The design of marine propellers with special reference to cavitation

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THE DESIGN OF MARINE PROPELLERS WITH SPECIAL REFERENCE TO CAVITATION.

INTRODUCTION

Lecture by Prof., L,C,Burrill, Dr.Sc, on Tuesday, ix/4/

The general approach to the problem of designing marine propellers. about which I propose to speak tonight is, I think, reasonably typical of the British approach to such matters, and I hope it will prove to be suf-ficiently challenging and i.n some measure unusual and controversial as to provoke a' certain amount of discussion., both here and after I have returned to England.

THE DESIGNERS PROBLEM

The task of the propeller designer is essentially a practical one.. In the first place, he has to decide the size of propeller requiredto absorb a given power at a given number of revolutions By size I do not mean on-ly the diameter, but diameter and pitch, because the power absorbed by a propeller is very largely determined by (pitch diameter) In the second place, he has to chose the best combination of diameter and pitch to give the highest efficiency

He can. for example, chose a large diameter and small pitch, or a small diameter and a large pitch, and still absorb the same power at given

rev-olutions, providing the sum (P D) is about the same, but there is one ratio of pitch to diameter' which will give the highest thrust for given shaft horsepower on a given ship.

Next he has t,o chose the minimum blade area to allow the propeller to wox k sat Isfact or ily without danger of cavitation or erosion, and finally, he has to chose the type of blade sections and also the radial distribu-tion of loading, which is very largely governed by the type of pitch varia-ton. from root, to tip which i.e adopted. He must, of course, also accept responsibility for the strength of the propeller and decide the number of blades, but these are matters about which he has lees choice than with the first ment ioned charact erist ics Now, the sources of informat ion which will help him to resolve these questions are, respectively, (i)experince with actual full size ships and propellers, () model experiments, a.nd(3) propeller theory These have been placed in this order deliberately, be-cause it is considered that full scale experience is the _{most important} source of information for future work, and model experimenta and theory are only valuable in so far as they help to interpret the results of full scale work and give a lead to future lines of development, _{In fact,it must} never be forgotten that the ultimate object of the naval _{architect isto} design full size ships which give good r esulis in service, and that the

results of model tests and theoretical speculations _{have no} valueinthem--selves, until they have been proven by the results obtained at sea Whena ship just fails to obtain the promised speed _{on trial or when the}

revolu'-tions of the engine are not correct, it is _{no use telling the owner that} your elaborate calculations give a different result _{or that you had been} guided by the result of the tests on model scale,

It is in fact true that many ships which have _{not had particularly good} model test results, have been judged to be most excellent in performance,

because the naval architect has been able to include a reasonable margin of power to meet the promised conditions of speed on trial and at sea

Adu. DW Taylor s well known dictum 'The time for pessimism is when the powering is being done, not when the trial is being runt! is well worth

Lremembering in this connect

ion.-Now, when we turn to the question of propeller revolutions, the same consideration does not apply in quite the same way.

If, for example. the shipbuilder has applied a certain margin ofpower and the engineer has added his own private m argin. the propeller designer has to strip away these margins and try to ascertain the true facts before

deciding the dimensions of the propeller, otherwise he may find his proppel-.-

-icr will run faster than expected and may not develop the necessary power, if the speed is greater than has been promised. In practice, thismeans three things.. firstly. the propeller designer must be able to make hi sown

estim-ates of power for different types of ships, secondly he must know his engineers and shipbuilders, and thirdly he must state clearly, when sub-mitting his final drawing the conditions for which the propeller has been designed; namely, the speed, power and revolutions to be expectedontrial and under average fair weather conditions at sea with a clean hull, This

should give the purchaser an opportunity of discussing the appropriateness of the design conditions chosen, and to criticise them should they not be in accordance with his wishes, This is most important if the trial trip is to be carried out under light-load conditions, as the full power may not be developed if the engineer specifies a limited number of revolutions inconsistent with the lighter load on the propeller.

It is also important to know whether the enginebuilder wil1-'emore dis-satisfied if the revolutions are a little low or a little fast - For a

diesel engine installation, for example :tt is most important that the

rev-olutions should not fall below the required number and in this instance it

T

aãvisable to keep a slight margin by designing the propeller to run about

i - revolutions faster than stated by the enginebailders.

