Transactions Scheepstechniek, Koninklijk Instituut van Ingenieurs, Sectie Scheepstechniek

P1985-3

TRANSACTIONS

1985

SECTIE VOCR

2

TABLE OF CONTENTS

H.J. \Nesters

"PREFACE"

A. Morrall

"Ships in breaking waves"

G.H. Elsley, D.J. Hardy

and M. Stevens

"Hovercraft - towards the second

quarter century"

C.A. Carlsen

"Offshore accidents and their impact

on Rules and Regulations"

Rink

_(MARIN)

and W. BetikeIrnan

(TH Delft)

"The SOS systematic series high

speed hull forms"

Jaarverslag

- S.S.T. 1984

Financieel jaarverslag 1984

Lecienlijst

- S.S.T. volgens

september 1985

(chairman K.I.V.I. -S.S.T.)

(N.M.I. - i_td)

(Britisch Hovercraft Corp.

Ltd)

//

SECTIE VOOR

KONINKLIJK INSTITUUT VAN INGENIEURS

SCHEEPSTECHNIEK

VOORWOORD

Zoals bekend mag warden verondersteld heeft het K.I.v.I. de nodige

problemen met "De Ingenieur" gehad, hetgeen een Lange tijd van

zoeken naar oplos,ingen, onderhandelingen daarover, en weer zoeken

naar andere oplossingen en samenwerkingsverbanden tot gevolg had.

Uiteindelijk zijn daarbij het voortbestaan van "De lngenieur" (in

gewijzigde opzet en het "Ingenieursnieuws" gekoppeld aan de

uit-gifte van eon viertal vakbladen op het gebied van Elektrotechniek/

Elektronika, Proces Technologie, Werktuigbouwkunde en Bouwkunde/

Civiele Techniek als resultaat uit de bus gekomen.

Voor de leden van de sektie Scheepstechniek (en niet alleen voor deze)

betekende dit dat voortaan binnen het K.I.v.I. geen passende

uitlaat-klep meer aanwezig zou zijn voor wat langere vaktechnische

verhande-lingen op typisch Maritiemtechnisch gebied.

De sektie voor Scheeps Techniek kon als kompensatie daarvoor trachten

zelf jets op publiciteitsgebied te organiseren waarvoor door het

K.I.v.I. dan een bedrag naar rato van het aantal leden der Sektie

ter beschikking kon worden gesteld.

hit bedrag was en is f 10,-- per lid. Voor dat bedrag blijkt het nl.

mogelijk om voor de leden van de afdelingen op het gebied van de

vier genoemde vakgebieden eon vakblad to bieden in samenwerking

met het NIRIA. Het betreft hier dus juist de grootste afdelingen,

nog versterkt met de carresponderende ledenaantallen van het

NIRIA.

Voor de kleinere afdclingen en sekties is zon'n uitgave eon

onhaal-bare kaart.

Pogingen om voor onze sektie relatief meer geld van het K.I.v.I.

toegewezen to krijgen

op

grond van de argumentatie dot uitgave van

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konden door het K.I.v.I. niet gehonoreerd worden zonder gevolgen

voor de begroting en/of dc hoogte van de kontributie.

SECTIE

VOOR

KONINKLIJK INSTITUUT VAN INGENIEURS

SCHEEPSTECHNIEK

Wel is ons eon 6enma1ige startsubsidie toegezegd voor de jaarlijkse

uitgave van "Transactions".

Op deze tot op heden wankele basis heeft het bestuur van de Sektie

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nemen van

het thans voorliggende geesteskind. Om dit kleintje met de

geboden

middelen alleen ter wereld te brengen block zelfs onmogelijk zonder

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en als vroede mannen de taak van vroedvrouw up zich genomen door de

bevalling zover voor te bereiden dat de drukker berciu was

_{on voor}

eon voor ens betaalbaar bedrag de gchoortc vurder af to wikk,len en

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Aan u de taak om het kind te bcoordelen en tc oordelen of het nog

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blijven.

Wij zullen het op liege prijs stellen als u ons dit corded]. kenbaar wilt

maken, hetzij schriftelijk, hetzij mondeling op komcnde lezingen

en

jaarvergadcring.

Jr. H.J. Westers voorzitter SST

SHIPS IN BREAKING WAVES

by

A. Morrall, NMI Ltd.

kivi

SHIPS IN BREAKING WAVES by A. Morral, NMI Ltd.

1. Introduction

The safety at sea for smaller vessels has always been at a level where inprovements are desirable. In the period 1970-1977, 11 vessels were lost in British waters and many were considered to have capsized. During the same period some 41 vessels were lost in Norwegian waters. Although it was not possible to explain all of these losses many of them were lost in bad weather and loss due to capsizing in extreme seas is probable in many cases.

In a number of the Norwegian losses where there were survivors, it has been confirmed that these particular vessels capsized in extreme sea conditions.

