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Sill11

C

dSeecvoelnodpmsyemotspoosifumtoteotnest

yacht architecture,

under the auspices of the Hiswa.

Edited by a committee

under the chairmanship of

Prof. Ir. J. Gerritsma

(2)

SYMPOSIUM

YACHT

ARCHITECTURE

'71

(3)

SY

Contents

J.D. van Manen and M.W.C. Oosterveld: High speed propulsion, page 6

Prof. R.W. Stuart Mitchell: High speed motors and gas turbines, page 16

J.B. Hadler and Nadine Hubble: Prediction

of

the power performance

of the series 62 parent hull, page 30

Pierre DeSaix: Systematic model series in the design of the sailing yacht

hull, page 42

Jerome H. Milgram: The distortion of sails due to fabric deformation, page 50

C.A. Marchaj: Instability of sailing craft-rolling, page 62

Ir. R.J. Schliekelmann: Join tdesign in the construction offibreglass reinforced yachts, page 84

Jakob Saethre: Experience with grp yacht construction, page 90

Dr. Ir. F.J. Kievits: Antifouling and prevention of corrosion, page

94

Published by: Uitgeverij Interdijk N.V., Schiphol-Oost, Holland,tel.: 020-45 37 51

Lay-out and cover: Uniepers N.V., Amsterdam, Holland

POSIUM YACHT

(4)

Second symposium on developments of interest to yacht architecture

Edited by a committee under the chairmanship of Prof. Jr. J. Gerritsma.

Members of the committee: Vice-chairman W. de Vries Lentsch Jr.; Jr. M.C. Meyer;

Ir. J.J. van den Bosch; Mr. J. van Vollenhoven; Prof. Jr. W. Draijer.

Introduction by Prof. Jr. J. Gerritsma, Delft Shipbuilding Laboratory.

This second Hiswa Symposium is intended to provide an opportunity for the exchange of thoughts and experience concerning technical and scientific aspects of yacht design and building. The economic importance of this branch of naval architecture and shipbuilding and the increasing application of series production necessitates the prediction of yacht performance during the design stage. Not in every case can these problems be solved by the use of

only empirical methods, and consequently the knowledge resulting from technical research and development could be very helpful to the yacht designer and yacht builder.

Very little research on yachts and yachting has been carried out, and consequently the

technical advancement in this field is relatively slow. This lack of research is due to the fact that such projects are expensive when compared with the cost of a yacht.

Systematic research not directed to one particular object, some striking examplesof which are given in this Symposium, is another very useful source of information, indicating certain trends and forming general conclusions. However, this systematic approach involves a more extensive investigation of the problem and even greater expenses. It seems to me that cooperation between individuals, organizations, and institutions could be extremely useful in this respect. And I would like to recommend such an integrated cooperation for the

accumulation of knowledge for the use of yacht designers and builders.

A Symposium like this provides an opportunity to present the results of such efforts in a concentrated form. Publications of the progress which has been made should be freely circulated without fear of competitors, because builders and designers must still produce their own interpretations of the increased knowledge. There is no doubt that the quality and performance of yachts would be improved by the use of more and better information. There is another point worth noting. A vast amount of technical knowledge used in naval architecture, aeronautics and mechanical engineering can be applied to yacht design problems and could

be presented in a form which is applicable to such problems.

The English language was chosen for usage in this Symposium to stimulate international participation and to facilitate the preparation of the papers presented. Nine authors, including five from outside the Netherlands, have contributed to this Symposium then expert knowledge of the subjects which are applicable to yachts and yachting. The organizing committee is grateful for their enthousiastic response and the superior quality of their contributions. Substantial technical knowledge is required for the understanding of all the papers. However, many of the results are presented in a form which is directly applicable by those who have little detailed knowledge of the theory.

Once again Hiswa has taken the burden of the organization and financing of the Symposium, once more showing their interest in the development and application of research in the field of yacht design and construction.

(5)

6

High speed

propulsion

by J.D.van Manen

and M.W.C.Oosterveld

(Netherlands Ship Model Basin)

Abstract

In this paper the results are given of an analysis of different types of propulsion devices when

applied to high-speed craft. The propulsion devices considered are super-cavitating

pro-pellers, waterjet propulsion systems, water-air

ramjets and shrouded and unshrouded

air-crews.

It was found that at lower speeds the super-cavitating propellers and the waterjet propul-sion systems were the best whereas at higher

vessel speeds the airscrews become more

favourable. From an economical point of view there is no reason for surrounding the airscrew

by a shroud. Over the whole range of con-sidered design speeds the water-air ramjet as propulsion device for high-speed craft is less

efficient. Introduction

During the past twenty years significant

ad-vances have been made in the development of

marine craft able to travel at high-speed over the water. These developments have led to three different types of craft capable of

ex-ceeding speeds of 40-50 knots. These different types are:

the semi-displacement ship, where a

sub-stantial proportion of the total weight of the ship is supported by forces acting on

under-water areas of the hull which behave as

planing surfaces;

the hydrofoilboat or foilcraft, where the hull

is lifted above the water surface on

under-water wings or hydrofoils, thus reducing the drag at high-speed;

the hovercraft, where the low drag is achieved

by raising and supporting the main hull

structure above the water surface by a self

generated air cushion.

Recently, the developments in high-speed

marine craft were extensively discussed by Silverleaf (1) and Silverleaf and Cook (2).

The different types of high-speed craft have in common that under-way a substantial portion

of the upward force supporting the weight of

the craft is provided by dynamic means. In the case of the semi-displacement ship and the foil craft this is achieved by a lift force generated by the forward speed of the ship. In the case of the hovercraft the lift is obtained by a cushion of air provided by fans.

If, in the near future there will be a market for high-speed craft, despite the fact that perhaps the air-water interface is not the best place for really fast travel, it is by no means certain that any one type will dominate it.

Propulsion devices for high-speed ships pose severe problems. Reasonable efficiency can be

maintained with conventional fullywetted or

non-cavitating propellers at speeds up to

almost 40 knots. At speeds above 40 knots it is extremely difficult to design propellers which

operate efficiently without suffering from

serious cavitation erosion.

Alternative propulsion devices which may be of use for high-speed craft are:

the fully cavitating or super-cavitating

propeller,

the waterjet propulsion system with internal pumps,

(6)

the water-air ramjet, the airscrew, the ducted airscrew.

In this paper the merits of these different pro-pulsion devices for application on high-speed

craft in the speed range of 40-80 knots are

investigated.

As a base for the study a non-amphibious

hovercraft has been considered. This type of

hovercraft, also called a rigid sidewall surface

effect craft, has rigid sidewalls which are

intended to remain in contact with the water,

thus prescribing it to be non-amphibious. For this craft, payloads in the range of 20-180 ton and design ranges of 100,200 and 400 nautical miles (nm) are considered.