Wi-th s-team-turbine installations the matter is not quite so simple be-,v-ause some engine designers adopt different procedures from others in fix-( ing the revolutions for full power.- Generally speaking, it is better to

design the propeller on the slow side (i.e.- _{about i lower than stateàTV} \ for such installations, but it is advisable to discuss th.s with the

engineers concerned.

DESIGN PROCEDURE.

Turning now to the question of design procedure there are two quite different methods of approach to propeller problems which may be labelled respectively (a) Thrust approach and (b) Torque approach _{The first of} these comes most naturally to the Naval Architect who works upwards from hi.s EH.P. curve, and the second i.e the Marine Engineers approach.because he very naturally works, as it were, downwards from _{his engine power (1. e} SH..P.) _{In the fina.l result, both aspects must he reconciled,}

but after a while the propeller designer usually choses the second _{approach as the} most sinple and at the same tine most reliable,, In other _{words, the}

prop--diet

designer uses the - 6 _{curves much more frequently than the} 6 presentation.

The reasons for this may be stated brie-fly _{as follows}

) The Engineer usually purchases and pays for the propeller and conseq-uently _{the first consideration is that the propeller}

must suit the engine.-

-) Information is readily available about the

power developed a± diff-erent. revolutions both in service and on trial; information _about

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thrust is hardly, if ever, obtainable.

Consequently, it is easier to check the value of slip wake fractions by the analysis of voyage data than it is to obtain information about the thrust deduction factor

) By designing for given SHP and revolutions all propellers considered

automatically satisfy this basic condition. At given revolutions, the change in ±orque for a given propeller at different speeds of advance is not very sensitive and consequently the effect of a moderate error in speed of advance is not great.

In general terms, it may be said that the B - ô approach is most useful for first estimates of power required for a given speed, but

that once the engine size and working revolutions have been fixed the B - 5 approach is the most satisfactory one from the propeller design-ers point of view, in fixing the final dimensions of the propeller.

. USE OF STANDARD-SERIES DIAGRAMS.

It is very fortunate that we now have standard-series diagrams for propellers covering a wide variation of blade area ratio, pitch ratio.etc.., and the propeller designers task is therefore much more easy than it was before such work was undertaken by the various research stations The most effective method of using such data is first of all to design a standard propeller to suit the required conditions, and then to examine this prop-eller for strength and cavitation, blade loading, etc., making such alter-ations as may be found necessary. In the early stages of design the iterative method is the most simple approach. Namely, a is first assumed from earlier experience, and then by entering the diagrams with the derived B value the efficiency of the best propeller can beobtained.. If this is consistent with the assumed Q.PC.. the task is _completed but

if not the assumed QP.C.. must be altered and the process continued until a correct balance is achieved. For a closer examination of theproblem after the engine size and the working revolutions have been fixed the following method is preferred.

In the first place a curve of estimated EHP is drawn to a base of

speed and from this the corresponding curves of _{Eperiai=model (EHPn + io%)} and E(seryjcey (EHP _{%) say to represent trial and service conditions,} respectively, may be derived. Suitable values of and hull _efficiency are then assumed and fixing the SHP and revolutions at the required level a series of B2 constants are determined, at say . knot intervals, The best efficiency can then be obtained from the B2 - S charts for each speed considered bearing in mi.nd the maximum diameter which can be fitted and

hence the corresponding THP-U=T2x DEP from which the value of E may be obtained using the expression.

E

hull efficiency. U

We can then draw a curve of B derived in this way on the same diagram as before, and where this cuts the two _{curves previously mentioned we} obtain the best speeds obtainable on trial and in _{service respectively.} Furthermore, if a third curve derived from the EHP _{for a lighter draft is} added then the corresponding speed for _{a light-loaded trial can be}

obtain-ed

It will be seem that the E curve derived from _{the propeller data is} re-latively flat and that the cutting points _{are well defined,}

This procedure is, of course, only a first approximation, because the revolutions have been maintained constant, but _{from the speed point of}

view the result is reasonably close, The next step would be to decide whether the propeller should be designed to work at the stated power and revolutions on trial, or in service, When this is done the final dimens ions can be fixed and the revolutions for the other conditionmay be deter-" mrned. Although this is a matter of detail, it is one of the greatest

import anne

If for example it is decided to design for a loaded voyage condition in average weather then the revolutions would have to be increased on a light trial in order to absorb the full power and the engineers should be aware of this before the trial is undertaken, On the other hand, if It is decided to design for the trial condition then the revolutions in service will be less and if the Mean Effective Pressure is limited the available

horsepower will also be reduced. The best method of investigating this problem i's to fix the diameter and power and vary the r p m, until the correct pitch'rati.o balance is achieved but this subject unfortunately lies outside the scope of a lecture such as this