Many navigators have experienced extreme sea states in areas where waves and current interact. When these events have resulted in a vessel capsizing the survivors have graphically described the moments before disaster struck: "a wall of water was run-ning towards us and collapsed over us".

In 1978 the Norwegians initiated a new project "SHIPS IN _{ROUGH SEAS" (SIS)} to improve the safety of smaller vessels. The aim was to use theoretical and experimental work and experience to improve the under-standing of manoeuvring, rolling and cap-sizing, i.e. the response of ships to severe conditions in the form of extreme movements. It was also within the scope of this project to try to establish new criteria for the stability of vessels on the basis of know-ledge of extreme environmental conditions and the motion properties of the vessel. This would be an improvement compared to the established stability citeria (i.e. IMO Intact Stability Criterion Resolutions A167 and A168) which considers the ship in calm water.

Nummerical Simulation of Breaking Waves

Recent accidents with fishing vessels in

breaking waves have focussed the attention

of naval architects on the dynamics of

extreme waves. For example, there has been a

lack of knowledge of the velocities and

accelerations in the structure of the breaking wave which are needed for

calcu-lating resulting forces on the vessel.

A number of theories for steady, finite

amplitude surface waves are available.

How-ever, in general they are only valid for

sym-metrical, progressive waves. Longuet-Higgins

and Cokelet (1) 1976 and Cokelet (2) 1978 have developed a numerical technique for

sol-ving the periodic, two-dimensional, deep water breaking wave problem. This method is based

on potential theory and a conformal mapping

of the physical plane inside a closed contour

in the mapped plane, and the equations of

mo-tions are solved in this plane. The wave form

in the physical plane is then found by an

in-version of the mapping.

In 1980 Vinje and Brevig (3) presented a

simi-lar method to that mentioned above, but with the exception that the problem is solved in the

physical plane and finite depth is introduced.

The advantage of this method is that certain

other effects can be included.

For example a floating cylinder can be

introdu-ced to represent the ship motion problem in

two-dimensional breaking waves.

The numerical simulation was carried out for

an initial wave which cannot remain steady

and where initially a sinusoidal wave was

given a large amplitude (steepness D/ =

0.13) and allowed to run freely in deep

water. As the wave progressed it became

asymmetric and the wave front steepened. A

smooth jet of fluid was ejected from the

wave crest and hit the forward face of the

wave. This describes the formation of a

plunging breaker. In shallow-water the wave

would develop much faster and with a larger

jet ejected than the deep-water wave. The

development of plunging breakers from

initial sinusoidal waves is illustrated in

figure 1. Figures 2a and 2b show the velocity

and acceleration fields when the wave front

has become vertical. The reference velocities

given on the figures are the phase velocities

according to linear theory and the reference

acceleration is the acceleration of gravity,

g. The horizontal velocities at the wave

crest are slightly larger than the phase

velocities, as should be expected. The

acceleration along the wave crest is smaller

than g/2. At the face of the wave the

acceleration of the deep water wave has a

magnitude of about 1.59.

The velocity and acceleration fields for the

deep water wave at 0.15T later (T is the wave

period according to linear theory) are shown

in figures 3a and lb. The maximum velocity at

the wave crest is about 1.5 times the phase

velocity and the maximum acceleration is about

3g at the vertical wave front. Notice that the forward part of the fluid jet has an

acceler-ation of about g, in the vertical direction.

This means that the jet is falling freely

under the influence of gravity, as should be

expected for a thin layer of fluid with the

same constant pressure acting on both sides.

Figure 4 illustrates a close-up of the wave

crest of the deep-water wave at different

stages. Figure 9 shows the development of a

plunging breaker generated in a wave flume,

ref. (4). _{It can be observed that there is}

good agreement between the wave generated in the laboratory and the nummerically simulated

wave.

Fig. 5 shows the variation with time of the

total energy of the deep water wave and of its

components. The horizontal momentum of the

fluid is also shown. It should be noted that

both the total energy and horizontal momentum

are practically constant with time.

Breaking waves

Definition:

Breaking waves are in general highly asymmetric

unsteady waves.

The description of a breaking wave, especially

in a random sea, has been rather arbitrary

and unclear. In a random sea breaking waves

will often occur when waves with different

dispersion directions interact. Kheldsen and

Mynhang (4) made recommendations for a

pre-cise definition of certain wave parameters

which can be used to give a more complete

description of the properties of asymmetric

unsteady waves for the two dimensional case;

this definition of wave parameters is given

in fig. (6). In this definition for each

wave the wave height is defined as the

vertical distance from the wave trough to

the following wave crest. With this defini-tion of wave height a parameter is obtained

that is very relevant to the analysis of

capsizing of fishing vessels and other small

vessels.

Criteria for Initation of Breaking Waves

In deep water the most common type of breaking

wave is the spilling breaker. These waves are

quite common and are often observed even when

the wave heights are reltively small, 1-2m.