The potentialities of the different types of

propulsion devices in combination with this craft are studied. The comparison between the different propulsion devices that is made

includes the following aspects : power requirement,

payload and weight,

economics.

In conclusion the ranges of optimum applica-bility of each propeller type is indicated.

Description of design program

In order to determine the effect of the choice

of the propulsion device on the power

re-quirement, the weight and the operating cost

of the vehicle, detailed information on the

characteristics of the vehicle must be available.

Simple preliminary design calculations must

be carried out for each configuration. The

3

Am--4 I I rd I I WWI I

in let

design calculations result in data on the

prin-cipal dimensions, the power to be installed,

weight, speed, endurance and performance in rough seas, etc. These factors in turn, influence

the costs of the hull, machinery equipment,

fuel and crew.

To facilitate the large number of calculations

required for each preliminary design, a

pro-gram was developed for use on NSMB's, CDC

3300 computer. When a computer program

for (ship)-design is available, it may be used as a measuring device to determine the effect of

the important design variables. In this case

special attention was paid to the choice of the propulsion device.

The design program was to a great extent

based on the data given in (4), (5) and (6). A flow diagram of the program is given in

figure 1. The program includes the following

items.

Calculation of the main dimensions of the rigid sidewall surface effect craft.

Calculation of the lift power or the power

required to maintain the air cushion in a given operating condition.

Estimation of the drag of the vehicle at

different weather conditions or sea states.

These calculations are carried out according

to (4) and (5).

Calculation of the required power for the different propulsion devices to develop the thrust needed to overcome the resultant drag

as estimated under item 3.

The characteristics of the different propulsion devices on which the calculations were based

mixing section

exhaust nozzle

figure 1

Flow diagram of computer program figure 2

Characteristics of 4-bladed supercavitating pro-pellers suitable for high-speed propulsion

figure 3

Scheme of water-air ramjet

are given in a following section.

Estimation of the weight of the different

components of the vehicle (hull, engines, etc.) and the total weight.

Estimation of the building cost of the vehicle

and the capital costs per year.

Estimation of the fixed costs per year. These

costs include the capital costs, insurance costs, wages for two crews, repair and maintenance

costs, costs of supplies and provisions,

over-head costs etc.

The calculation of the weight of the vehicle, the building- and capital costs and the esti-mation of the fixed costs per year have been

carried out according to the data given in (6).

Estimation of the total operating costs per year. It has been assumed that the craft will operate in the southern part of the North Sea

over design ranges of 100,200 and 400 nautical

miles and that the craft will be in service 12

hours per day and 300 days per year. The

harbour time and the effect of the weather conditions on the craft speed and fuel

con-sumption are also taken into account. The results of the economical study are given

in the form of values for the average annual

costs of vehicle operation and of the required

freight rate, RFR, defined as the average

7

2 0.6 as A 0.4 0.3 50 8 06 07 0.4 BAR .7o 0.6 02 as 0 50 11.1. c oo,

4

0.05 o 0 I 111111111111 Illbh. 0 07411V-4 _az, lippri'-lipp.,

4

rig.,;,P

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El

V

0204

Pr

100 200 a. 500 1000 100 200 or- 500 1000 r leg, ducted L., "I'-.'"w II pa wool I

deeign epeed end a go can balibt at which thie cpwwd

be reached at ve Ideo gm name pe ot opu on a, . L-I .,,,_,.c. --m o dlmenaiore lift power drag =7lehwh,,ijwt eter-eir ramjet L _ _ design or . propulsion

Idevice

Pr",g,t1:7 rnr m e i Fr- - - -Gr'''`---'--'---1 e aueuel co t -o 1 performance :=Z1Mreo= I I 0 0

(7)

8

annual costs divided by the annual capacity. Characteristics of propulsion devices

Supercavitating propellers

In order to select optimum supercavitating

propellers for application on the here consid-ered rigid sidewall surface effect craft, it would

be useful if just as in the case with

non-cavitating propellers (see (7)), the results of

tests with systematically varied series of screw

models were available. At this moment these results are not available however, so that use

had to be made of calculated performance

curves.

Important design parameters for super-cavitating propellers are:

the thrust coefficient CT;

CT

-1/20/274D2

where;

T = thrust,

V = speed,

D = diameter,

the stress coefficient a;

1/2ev2 '

where; a = allowable design stress, the advance ratio 2;

V

=

=

it nnD

where;

n = number of revolutions per

second,

J = advance coefficient

the expanded blade area ratio; BAR.

Systematic calculations of performance and efficiency have been made by Barr (8). From

his results

the optimum propeller design

parameters, from the viewpoint of efficiency were determined for four-bladed propellers.

The results are given in figure 2. On a base of

the stress coefficient a and with the thrust

coefficient C, as parameter, the expanded

blade area ratio BAR, and the advance

coefficient 2 are given for maximum propeller

efficiency. In addition, the corresponding

maximum propeller efficiencies are given.

These results were used for the selection of optimum supercavitating propellers for the

rigid sidewall surface effect craft. The effect of

shaft inclination on thrust and the drag of

shaft and struts are also taken into account in the design program.

In the calculations it was assumed that the

vehicle is fitted with two supercavitating

pro-pellers and that the propeller material has an allowable design stress of 25,000 psi. This

figure is typical for supercavitating propellers. The effect of large hump speed thrusts, which may be required for the fixed sidewall surface effect craft, has not been taken into account in figure 4

Relationship between thrust coefficient CT, efficiency -q, and speed V of a waterramjet

figure 5

Relationship between volume ratio q thrust

co-efficient CT and speed V of a waterramjet 0.64 an 056 052 0.48 0.44 0.40 4

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(8)

the design of the supercavitating propellers. At off-design conditions (bad weather con-ditions) it is assumed that the thrust of the propellers at full power remains the same as

in the design condition. Waterjet

The design of the waterjet propulsion system

with internal pumps was based on the data

given by Contractor and Johnson (9). For the

thrust T, the overall efficiency rite, and the

power P of the waterjet propulsion system the following relations hold:

(1 K)

TV

P =

where; D is the diameter of the jet,

Vietis the jet velocity, and

lip is the pump efficiency.

The head loss of the inlet of the system and the

internal head loss are taken into account by the loss coefficient K. It is assumed that the approach flow into the inlet of the system is uniform and that the velocity is equal to the

vehicle speed V.

In the calculations with the design program it

was assumed that the rigid sidewall surface effect craft is fitted with two waterjet

pro-pulsion systems. The internal pumps of these

systems are assumed to have an efficiency = 0.80 and the loss coefficient K for the two systems is assumed to be K = 0.25. An

estimate of the weight of the water in the

systems has been made and is taken into

account in the weight calculation of the vessel.