. ANALYSIS OF TRIAL TRIP AND VOYAGE DATA

In order to apply the design methods discussed above it is necessary to assume a wake fraction and a hull efficiency This can be best done, by analysing the trial and voyage data for actual ships with the same prop eller diagrams as will later be used for design purposes. If B - 5 diagrams are used then the analysis wakes should be obtained on the basis of torque identity and such wake fractions are usually lower than those obtained from tank tests on the basis of thrust i.dentity. In order' to cx plarn the principle of such analysis work the following simple example can be given. Before a ship goes on trial a KQ _- J diagram for the actual propeller can be drawn in 'the usual form This may be based on

standard-series data or alternatively on the open-water tests with a model of the actual propeller Knowing the actualdiameter this diagram may readily be converted into a curve of _{against V1N and when each pair of runs}

N

is completed the ship values of BHP/N3 and V/N may be inserted as below

and the required analysis wake fraction is given by the _{ratio ABAC. If} desired, lines of equal wake fraction may be drawn i.n 'the diagram before the trials so that the wake fraction can be determined visually _{for each} pai.r of runs as the trial proceeds

It w:ll be found that the wake fraction _{so determined varies very} little from run to run; so that this provides _{a powerful check on the} powers returned by the engineers after each pair of runs.

The analysis hull efficiency is given by

Hull Efficiency -- QP.C/ Prop effey (behind) EHP - allowances

and the Q-P.0 - while the propeller efficiency

BliP - shaft friction

may be taken from -the standard-series diagrams Usually the effect of relative rotative efficiency may be neglected. so that the analysis QPC may be said to include this effect.. When the results of a large number of trial and voyage results have been analysed in this way the derived wake fractions and hull efficiency values may be used with confidence in

designing new propellers. The standard series diagrams are in effect only used as a means of interpreting and interpolating the full size ship data..

6. THEORETICAL APPROACH TO PROPELLER DESIGN

At the beginning of this century the most usual method of designing propellers was to assume an appropriate value of apparent slipe based on previous experience. and to use this to fix the required face pit ch the

diameter being determined by very simple momentum consideratioas That is to say. it had been found from experience that successful propellers gave an apparent slip in service of approximately o to iper cent.

andconseq-uently the designer- decided in advance that his propeller should work at this siip so that if V was the expected ship speed. and N the intended revolutions per minute, then the so-called "speed of propeller'wasgiven

by NP. and a suitable value of P, the pitch of the propeller was obtained in this way. by applying the expected slip,

NP

V (o

1 e slip

S

NP

This pitch was then associated with a diameter slightly less than the loaded draft of the ship, and an assessment of the thrust was made on the basrs of a very simple form of the momentum theory. using a volume of water pro,ected from the shp in cubic feet per second given by the simple

formula (vOlume) RP(A - a) where R r P pitch in feet and (A a the area of the screw disc less the area of the boss, in conjunction with

(RP v, where vwas the ship speed in feet per sec.

This thrust was then compared with the estimated resistanceoflhe ship at the expected speed and if too large Or too small, then the diameter was adjusted accordingly until a balance was obtained, The efficiency of propulsion was then either assumed by taking an arbitrary figure of 50 to

6o per cent based on previous experience, or _{an estimate was made of the} toia,I bladefr-ic.tion. Which was added to the "loss due to slipe,,

There were no clear ideas about suitable diameters, and it was not

until muc.h later that the conception of an "optimun diameter,in terms of slip or loading, emerged In general it was considered that the pitch-ratio should be made as large as possible. as it was considered that, high pitch--ratios gave the best efficiency, but ii. is not clear how _{., this high}

pitch-ratio was always to be obtained,

The blade areas in common practice were extremely snail,

basedpresum-ably on the idea. of reducing the loss due to friction.. _{It was also quite}

usual to make the diameter slightly larger than thought _{necessary by}

cal-culation, _{so that the propeller could be cut down if required.}

Present day -theoretical methods of propeller design are, of course, mnch moxe powerful, as we now have a much more detailed and intimate knowledge of propeller action, but, in principle _{they are based on the}

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same considerations; namely that the forces at the blades are produced by setting the sections at am angle of incidence to the resultant flow at each radius and that th.ese forces must _be balanced against the mometum changes arising from the increased velocities in the slip-stream behind the propeller

There are, however, many problems such as flow curvature within the limits of the blades, blade interference, the effect of boss size, and the

influence of pressure drop due to rotation in the slipstream. to which on-ly partil solutions have been found The results of even themost advanced

calculations in this field must therefore be regarded as comparative rather than absolute, and it is, in my view, still necessary to link the results of such calculations closely with the results obtained by standard-series methods of design and with observations drawn from full-scale practice.

