Anti-roll stabilisera, such as bilge}

keels, anti-rolling tanks and active fins, _{are now in common use, and}

active fins are now almost a standard fitting on new passenger ships. The

reduction of pitch by such means is much more difficult, because of the greater forces involved and the liability to damage if such fins are placed

at the bow, where their effect is greatest. _{Nevertheless, the benefits}

to be derived from reducing pitch are so great, both commercially and

militarily, that this research must be vigorously pursued.

There is no doubt that we stand only on the threshold of our Iciowledge

of the seagoing qualities of ships, and the next few years promise to be among the most interesting experienced in ship research for a long time.

The problems to be solved are many, but the following list may serve as a first guide:

the gathering of information on sea states on the different

ocean routes;

developing the means of producing any such required sea state in

an experiment tank and of measuring it;

e) development of model techniques for testing in irregular seas;

full-scale and model trials for correlation purposes;

determination of virtual mass factor8, damping coefficients and

forcing furions in waves;

determination of the motions of a given ship in a given sea;

methodical experiments on models in rough seas, to determine the

effeciupon behaviour of systematic changes in fullness, proportions, freeboard, flare and similar features

and h) the reduction of

motion, especially heave and pitch, by stabilisera.

-The future

Most of the problema in hydrodynamics as applied to ship design

which one can fbresee in the immediate future are those concerned with

the attainment of higher speeds and the delivery of greater horsepower.

In all other f orias of transport the last few decades have seen a great

increase in speed, and there is a continual pressure on shipping to follow

this trend.

Ships have the great advantage that they obtain their "lift" by

buoyancy and have not to pay for it in drag.

Thus the medium speed

cargo ship nay have a lift-drag ratio as high as 200, and ships of this

kind are still the principal means of carrying bulk cargoes around the

world.

In an effort to obtain ever greater efficiency and lower unit

costs, such ships have increased in size until we have tankers carrying

over 100,000 tons of oil.

For their length these ships are comparative3)'

slow and pase no great problem as regaris hull form, although they are

so full that methodical series data no longer cover their requirements.

One future research problem is thus to extend such series up to block

coefficients of 0.85.

In the matter of propulsion, however, they raise

serious question5.

If they are single screw, the powers necessary call

for propefler diameters which have reached almost the limit of those

which can be accommodated on the maxinnim draft or manufactured in existing

facilities.

If tankers grow any bigger or faster, it will be necessary

to go to twin screw designs, and this will call for further researoh into

the best form - conventional, buttoci-flow or twin skeg.

The high powers

per shaft also exaggerate the vibration problem as

already mentioned, and

continual attention must be paid to this matter.

Turning to passenger ships an cargo liners, speed can be increased

-if power is available, ships are made longer and finer, arxi -if the economics

of the world call for such a speeding up in sea transport. As speed is

increased, wave-making resistance increases very rapidly, arid we approach

a kind, of barrier akin to the sound barrier in aircraft. For the large

displacement-type ship this can only be delayed to higher 8peed by making

her longer, so increasing first cost as well as running costs.

Much is known about the smooth water performance of such ships, and

there is not much scope for spectacular improvements in hull form. But

savings are possible by attention to the cona.ition of the hull surface,

paying proper attention to its structure, preparation and. painting - maybe

to building ships of corrosionresistant material.

In rough water, there is a great deal more scope for improving sea-going

qualities by reducing motion, so leading to greater comfort and, higher

sustained sea speed. Research in this field is only in its infancy.

In an attempt to reduce resistance and avoid the major effects of

waves, a great deal of effort has been devoted to the development of

Frdrofoil craft. On a small size, they can attain the same speed as a

conventional high speed motor boat for about one half of the power and are

better able to cope with moderate seas. They have attained a certain

success as small passenger craft on lakes and in sheltered waters, but in

larger sizes the foils and their supporting struts become very heavy, there

are difficulties in transmitting the high powers to propellers immersed

maiy feet below the hull, and. the payload suffers greatly in consequence.

When floating on the water, the foils also require very deep bertha, or

else have to be made retractable, with a still further increase in weight.

Such craft may have a future for certain passenger and military purposes,

and research should continue into the means of obtaining more efficient foils,

better struts to avoid cavitation and aeration and. ways of getting high

powers to the propellers.

-Still more recent are such ventures as the Hovercraft, _supporting

itself just above the water on a cushion of air. _{This system is designed}

to reduce the drag, and can only be Used over relatively smooth _{surfaces of}

land or water. _{Nuclear power plants for marine use are already well}

established technologically - their adoption by the merchant marine is

one of economics. _{So long as they merely replace conventional plants of}

the same power they do not introduce any new hydrodynamic problems. _However,

when their first cost is reduced, their cascity for developing full power

for long periods and the absence of any need. to carty large quantities of

fue]. may change the picture and call for faster ships. _{This will mean}

finer and longer ships, calling for new considerations _{in design and seagoing}

performance will become more Important than ever.

This possibility raises the problem of how these greater powers can

be delivered without serious cavitation. _{The diameter and blade area of}

propellers has about reached the limit on modern liners and. warships, and

research must be directed towaris new ideas in propulsive devices. _These

inclnde ducted propellers, in which cavitation may be delayed by the use

of multi-bladed pump runners and guide vanes, contra-rotating propellers,

modified stern lines and appendages and fully-cavitating propellers. These latter have been shown to be feasible for high speed motor boats for speeds of 40 knots or more and for large liners at speeds of 50 knots. They avoid erosion and since they require high revolutions may be used with gas turbines and other liit, fast tinning machinery. _{Mention has already been made of the}

wave-making barrier experienced by surface ships as speed is increased. The

advent of nuclear propulsion and its special advantages in submarines has

not unnaturally turned many peoples thoughts _{to these craft as the possible}

ships of the future, avoiding both their own wavemaking and ocean waves by running well below the surface.

There are mazy operational problema associated with such ideas,

-including the excessive drsfts for berthing and docking, the difficulties

of navigating, the building of a pressure hull and the difficulties of

ballasting, to mention only a few, all leading to a vessel of considerably

greater displacement and size than the surface ship of the same deadweight

capacity. These, however, are quesfioxis of economics, and this is no

place to discuss them. Hydrodynamically, however, there are also some

serious problems to overcome. At low and moderate speeds the submarine

has more resistance than the surface ship, because of the greatly increased

wetted surface, and it is only at comparatively high speeds that she shows

superiority. At these high speeds, however, directional stability and

contro], become very important, as does the q,uestion of propeller design.

There seems no economic justification for such a venture for normal

commercial use, but such craft may be ballt for reasons of defence, national

prestige or as an experiment, but for hatever reason such a ship was

built, a great deal of difficult but intensely interesting research would

have to go into the design.

In this paper an attempt has been made to survey the principal fields

of hydrodynamics as they affect ship 'esign problems, now and in the future.

It is obvious that there are a multitude of such problems, some old and

yet unsolved, others new and raised by the remarkable rate of development

in all branches of technology. All over the world new facilities for

research are being built today, and naval architects and scientists can

look forwani to a future which is full of challenge and to the possibility

of seeing revolutionary changes brought to successful application in the

ships of tomorrow.

Aclntowledgment

This survey has been carried out as part of the research prograxnnie

of the National Physical Laboratory, and the paper is published by

permission of the Director of the Laboratory.