It is assumed that the manoeuvrability of the

vessel is achieved by directing the jets. Thus, no rudders are needed resulting in a reduction

of the total drag of the vessel. The waterjet propulsion systems are designed for the

cruising condition of the craft and, once again,

no attention has been paid to the "hump

resistance- condition. It is assumed that at

off-design conditions the thrust of the system at full power remains the same as in the design

condition. Waterramjet

An application of two-phase flow for naval purposes is found in a special type of

pro-pulsion; the water-air ramjet. A scheme of the

system is given in figure 3. Water enters a

diffusor at an undisturbed stream velocity. In

the diffusor the kinetic energy of the flow is

figure 6

Relationship between pressure ratio and volume

ratio q of a waterram jet figure 7

Characteristics of 4-bladed airscrews figure 8

Characteristics of 3-bladed shrouded airscrews

18 1,6 1.0 0. 0. 0, 0 .8 01 So 06 05 03 02 01 FLOW PARAMETERS

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(9)

10

figure 9

Shroud profile of ducted airscrew figure 10

Specific power of an 80 ton payload hovercraft

fitted with different propulsion devices

figure 11

Specific power of different types of high-speed craft figure 12

Weight ratios W' W and W, + W1/ W of the rigid

sidewall surface effect craft

1. VW 11 + WP=160 tons x =80 tons o .40 tons o .20 tons silverleaf

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---WP .410 f

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IreoHt.Pton) .G., .."..., 10 20 vi,/6,ktna,i 30 s 0 0.8 0.6 Wp 0.4 02 0 12 -...,..., 14/P. 160 60 Wp +Wf ....,...4WP,160. 80---.

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(10)

converted into pressure. The diffusor is fol-lowed by a mixing section in which the high pressure waterstream is mixed with a large

amount of air. The air normally enters through

orifices situated in the wall of the mixing

section. The water-air mixture leaving the mixing section is accelerated in the exhaust

nozzle where pressure is converted into kinetic

energy.

This system is completely analogous to the air ramjet. In both cases the thrust is developed by

the reduction of the density of the working medium after the pressure has been raised in

the diffusor. In the air ramjet this density

reduction is realized by combustion, whereas in the waterramjet this reduction is caused by the introduction of gas bubbles into the mixing

section. Due to this lower density, the

ex-pansion in the exhaust nozzle to the ambient pressure results in an exhaust speed that is

higher than the undisturbed stream speed.

In the absence of systematic test data for

water-air ramjets, calculated performance curves are necessary in determining optimum waterramjet design parameters and in making preliminary performance estimates. Extensive

investi-gations on the water-air ramjets have been

performed at the NSMB. This work was

started by Van der Walle (10) in 1963. In this

paper use is made of the results of experimental and theoretical investigations of Van Gent (11).

Important design parameters for the water-air ramjet are:

the thrust coefficient CT;

CT = 1/2 v- 7/4D

where;

T = thrust,

V = speed,

= diameter of mixing section. the ratio between air- and water volume in

the mixing section q1,

the ideal efficiency ;7 i;

TV

= N

where; Ni= power delivered by the

com-pressed air if expanded

iso-thermally to the ambient

pressure.

A complete set of design parameters for a

ramjet with a converging exhaust nozzle is

given in figures 4, 5 and 6. The outlet to inlet area ratio of the nozzle is 0.70. At low speeds

the flow in the nozzle is subcritical, while at higher speeds the flow is critical. Due to the absence of a diverging part of the nozzle the

flow is not completely expanded to the ambient

pressure. The range of ch values which is

possible from the hydrodynamical point of

view lies between 0.2 and 2.8. On a base of the

thrust coefficient and with the speed V as

figure 13

Weight ratios Wyl W of conventional ships,

hover-crafts, helicopters and airplanes figure 14

Economics of hovercraft fitted with supercavitating

propellers 11

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0 o 50 100 V (knot ) 150 200 250 13 2.0 15 RFRA,-1.0 0.5 0 40 14 supercavitat design range ing propeller 100rim

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(11)

12

parameter the ratio q, and ideal efficiency IL

are given in figures 4 and 5 respectively. The flow parameters from which the necessary air

volume flow rate and the exhaust pressure of

the air supply system can be derived are given

in figure 6 on a base of sql, where s is the mixing efficiency.

In the calculations with the design program it

was assumed that the compressor delivering

the air has an efficiency of Ni1shaft HP =0.75.

Further, it was assumed that the vessel is

fitted with two water-air ramjets. The ramjets

were designed for the cruising speed of the

vessel. In addition, it was assumed that at off-design condition the thrust of the system at full power remains the same as in the design con-dition.

Airscrew and ducted airscrew

For the design of the airscrew and the ducted

airscrew use has been made of the results of

tests with systematic series of unshrouded and

shrouded propellers as given in (12) and (13)

respectively. The results of these tests are

presented in figures 7 and 8. In these diagrams

the thrust coefficient Cr and the efficiency q,

are given as a function of the advance coefficient

J with the pitch angle /3 of the screw as

para-meter. In the case of the ducted propeller the thrust coefficient CT was based on the total

thrust of the system or the sum of the propeller

and nozzle thrusts. The shroud profile of the

ducted airscrew is given in figure 9.

In the design program the following restrictions

were made in the choice of the diameter and

the RPM of the airscrew and the ducted

airscrew.

The tip speed of the propeller must be lower

than 0.8 of the sound velocity of air at

at-mospheric pressure. If the velocity relative to the propeller exceeds the critical velocities

of the airfoil sections an ensueing loss in

propeller efficiency will be found.

The diameters of the airscrews are limited

by the dimensions of the vessel.

In the design calculations it was assumed that

the vessel is fitted with 1, 2 or 4 airscrews. These screws were designed for the cruising

speed of the vessel and do not reflect the effect

of large hump speed thrusts. It was assumed

that the blades of the airscrews were adjustable.

Consequently, at off-design conditions (bad weather conditions) the screw propellers can absorb full power at the same RPM as in the design condition and the increase in thrust

can be determined with the aid of figures7 and

8 for respectively unshrouded and shrouded

airscrews.

Results and conclusions

As already mentioned, the comparison of the potentialities of the different propulsion de-vices was considered in combination with a

rigid sidewall surface effect craft. The ranges figure 15

Economics of hovercraft fitted with waterjet pro-pulsion systems with internal pumps

figure 16

Economics of hoN ercraft fitted with water-air ramjets

2.0 waterjet design range 100nm 1.5

11

R F RA 1.0 05 rH flAan nm ) ,:i.`411°

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9 0 50 60 70 80 90 VI knot ) 40 15 20 1.5 RFRA 1.0 0.5 040 16

waterair

design

ramjet

range 1013 nm (Hfl/ton.nm) A4q o

o'

50 V (knot ) 60 70 80

(12)

of design parameters considered being:

payload IV, , Wp 20-160 ton

speed V

, V

-- 40- 80 knots

design range, - 100-400 nautical miles.