In other words, I would prefer in the first place to design a propeller in the usual way using standard-series data and general experience and

then to vary the details of design in accordance with the results of vortex-theory calculations, rather than to design each new propeller completely by pure theory from the ground up. Generally speaking. it is

sufficient to examine the conditions at four characteristic radii (namely

and and then to adjust the pitch and centreline camber to give optimum loading distribution and minimum drag conditions at these radii. the intermediate blade sections being adjusted by fairing between these characteristic sections. By proceeding in this way i.t Is possible

to adjust the radial pitches and section camber's to suit the expected wake distribution, and at the same time to control the lift coefficient at which each section will work If the standard propeller is well chosen the amounts of adjusment required at the several radii will be snall and

it is a simple matter to ensure that the final propeller is equivalent in power absorption to the normal standard design. thus making sure that the propeller will develop the required power at the correct revolutions. The method is also one of great flexibility, as the local pitches and camber's may be chosen to avoid awkward blade shapes and thus make the propeller easy to manufacture.

7 MODERN APPROACH TO CAVITATION PROBLEMS

Since S W Barnaby first proposed a limiting thrust per square inch of blade surface as a suit able criterion for the avoidance of cavit atiozi our knowledge of the action of a marine propeller has increased very

con-siderably and this has led to a different approach to the problem.

Admiral DW Taylor in 1909 carried out a number of tests in open water with propellers specially designed to cavitate and he then proposed a limiting tip speed of

i,000

feet per minute (or oo feet/sec ) Several other investigators since that time, and notably Irish in

_gq

have suggested the combination of a suitable limiting thrust per square _inch:: and a limiting tip speed Such global criteria do not however,, allowfor possible variations in blade shape or section design and modern considerä4-tions of cavitation are therefore based mainly on a comszderatron of the; lift-coefficients at which the several sections at different raddi of a propeller blade will be working in service, judged in relation to acaviia-tion number

_a

p-e/q, where hp" is the static pressure due to _(water head atmospheric head. 're" is the appropriate vapour pressure and

is the dynamic pressure-p V arising from the stream velocity V

In order to explain this procedure in simple terms, it is necessary first of ai.l to state that a propeller blade acts precisely _{in the same} manner as an aeroplane wing in flight if due allowance is made for the

induced velocities occasioned by the working of the propeller as a propulsive unit _{If this can be accepted, then we can consider first of}

all what would happen if a simple aerofoil (or aeroplane wing) were to be towed under water at different speeds of advance. The pressure distribu-tion around such an aerofoil would

depend

upon (a) its cross-sectional shape and (b) Its angle of incidence relative to the line of advance,, Fig shows, for example, the pressure distribution around two

alternative forms of sections at a small positive angle of incidence, So long as the angle of incidence remains constant the shape of the pressure distribution curve will remain the same; it is usually plotted in relation to the stagnation pressure

pV

at the nose (i,e. at the point where the face and back stream divide), If therefore, the speed of advance is doubled then the maximum suction will be increased four times, if it is trebled then the suction will be increased nine t.imes, and so on. It will thus be obvious that as the speed is increased the back-suction peak increases rapidly and will soon reach a point when the local pressure falls to the level of the vapour pressure. When this occurs, any further increase in

suction will cause a sudden rupture of the water surface in contact with the section and a bubble or cavity will be formed,

As the flow is continuous this bubble will be entrained with the fluid and thus enter a region where the pressure is increasing, and Professor G Knapp, of the California Institute of Technology, and others, have shown by means of extremely high speed cine-pho-tography that each bubble

collapses, reappears by a kind of phenomenon, collapses again,

and then reappears several times before it finally disappears

From the two pressure-distribution diagrams I have shown, namely. for a. round-back section and an aer'ofoil section, respectively, it will be

obvious that the maximum suction on the latter exceeds that on the former; so that cavitation will occur at a lower speed of advance with the aero-foil section than with the corresponding round-back section