It is noted that a design range of 100 nm is

typical for service over the Channel. The com-parison included the following items:

power requirement, weight and payload, economic factors.

With respect to the results of the power

re-quirement calculations the method of

presen-tation used by Crewe (3, 4), Silverleaf (1),

Silverleaf and Cook (2) and Van Manen (14)

has been followed. Here the overall hydro-dynamic efficiency or the specific power of

the craft is given on a base of the speed

coeffi-cient. These coefficients are defined as follows:

overall hydrodynamic efficiency or specific power = PIWV,

speed coefficient = VIW116,

where; P = total power (in metric HP),

W = weight of craft (in metric

tons), and

V = speed (in knots).

The speed coefficient may be regarded as a

form of the Froude number used in many

ship-powering analyses.

The power requirement for an 80 tons payload,

rigid side wall surface effect craft fitted with

successively super-cavitating propellers,

water-jet propulsion system with internal pumps,

water-air ramjet, airscrew and ducted airscrew

is given in figure 10.

For comparison purposes

the power

re-quirement of the here considered rigid sidewall surface effect crafts with payloads in the range

of 20-160 ton and design ranges of 100-400

nautical miles are given in figure 11. In addition

the power requirement of semi-displacement

crafts, crafts with surface piercing or

sub-merged foils and

amphibious and

non-amphibious hovercrafts according to Silver-leaf (1) and SilverSilver-leaf and Cook (2) are also

given in this diagram.

It is

noted that conventional ships have

specific powers in the range of 0.01-0.05 HP/ton

knot. The value of the specific power of the

different types of high-speed craft is at least 20 times the specific power value of conventional

ships. Due to the much greater power

re-quirements of the different types of highspeed craft it may be concluded that these crafts are essentially suitable for short range operations.

Further, it can be seen that the power

re-quirement for the rigid sidewall surface effect craft (which is a non-amphibious hovercraft) agrees very well with the data given by

Silver-leaf and Cook.

The weight ratios WpIW and TY, W11W for the considered rigid sidewall surface effect crafts with payloads in the range of 20-160 ton, speeds in the range of 40-80 knot and a design figure 17

Economics of hovercraft fitted with airscrews

figure 18

Economics of hovercraft fittedwith shrouded

air-screws 13 2.0 1.5 R F Rh,m 1.0 0.5 0 17 airscrew design range 100 nm - ( H fliton.nm)

0

* 160 50 60 70 V (knot) 80 90 2.0 1.5 RFRinm-(H 1.0 0.5 0 30 18 shrouded design range 100nm air_screw fl/ton.nm I

Mid

..ijopdllrAl

Mil

Al

.

wr

...

---. \b° 40 50 V (knot) 60 70 BO

(13)

range of 100 nautical miles are given in figure

12. W,,, W1 and W denote respectively the

payload, the weight of the required amount of fuel (for a design range of 100 nm. with a spare

of 40 percent) and the total weight of the vessel.

From this diagram it can be seen that at low

payloads and high vessel speeds these weight

ratios become less serviceable. In figure 13 the payload-weight ratios Wp/ W of

con-ventional ships, hovercrafts and aircraft according to Crewe and Eggington (4), and of

the considered rigid sidewall surface effect

crafts are given.

The choice of the type of high-speed craft for a specific application as well as the propulsion

devices which are to be used, will in first

in-stance be selected for economic reasons rather than for technical efficiency alone. The econo-mics of different commercial crafts can best be

compared on a base of the total operating

costs or the average annual costs. Here the

results of the economic analysis are expressed in the form of the required freight rate (R FR) divided by the design range of the vessel. The required freight rate is defined as the average

annual costs divided by the annual capacity.

Thus the results of the economic analysis are expressed in the costs (in Dutch guilders) per ton nautical mile.

The economical data, calculated with the

program developed for the rigid sidewall

surface effect craft, are given in figures 14

through 18 for respectively the supercavitating

propellers, the waterjet propulsion systems

with internal pumps, the water-air ramjet and

the unshrouded- and shrouded airscrews. For comparison purposes the results with the

different types of propulsion devices are given

in figures 19, 20 and 21 for vessel speeds of respectively 60, 70 and 80 knots. From the

fact that the calculations for the different

propeller types have been made with the aid

of a high-speed ship design program, in which design conditions and assumptions have been kept constant, the diagrams have a high value

from a view point of comparison of those

different propeller types.

For this reason the following conclusions can be drawn.

At lower vessel speeds the super-cavitating propellers

and the

waterjet propulsion

system with internal pumps are the best

whereas at higher vessel speeds the airscrew become more favourable.

From an economical point of view there is

no reason for surrounding the airscrew by a shroud of nozzle at higher vessel speeds. Over the whole range of considered design

speeds the performance of the hovercraft fitted with a water-air ramjet is moderate. This fact can be explained by the relative

low efficiency of the air compression process

needed for this propulsion system. The

efficiency of the water-air

ramjet can

probably be increased by supercharging.

figure 19

Economics of a hovercraft fitted with different

pro-pulsion devices designed for a speed of 60 knots figure 20

Economics of a hovercraft fitted with different

pro-14 pulsion devices designed for a speed of 70 knots

19 2.5 2.0 RFRA-, 1.5 1.0 05 00 speed V-supercavitating 60 knots airscrews waterjets water-air airscrews ramjets propellers ducted

--

---

..

--(Hflitom)

I I I 1 k

\

\

\

\

\

N. ...._

-.m,-.--..-,_-_---50 100 wp (ton) 150 200 20 2.0 1.5 1.0 RFRk, 0.5 0

25

i I speed V .70 knots waterjets water-air airscrews ducted supercavitating I ramjets atrscrews propellers I 1 \

--

-

----1 k 1 1 \ \ \ \

\

\ \

\

\

\

\

( H fLit mom ,

\

.

\\\.

\

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.

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...,_ --....___ . .._---0 50 100 w (t)on P 150 200

(14)

figure 21

Economics of a hovercraft fitted with different pro-pulsion devices designed for a speed of 80 knots

When a final selection has to be made, i.e.

whether air or water propulsion should be

applied, the aspect of low speed ship handling operations may play a role.

Acknowledgement

The authors wish to thank the members of the staff of the Netherlands Ship Model Basin for

their contribution to the preparation of this paper. Especially the help of Mr. J. Holtrop

has been greatly appreciated.

References

Silverleaf, A., "Developments in high-speed

marine craft," 42nd Thomas Lowe Gray

Lecture, Inst. Mech. Eng., January 1970. Silverleaf, A. and Cook, F. G. R., -A com-parison of some features of high speed marine

craft,- Trans. R.I.N.A., 1970, vol. 112. Crewe, P. R., "The Hydrofoil Boats; its history and future prospects," Trans. I.N.A.,

1958.

Crewe, P. R. and Eggington, W. J., "The Hovercraft. A new concept in marine trans-port," Trans. R.I.N.A., vol. 102, no. 3, July

1963.

Crewe, P. R. and Eggington, W. J., "Esti-mation of power and drag for marine

hover-craft," Hovercraft Unit Report 10, September 1969.

Miller, E. R., "An analysis of rigid sidewall

surface effect craft for high-speed personnel

transportation," Chesapeake Section, SNAME

April 4, 1968.

van Lammeren, W. P. A., van Manen, J. D. and Oosterveld, M. W. C., -The Wageningen B-screw series,- Trans. SNAME, 1969.

Barr, Roderick A. "Super-cavitating and super-ventilated propellers," Trans. SNAME

1970.

Contractor, D. N. and Johnson, Virgil E. Jr.,

"Waterjet propulsion," AIAA/SNAME

Ad-vanced Marine Vehicles Meeting, 1967. v. d. Walle, F., "Theoretical derivation of

the performance and geometry of a

water-ramjet," NSMB Lab. Memo nr. 8, 1962.

van Gent, W., "Investigations on the

characteristics of an air-water ramjet," to be

published.

Hartman, E. P. and Biermann, D., "The

aerodynamic characteristics of full scale

pro-pellers having 2, 3 and 4 blades of clark and RAF 6 airfoil sections," NACA Report 640,

1938.

Black, D. M., Wamanski, H. S. and

Rohrbach, C., "Shrouded propellers-A com-prehensive performance study," A1AA, 5th

Annual Meeting and Technical Display, 1968, paper 68-994.

van Manen, J. D., "Size, type and speed of

ships in the future," 3rd Symp. on Naval

Hydrodynamics, 1960. 15 1 2.5 20 15 RFR/nm/ 1.0 0.5 0 \ \\ \ 1 \ speed I V.80knots supercavitating waterjets water-air airscrews ducted airscrews propellers ramjets i i i \ \ \

\

--

-

----\\

---\

\

\

\ 1 \ .1 \

\\

\

\

\\

\

(H / - fiton

\

\

\

\

\\\\\

\

\

\

\

\

\

i.nrn)

\

\ \

\

N

\ \

\

N.

11.1. I

,...

----... . -.

Miro

0 50 100 Wp (ton) 150 200

(15)

16

High speed motors

and

gas turbines

When I was given the invitation to read this paper to the Congress, I accepted, unhesitat-ingly, for two reasons. Firstly, I was

appre-ciative of the honour done me and secondly I was smugly secure in the knowledge that I had,

a "few" years previously, written a series of

articles for a British marine journal, on a very similar, albeit somewhat more restricted topic

(Mitchell 1956). However, when I came to

look up my previous effort, two things became

abundantly clear, a) the "few" years ago was

fourteen and b) in that intervening period,

progress has been so rapid and changes so

complete as to make the earlier articles almost useless as a basis for this paper!

At first, it was my intention to present you with a survey of current practice but as my thoughts

matured, I came to the conclusion, that you could just as well consult the current engine

catalogues as I could and so the idea developed

of indulging in rather

more "crystalball

gazing- hoping thereby to stimulate discus-sion and get confirmation or otherwise that,

that section of the boat industry here today, is

happy with the direction in which its propulsion

units are likely to develop and progress in the foreseeable future.

The power range chosen

is 100 s.h.p. to

1000 s.h.p. - somewhat arbitrary limits

I

agree but I think, covering the requirements of

almost all but the smallest pleasure craft and

a very wide range of small and medium sized work boats as well.

Starting from the lower horse power end of the

range, I find, consulting my earlier article,

that I was then discussing the gradual demise

of the "tailor made", small-quantity

produc-tion, hundred percent specialised marine diesel

engine and its replacement by the converted,

high speed, mass-produced, automotive type,

diesel engine. This trend has certainly conti-nued, in fact accelerated, until today in the horse power range 100-400 the latter type of

engine is almost universally applied to marine propulsion. In this context, the American term

"automotive type" is

applied to a

multi-cylinder engine of multi-cylinder bore not more than 6" diameter, with maximum power developed

at not less than about 90 p.s.i. (6.3 kg/cm2) b.m.e.p. and 1700 r.p.m. crankshaft speed.

The specific weight less transmission and

cooling system but including electrical

equip-ment is of the order of 10-12 lb/b.h.p. (4.5-5.5 kg/pk). The reasons for this continuing swing to the automotive type engine are not

hard to seek.

Although, over the years, the marine engine of

say 900-1000 r.p.m. and 40-50 lb/b.h.p.

(18-23 kg/pk) used for pleasure and work

boats alike had made for itself an outstanding

reputation for reliability and fitness for its

purpose, the cold winds of modern economics, if nothing else, have almost literally blown it into obsolescence. Firstly, there is the question

of price. Because of the large quantities pro-duced for commercial automotive applica-tions, - in the larger factories up to 7,000 per

week, automotive type engines have been sub-jected to the most intensive production devel-opment. As a result, the selling price, complete with reverse-reduction gear is remarkably low

by R. W. Stuart Mitchell

University of Technology

Department of Mechanical Engineering

(16)

and of course, extremely competitive. Secondly,

there is the aspect of service and spares. The automotive application has resulted in the

engine manufacturers or their agents

main-taining world wide service organisations and spares are, again because of quantity produc-tion, relatively cheap. Thirdly, there is the light weight and small bulk resulting from the high

rotational speed of the engine and its high

specific output. Lastly, but certainly not least

in importance, is the fact that, concurrently with the above mentioned production

devel-opment, there has been equally intensive

technical development, resulting in engines today, just as reliable and long lived as their slower speed, heavier counter parts of 15-20

years ago.

It must be appreciated however, that the

marine application of those engines is a

con-version. In fact, it has given rise to the verb

"to marinise" and such "marinisation-

is

frequently carried out by firms who take stand-ard automotive type engines, from the

manu-facturers and make the necessary

modifi-cations and additions for marine service. The

skill and experience with which this conversion

is made can make or mar the marine propul-sion application of a particular automotive

engine.

One of the more important differences between

automotive service and marine service concerns

the engine rating. In the former, the engine is not called upon to develop its full rated power and speed, except at intervals and for relatively short periods; furthermore, high specific

output and in particular, a high power to

weight ratio are essential to help give the

rapid acceleration required. In both types of service, but even more especially in marine, absolute reliability is frequently, the domi-nating characteristic. Marine engines have little demand for rapid acceleration but full

power must be capable of being sustained for

long continuous periods. Automotive type engines, when applied to marine service, are therefore derated both in b.m.e.p. and

crank-shaft speed.