In fact, the total area under the pressure curve, including both the subpressure on the back and over-pressure on the face, represents for all practical purposes the lifting force acting on the section, or, if these

are plotted in relation to the stagnation pressure and the chord is taken as unity, it represents the lift coefficient where

CL lift coefficient

and as the maximum suction may also be expressed as

in ax

max, suction coefficient. p V2

we have, approximately, for the round-back section

m ax

i2O

CL

p V2

and for the aerofoil section

inax i46 CL - p V2 lifting force p AV2 2

(where A - r.o in this case) 7

and furthermore, since cavitation occurs when

max (p

we can see that cavitation will occur with the round-back section when

p e

1,2o

CL

and for the aerofoil section when

0 (aero) 1.46 CL

In other words, for a given value of 0 the permissible lift-coefficient is given by a and 92 or' 0 ₂₇₄ 6g

and under these conditions cavitation would occur at a lift coefficient of

i88

for an aex-ofojl section

228 for a round-back section

1-20

Alternatively, if the lift-coefficient was _{2 for each type of section} then the aerofoal section would cavitate at g6 fps. and the _round-back section at o6 8

fps.

Now let us compare this with the section of a propeller blade situated at a radius of feet and rotating at 100 rpm. with a speed of advance of knots. In this case. we have

permissible CL =

permissible CL

34 3

i44

rotational speed

(round back section),

a

(aerofoil section

1 46

If for example. the lifting plane (or wing) was travelling at _{a distance} of to feet below the water surface at a speed of _{ioo feet per sec the} atmospheric pressure would be, say, 14.7 lbs./sq in, the water bead

lbs/sq. in. and the vapour pressure 6 lbs/sq. inC, so that

(p - e) 14.7 - 4"5

'26

1892

lbs/sq. in 1 64 ioo and .:. p V -. x x _{69 lbs "sq in} 100

2X9X

-x It

6o

94-2

fps 8 2'74 CL CL

1 48

274 0I

and the resultant speed 8874 410

g63

fps.

so that for a lift coefficient of o the aerofoil section would be just on the point of cavitating whereas the round-back section would have a

margin of about o rp.n. before this occurred

This is a very simple example, but it illustrates the manner in which a modern non-cavitating propeller may be designed. In practice the

resultant velocity at each radius relative to the blade section is found by combining the axial velocity and the rotational velocity as above and then making an additional allowance for the inflow velocities caused by the action of the propeller itself The blade section must then act at a small angle of incidence to this final resultant velocity, at each radius. This accounts for the changing pitch angle of the blades, from radius to to radius and the designer by choosing the diameter and pitch for a given application automatically fixes the angle of incidence at which each

section will work in service If, for example, he chooses a small diameter and a high pitch for a given job the angles of incidence will be greater than if he chooses a large diameter and a small pitch although the power absorbed and the speed of rotation may be the same in each case From the simple considerations outlined above it will be obvious that in order to

avoid back cavitation it is advisable to adopt a low angle of incidence where possible, in order to reduce the working lift-coefficient,but there

is a practical limit to this procedure, because when the incidence is reduced beyond a certain point the shape of the pressure distribution diagram changes and important suction peaks occur on the face near the

leading edge; consequently there is a danger of face cavitation. So far

as the thick root sections are concerned these can usually carry a higher suction than the thinner outer sections because of the increased head of water available, and furthermore, the danger of face cavitation occurs at a higher angle of incidence as the thickness is increased,. For these reasons it is usual to arrange for the inner blade sections to work at a higher lift-coefficient than those in the outer parts.. For example, while the lift-coefficient may be only _{io for the sections near the tip,} the lift-coefficient of the inner sections may be as high as 6o. When the

propeller is working behind a ship, it so happens that the axial speed of the water near the boss is considerably reduced by the wake-concentration which occurs in this region. This automatically increases the angles of

incidence towards the root of the blade, so that, even allowing for an increased liftcoefficient for these sections, it is usual to adopt a pitch-reduction towards the root of the blades.