A few words of explanation in the

inter-pretation of the marine engine performance

curves may be helpful here. In ship propulsion

(with constant engine/propeller speed ratio) the horse power, absorbed by the propeller will vary approximately as the cube of the

propeller r.p.m. and the torque approximately as the square of the r.p.m. This required engine

power is, up to the point where the engine is matched to the propeller, considerably less than the rated power the engine is capable of

developing at a given r.p.m. This latter, is the curve usually quoted as the engine power curve

and for marine propulsion will be either the "continuous" or "12 hour" rating, whereas for the automotive application it will appro-ximate to the maximum power at full throttle. Figure 1 shows typical performance curves for marine service for an engine of the type under discussion whilst the automotive performance of the same engine is shown for comparison.

A mention of the method of recording fuel consumption may be of interest. The engine designer or development engineer thinks in

terms of specific fuel consumption which is

the inverse of thermal efficiency and measured

in weight of fuel consumed per b.h.p.-hr. (in

automotive engines because they all normally

burn fuel to the same basic specification,

volume is frequently substituted for weight). However, specific fuel consumption is of little practical interest to the boatman or yachtsman

anxious to know the possible range of an

afternoon's sailing. Thus has arisen the custom of quoting volume per hour for different power settings of the engine.

The derating will of course increase the

spe-cific weight which for the bare engine may in-crease to the order of 16 to 171b/b.h.p. (7.3-7.7

kg/pk) whilst the addition of the necessary marine accessories such as pumps, marine

cooling system, reverse-reduction gearbox etc.

will bring the complete "machinery" weight up to something in the region of 25 lb/b.h.p. (11.4 kg/pk). This is

still half or less the

specific weight of the earlier "pure" marine

engines.

In the marine conversion, an important feature

is the cooling system adopted. The earlier medium speed engines frequently had direct sea water cooling, no doubt because of its

simplicity. With the modern high speed engine, this has been replaced by closed circuit fresh

water cooling, notwithstanding the necessity

to provide one or two pumps and a heat

exchanger. The latter usually takes the form of a sea water cooled pump circulation type but may be a "keel cooler" which consists of a tube or nest of tubes, fitted externally to the hull of

figure 1 Turbocharged engine 1 Sa: O.' 250 200 150 100 50 ,la . 650 600 550 500

li

-.2

I

40 .38 .36 DIESELMOTOR - DIESEL 6 cyl. 5.00o5.71 in 673 ENGINE coin lot MIR S,ttrng

illfarAlIGEMI

AirAita.°101.1..."

%

1

,

4._

e

e

-... Speedr.P.,,,, 800 1000 1200 1400 1600 1800 2000 2200 The curve for continuous service conforms to Dauer- Curve for intermittent service conforms to the 10 7.-leistung 8 of DIN 6270 in cotes where engine is used overlood-curve according to the lost-mentioned per-without forexists, it olso very closely conforms to 8.5. 60.1958.Where satisfactory overload protection to or standard. It also conforms to SMUT No IR.

the boat. Its major advantage is that it

elimi-nates the need for a sea water circulating pump but against this is its relatively low efficiency and vulnerability to damage. The closed circuit cooling system is thermostatically controlled to give rapid warming up. Overcooling or slow

warming up of diesel engines is now recognised

as a primary cause of excessive cylinder bore

wear and of piston ring troubles. Direct sea

water cooling is conducive to these two cooling

defects and this, together with its potential for corrosion of and deposits in, the cooling water

jackets and passages has led to its almost

complete disappearance from the modern

engine installation. Figure 2 shows a typical system as used for the converted automotive type engine. Air cooling, at first sight

attrac-tive, seems to have made relatively little prog-ress over the years.

The greater number of high speed diesel engines

for marine service, in the power range 100-400 s.h.p. are four stroke engines. This once again

emphasises the automotive background of

those engines and contrasts with the situation

at the other extreme of marine diesel engine application i.e. the very large, slow speed, "cathedral", type of engine where to the best

of my knowledge there is no four stroke engine

in

production because marine propulsion

conditions "per se- are favourable to the two stroke system. Speaking generally and one

(17)

18 3 SALT WATER rHERMOSTAT ENGINE OIL FILTER EXHAUST

-11. SCAVENGE AND INLET FRESH WATER MANIFOLD UNIFLOW WITH POPPET VALVE SALT WATER PUMP FRESH WATER SALT WATER OIL

In afresh-water closed-circuit cooling system sea water is pumped through a heat-exchanger which replaces the normal automotive air-cooled radiator. A heat-exchanger unit for the lubricating oil is-also incorporated. In this example gear-box cooling is effected by leading the sea-water intake through

a water jacket 2 LOOP FRESH WATER HEADER TANK COOLING TUBES OIL COOLER TO FRESH WATER PUMP POWER TAKEOFF

°

1

UNIFLOW WITH OPPOSED PISTON

ii

. CROSS, LOOP & UNIFLOW SCAVENGE SYSTEMS

of successful high speed automotive type two stroke engines, the characteristics required of

the automotive application are more difficult to satisfy with a two stroke engine, primarily

perhaps with respect to specific fuel consump-tion. Again, the high speed two stroke engine is relatively expensive to manufacture, even in large quantities, and therefore has difficulty in

competing, in terms of price, with the

mass-produced four stroke. From the technical

aspect, the performance of a two stroke engine

is to a large degree dependent upon the form of cylinder scavenging adopted. "Uniflow" scavenging (figure 3) has produced the best

results and the highest b.m.e.p.'s but it requires

exhaust valves in the head or the opposed

piston form. In the high speed engine, the valve

gear design problems of the former arrange-ment, may be difficult to solve remembering that the gear has to operate at engine speed.

In the latter arrangement, problems of overall dimensions may intervene, since for high speed operation, the double crankshaft arrangement has been the one most favoured. Nevertheless

the British Admiralty has had satisfactory

marine service for many years from a high speed two stroke valve-in-the-head engine, the latest example of which, in 12 cylinder

double bank form, develops 210 b.h.p. at 1800 r.p.m.

Another British manufacturer has in

produc-tion for automotive, marine and industrial

applications an opposed piston, two stroke

engine with one crankshaft and a linkage

system employing bell-crank levers. The

marine version is rated at 75 b.h.p. and 1800

r.p.m.

Mention has been made of the vital importance of good specific fuel consumption in the auto-motive engine. This has led, in recent years for

this type of engine, to the ever-increasing

adoption of the "open" direct injection

com-bustion chamber, pioneered by British engine

builders, in contrast to the separate chamber indirect injection systems more favoured in Germany and the U.S.A. The characteristics

of the fuel injection equipment for the "direct" and "indirect" systems are somewhat different. The direct system requires a multi-hole nozzle

with a relatively high injection pressure, is

sensitive to condition and thus requires a high standard of maintenance. The indirect systems

have much easier fuel injection conditions,

lower injection pressure and a single hole

pintle nozzle suffices, with the further

advan-tages of a lower rate of wear in the injection system and less power required to drive the

fuel pumps. The pintle nozzle is less sensitive

to carbonisation than the multi-hole type and

can tolerate a lower standard of maintenance.

The open combustion chamber gives a lower

fuel consumption, primarily because of the air

pumping losses of the separate chamber sys-tems, gives a lower rate of crankcase fouling

particularly in high speed engines, but is much

more sensitive to the quality of fuel which it

can satisfactorily digest. Separate combus-tion chambers are usually quieter as regards

combustion noise and whilst the compression

figure 2 figure 3

(18)

pressure is higher, the lower maximum cylinder pressure obtaining, contributes to

overall smoother running.

One of the most important features of any marine engine is its ability, or otherwise, to

cold start. In the diesel engine, the fundamental requirement for starting is that the temperature of the air after compression should be higher

than the spontaneous ignition temperature of

the liquid fuel. This in turn depends upon the

compression ratio employed and the rate of

heat loss to the cylinder and combustion

chamber walls during compression. Allowance

must also be made for the fact that engine crankshaft speed, at the instant of starting is

very low, probably of the order of 100 r.p.m.

The small engine, because it has the larger surface/volume ratio, dissipates heat more

readily than the larger engine.

This has advantages in certain respects but from the point of view of satisfactory

oper-ation and starting calls for a higher

compres-sion ratio. The form of combustion chamber

has an important influence and the open type

loses least heat. It therefore gives easier cold

starting, other things being equal. Thus, whilst the small high speed "direct" injection engine

may operate at a compression ratio of say

15 : 1, the corresponding -indirect- injection

engine will require a ratio of 20 : 1 and will normally have to be provided with some form of starting aid - a heater plug or valve or screw plug to increase the effective compression ratio temporarily, during starting.

Small high speed engines, up to say 41"

(114 mm) cylinder diameter may be hand

started. This however, in marine engines, is

always a matter of some difficulty. because the

disposition of the engine in the boat necessitates

building up the starting handle on some form of "tower" and connecting it to the crankshaft,

usually by chain. It may still, however, be

difficult to exert adequate effort on the handle

and movement of the boat, even in relatively

calm water does not help.

Earlier prejudice against electrical starting is

fast disappearing and this is now the normal method for the smaller high speed engines. Care must be taken in the installation, with the location of the electrical equipment and

usually 24 volt systems are used. Air starting

motors, supplied from stored compressed air enjoyed some vogue a few years ago. Many

boat owners and some engine builders, in their constant quest for the highest possible degree of reliability are still unhappy at the possibility of storage batteries becoming run down or air

bottles exhausted and from time to time,

forms of self-sufficient hand operated systems appear on the market e.g. inertia or hydraulic

but these never seem to 'catch on' and there does seem to be a presently unfulfilled need

for really satisfactory and reliable equipment of this nature.

The converted automotive type engine is in-variably of the underslung crankshaft

con-struction, with the main bearings supported in the crankcase and the sump an unstressed com-ponent containing the lubricating oil. This is in

contrast, to the earlier medium speed type of

engine, with its deep section bedplate carrying

ENGINE EXHAUST TO TURBO EXHAUST FROM TURBO

C=E

NOZZLE SEGMENT

0

BLOWER IMPELLER NOZZLE SEGMENT ENGINE EXHAUST TO TURBO

the main bearings, crankshaft, oil sump etc. which in conjunction with a separate short

crankcase and frequently, individual cylinders,

allowed major overhauls to be carried out,

without removing the engine from the boat. At

first, the alternative construction was

con-sidered to be disadvantageous, necessitating as it did, removal of the engine for overhaul but

very quickly it was realised that, the lightweight

and small bulk of the engine type and the ready availability of service facilities at commercial

vehicle garages discounted earlier fears and the monobloc construction of crankcase and cylinder block with its much greater rigidity contributed to better oil tightness and con-sequent general engine cleanliness.

Never-theless, marine engine conversion firms must pay due attention to accessibility and routine maintenance facilities, "in situ".

An important feature found on a number of

high speed marine engines is the "handability" of the main structure. Thus, auxiliaries can be fitted to either side of the block and crankcase end covers can be changed, end for end. This facilitates the production of port and starboard

engines and is particularly valuable in twin screw installations or where two engines are

geared to one propeller shaft.

An installation feature which must be borne in

mind in the marinisation of the automotive

type of engine is its ability to run satisfactorily when installed with any required angles of heel

and list. This is frequently particularly im-portant for the smaller pleasure boat instal-lations and may call for modifications to the

AIR INTAKE MUFFLER

BLOWER DISCHARGE

TO ENGINE INLET VALVES

EXHAUST FROM TURBO

Turbo blowercentrifugal type driven by engine exhaust

4

figure 4

(H.M.S.O. 1962)

General arrangement engine room

automotive engine

crankshaft and sump

arrangements.

In the diesel engine field in general and more

especially in the medium power, medium

speed/high speed, engine ranges, competition between the four stroke and two stroke cycles

of operation is, more or less, as old as the

industry itself. This is so, because in the

com-pression ignition, fuel injection, type of engine,

the two stroke engine comes much closer to

fulfilling its theoretical superiority in specific output, than it does, for instance, in the spark ignition, carburettor, type of engine. Over the

years, this "cycle" competition and contro-versy has remained consistently acute and topical and the position, in these respects, is today, little different from what it was, say

forty years ago, although there have been

relatively short periods when one

or other

cycle appeared to be in the ascendant. Without

doubt, one of the most important technical

features which has kept the four stroke cycle in

the race has been exhaust gas turbocharging

(figure 4). This has a background dating back to about 1905 when Dr. Alfred Biichi took out his

first patents but had perhaps its most

spectacular development period in the decade

immediately following the end of World War II.

Today, in the medium power, medium speed/

high speed engine range, the turbocharged version is the rule rather than the exception

BLOWER IMPELLER AIR INTAKE

(19)

and turbocharged four stroke engines are in

production with brake mean effective pressures

up to 250 p.s.i. (18 kg/cm2) and two stroke

engines up to 160 p.s.i. (12 kg/cm2). It might be

added that the successful turbocharging of two stroke cycle engines has a much shorter history than that of four stroke cycle engines and can

be said to date back no longer than the last

15-20 years.

However, for marine engines, the above picture

is somewhat distorted at the two extreme ends

of the power range. At the top end, with the

slow speed engine, the successful turbocharging

of the two stroke cycle has consolidated this

type of engine for high power installations, to

the complete exclusion of the four stroke. At the lower end, which we are presently

dis-cussing,

the automotive application

back-ground of the engines concerned has retarded until very recently the satisfactory turbocharg-ing of the four stroke cycle and as has already been stated, greatly limited the use of the two stroke cycle. Two or three basic problems have made the automotive application less amenable

to successful turbocharging than larger non-automotive engines. First and foremost was

the early difficulty of achieving a satisfactory

traction type torque-speed relationship i.e.

small variation in torque values over a wide

engine speed range, maximum torque occurring

at an engine speed well below the maximum power speed and a "drooping- torque speed curve between these two engine speeds.