Finally, it should be mentioned that a given lift-coefficient _{may be} obtained either by giving the sections a small centreline camber and a relatively large angle of incidence, or vice _{versa, and that there is an} optimum centreline camber for each lift-coefficient _{which places the} stagnation point exactly on the noses It is also possible to vary the

shape of the contour of the blade sections so as to obtain an almost uniform suction on the back _{In these circumstance the formula for}

A may be reduced

A_{p max.} 9max

in theory to _{8o CL although a more practical value to assume} would be A max C

One particular application o.f the above principles _{is to design} the blade sections of the propeller so that the total lift is de'veloped by means of centreline camber, and not by angle of incidence relative to the resultant flow velocity.. This procedure leads to _{sections having a large}

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amount of cen-treline camber, and in the outer parts of the blades where the thicknesses are very small, the sections designed in this way will have a hollow face (ie. crescent-shaped sections) For sections having a centreline camber which is disposed symmetrically about the mid-chord position the theoritical condition for placing the stagnation point on the nose is precisely that the total lift is developed by the centreline camber. Two examples are the socalled Karman--Treffz profiles formed by two

circular arcs and the NACA series sections based on the a io camber

line, both of which types have been applied in this way.

Experience with such sections shows, however, that the flow does not

divide exactly on the nose as predicted by theory and that it is better practice to design the sections to work at a small angle of incidence. Very satisfactory results have been obtained with hollow-faced outer

sections designed in this way for propellers having very wide blades and working under high-loading conditions., but, on the other hand, the

applications of such sections to merchant-ship screws of small blade- area ratio working under moderate-loading conditions has not been entirely suc-cessful. either from the viewpoint of efficiency or freedom from erosion. The reason for this does not nesessarily lie in the principle involved, but rather in the means by which the necessary cambers have been deter-mined, the actual curvature of flow near the tips being less than is suggested by the theoretical assumptions made.

8 RECENT EXPERIENCE AND MEANS OF AVOIDING CAVITATION

In recent years, owing to the increased speeds of merchant ships which have been achieved mainly by the adoption of higher powers and higher

rev--1utions per minute, it has become increasingly difficult to avoid the

effects

of cavitation,

This is particularly so in the case of high speed cargo-liners and

tankers where the increased powers have been transmitted on asingle shaft, because in the single-screw arrangement and with a relatively full after body the propeller is working in an extremely high and variable wake; consequently the angles of incidence at which the blade sections are working alter very considerably during the course of each revolution..

For example, it is possible for each propeller blade to be quite free from cavitation in the outboard position (i,e when at right-angles to the vertical centreline of the ship) where the velocities are relatively high and then to cavitate on the back when passing through the upper part of the aperture, where, owing to a frictional wake belt the axial velocities are low.

Alternatively, if the propeller blade tips are free from back ca-vita-tion when passing through the aperture, they may suffer from face cavita-tion during the remainder of the revolucavita-tion,

The means whereby the designer can meet this challenge of higher rev-olutions and ever increasing loading are as

follows:;-. To increase the amount of blade area and thus reduce the thrust/sqin

of surface.

To diminish the blade angles and angles of incidence by _adopting slightly larger diameters or by using increased centreline cambers.

- To adopt different pitches at different radii of the propeller in

order to diminish the loading in critical regions

. To avoid the ocurrence of unduly high suctions on the back of the

blades by using section shapes which give a more uniform distribution of pressure

-. To avoid the incidence of local suction peaks

near the leading edge by usIng suitable amounts of centreline camber and _{a suitable shape of} entr ance,

6 To reduce the thickness of the blades,

NEW MATERIALS FOR PROPELLERS,,

The last item in the above table is closely related to the possibility of using new materials which are stronger and more resistant to the effects of cavitation Briefly there is no metal including the hardest steels which is completely resistant -to the destructive action of cavitation, but recent work In the metallurgical field has led to the development of

several new alloys, such as the nickel-aluminium bronze alloy which gives a breaking strength of tons/sq in as compared with about tOns/sq. in,, for normal manganese bronze and about 18 tons/sq. in.for cast iron, and which at the same time has a very much, increased resistamee to both erosion and corrosion. This makes possible the use of thinner blade

sections and thus extends t.he range of thrust--loading which can be accepted by the designer without risk of erosion damage to the blades.

The problem of avoiding cavitation is still essentially a question of design but if the metallurgist can resolve the physical and chemical

problems of the erosive action of the collapsing bubbles and produce still further new materials which cam sustain the severe blows without serious deterioration of the sürface this will most surely lead the way to

further advances in propeller performance and ship speeds.

2435')