Secondly, was the problem of lack of

acceler-ation, due to the high inertia of the then con-ventional turbocharger rotors. Thirdly, there

were the economics and space problems. Asthe

engine becomes smaller, the cost and size of

the turbocharger assume proportionately

higher relative values. This meant that in the

early days of automotive engine turbocharging

- say 20 years ago -it was frequently cheaper and less space consuming, to buy a six cilinder

version of a particular engine range than to turbocharge the four cylinder version.

How-ever, in the period of say, the lastfifteen years,

automotive engine turbocharging has been

steadily gaining ground, as solutions or partial

solutions have been found to the problems, albeit more slowly and with much lower

in-creases in specific output than was the case in

the larger engines. This has been reflected in the

marine conversion world where

turbocharg-ed engines with specific outputs of 25 %-30 %

higher than the naturally aspirated, say 100

p.s.i. - 110 p.s.i. b.m.e.p. (7-7.7 kg/cm2) have

been on the market for some time, andspecific

outputs have been gradually increasing to

120 p.s.i. - 130 p.s.i. b.m.e.p. (8.4-9.1 kg/cm2). At the same time, crankshaft speeds have been slowly increasing, as simultaneously, the com-bustion problems associated with the "square" engine have been better understood and solved and similarly with cylinder gas flow and

lubri-cation phenomena, which have raised mean

piston speeds.

Very recently, in the last two or three years,

turbocharging of the automotive engine seems

to have surged forward suddenly and new

designs are appearing with specific outputs of

20 the order of 33 bhp/ft3 bulk (1160 pk/m3).

This order of performance has not yet found

its place in the marine world but no doubt will do so in due course.

This improvement in specific performance has not been obtained at the expense of increased

maintenance or lower degree of reliability. In fact, to the contrary, the converted

auto-motive engine can be regarded today as a

totally dependable machine able ro run for

10,000 hours - 15,000 hours between overhauls.

From all the foregoing, it might appear that in the horsepower range 100 b.h.p. - 400 b.h.p.,

the converted automotive engine had put the

specialist marine enginebuilder completely

out of business. This is not so but the great majority of those who have survived have

done so, only because they have changed their policy and are now building engines with most of the features and characteristics of the

con-verted automotive engine and especially its

high level of specific output. The problems of

production quantities and price

competiti-veness for these engines have been tackled by adapting them to marine auxiliary and indus-trial electric generator drives and who knows,

we might one day see them adapted to the

heavier automotive drives such as the heavier

earth moving equipment. The wheel would

then have made a complete turn, because the

first automotive diesel engine in the U.K. in

1929 was aconverted marine engine of 50 b.h.p.!

If now we turn to the higher part of our chosen power range, viz. 400 b.h.p. - 1000 b.h.p. we find a situation almost completely analogous to that of the lower power range and differing in a number of respects only in degree or in the rate at which characteristic trends are

develop-ing.

Broadly speaking, it can be said that the

auto-motive engine of the lower power range

becomes the rail traction engine of the higher

power range somewhat similarly derated for

marine propulsion duties but of higher specific output in terms of b.m.e.p. for the four stroke because of the more highly developed state of turbocharging, as previously mentioned;

b.m.e.p.'s up to 220 p.s.i. (15.5 kg/cm2) are commonly in production and service for this

type of engine. The two stroke engine is very

much more in evidence and this also is

tur-bocharged up to b.m.e.p.'s of the order of

160 p.s.i. (11.2 kg/cm2). Crankshaft speeds are lower than for the automotive engine, being in

the range 1000 r.p.m. - 1500 r.p.m. This is because of the larger bore and stroke

dimensions the former probably in the range 6"dia -9" dia (152-228 mm) with "square" or slightly

"under square- stroke/bore relationship. The

general design characteristics are very similar to those of the automotive engine and most of

what has been stated in relation to the latter type is equally applicable to the rail traction

type of engine and its marine propulsion ver-sion. The word "version" here is used

deliber-ately in preference to "convetsion" because the production quantities are very much less

than for the automotive engine and the normal procedure is for the basic design to be adapted

for marine propulsion by the engine

manu-facturer rather than a supply of engines sent to

an independent company for "conversion"

usually by the modification and or addition of auxiliary equipment. Thus starting in the larger

sizes is sometimes by direct application of compressed air, although this has the disad-vantage of initially chilling the cylinder

con-tents, and if compressed air is used, it is more

often in some form of separate air starting

motor.

The demise of the "tailor made" marine engine,

especially at the higher end of the power bracket

is much slower than is the case in the lower

power range although the trend is equally

evident as is the progressive adaption of high

speed engine design and performance charac-teristics by such specialist engine builders. Having discussed the diesel engines and their performance, currently available and likely to

be so in the immediate future, it is necessary to discuss the transmission problems, in the

first instance, on the use of the fixed pitch

propeller. The first and broadest classification

of transmissions is based on the location of the engine in the boat and designated a)

out-board

or b)

inboard with the combination

c) inboard/outboard sometimes called the

transomor "Z"drive.

For the power range with which this paper is

concerned, viz. 100 s.h.p. - 1000 s.h.p. it is not

proposed to discuss the first mentioned

ar-rangement, because for the most part it

is confined to the lower end of the power range, is not associated with the types of engine which

have been discussed and is applied to the

smaller and lighter types of pleasure craft.

Much the same could be said for the "Z"

drive, except in at least one instance known to

me, where a heavy duty version is available

and can be installed in pleasure craft and work

boats alike up to the top end of our chosen

power range, in a series of nine sizes 40 s.h.p.

-1250 s.h.p. These units are strictly speaking, steerable right angle drives combining two

systems - propulsion and steering. The power transmission which transmits the engine power

to the propeller, incorporates a speed

reduc-tion in the system and allows the use of a high

speed engine, with a propeller r.p.m. close to

the optimum, with respect to the propeller

diameter; a reduction ratio up to 4:1 can be

incorporated. Steering follows from the

capa-bility of the propeller to be turned about its

vertical axis through 360 degrees either to port

or star board and gives a high degree of

manoeu-vrability. In one installation with 1250 s.h.p.

high speed diesel engines and propeller speed 365 r.p.m. it is claimed that the propellers can

be rotated continuously in either direction in

less than 20 seconds for 360 degrees. By turning

the propeller through 180 degrees from ahead, full astern power can be obtained very quickly and stopping time is said to be less than that of a conventional system. Steering control can be

local or remote with electro hydraulic, fully

hydraulic or fully electric power systems for the

latter. The propeller stem can be raised for in-spection and maintenance without drydocking

the boat. Figure 5 shows a possible

arrange-ment of the system in semi diagrammatic

form.

A variation of the system incorporates the

rudder-propeller in a chassis on which the

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