In some cases they break for a relatively

long time, while others suddenly stop a

fraction of a second after breaking.

In reference (4) the initation of breaking

waves was defined as the physical state

where the very first air entrainment is

visible on the water surface. This

defi-nition is in good agreement with

observa-tions from aireal photographs of breaking

waves.

It is of interest to note Longuet-Higgins'

theoretical breaking criteria for gravity

water waves, namely, that the downward

accel-eration in the wave crest exceeds1/2g.

At least four types of breakers are known to

exist in deep water and these are illustrated

in fig. (9). A Spilling Breaker

The spilling breaker is the most common breaker form both in deep water and on the

beaches. It is characterised as a nearly

symmetric steep wave that breaks at the crest where air is entrained. This entrapped

air forms an air-current jet that runs down

the forward face of the wave. Fig. (9), shows

a typical spilling breaker in deep water.

A Plunging breaker

The plunging breaker is a very steep

asym-metric wave that is well known on beaches

and from time to time also occurs in deep

water. It is characterised by the phenomenon

that the steepening of the wave suddenly

leads to the shooting of a very thin sheet

of water from the wave crest, and this water

forms under the influence of gravity the

well known overhanging crests until the

water plunges down as a jet on the front of

the wave near the mean water level. Air

entrainment can occur at an early stage when

the jet is found, or at a later stage when

A typical plunging breaker is shown in fig (9).

The Capsizing of MiS Helland-Hansen

Dahle, ref (5) investigated the capsize of

the MS Helland-Hansen using model

experi-ments in breaking waves. The capsizing of

the vessel took place in deep water in the

ballast condition and was caused by a

breaking wave which hit the vessel

broad-side.

The accident took place when the wind force

was about Beaufort Force 8 and the wind

direction corresponded tot the wave direction.

A current of varying strength was running in

the opposite direction. The significant wave

height was about 3.5m. The survivors recalled

that while underway at 6 knots the skipper

observed the breaking wave with a height of

about 5m approaching the vessel from the port

side. The vessel was then hit by the wave and the slamming impact was felt but not heard by

the survivors. Within seconds the vessel listed

to about 60' and soon afterwards listed at a

stable position of about 80' until it sank 20

minutes later.

Righting moment and righting arm curves of the

MS Helland-Hansen used in the investigation

are illustrated in figure 10.

As part of Dahle's investigation into the

cap-sizing of the Holland-Hansen he examined the

equation for a vessel being hit by a breaking

wave given by Kholodolin and Tovstikh (6). viz.

( 14 + A44 )) +

= 0

Where C = phase velocity of wave

CD = drag coefficient of superstructure

A = area of wave impact

a = moment arm = density of water Where 14 144 M1 M,7 M3 N,

= mass moment of inertia

added mass moment of inertia = moment of damping

= moment due to wave (except impact)

= moment caused by wave impact

= righting moment

0, 0, 0

= heeling angle and its derivatives

In ref. (6) the energy transfer during the

impact period t only is regarded as im-portant. M1 and M2 are therefore neglected

because they are regarded as much smaller than

M3.

Throught simple energy considerations, the

fol-lowing expression for energy transfer from the

breaking wave to the vessel during the impact

period t is derived

1.2

[.[gt3M3dt )2

t3

*(I

+ A-44 )W0 = 2 ( I4+A44 )

Oa = angular velocity after wave impact (the

increase of angular velocity is assumed

lineair during the interval

0

t3 = duration of wave impact. It is assumed

that initially 0 = 0 and 0 = 0

E3 can also be expressed as follows:

[,./gt3 ip

CDAc2a dt ]2

E3

_{2 14 + A44)}

The integral of equation (3) is difficult to

evaluate because A, a and possibly CD are

functions of time and C also varies vertically.

Furthermore, 6t3 cannot be calculated. Therefore

to use equation (3) Dahle employed the results

from model experiments, using Froude scaling.

The energy content of the impact pulse can be

presented in a simple way by a triangle with

height M3 and duration i t3. Equation (3) then gives:

i.

[M3 At3)

2

El =

g IT

4

A44)

At2 = duration of exposure

E2 can also be expressed as follows:

[igt2m2 dt ] 2

p0

E2 = A ja Gzd, =

_{2( 14 + A44)}

a

= surface inclination of wave

at time of impact

The total energy transfer during the time of

exposure for a steep, breaking wave is as

follows: ET = E2 + E3

= E1 _{+ E4}

Where E, = energy from steep wave

E3 = energy from wave impact

El = energy dissipated by damping

E4 = engergy content of heeled vessel

By assuming that E3 is independent of

stabili-ty, and using the data obtained from the

experi-ments, the relative importance of E, and E3

can be indicated.

Due to the dominating influence of C in equation

(3) E3 for the waves below 6.5m are only a

fraction of E3 for the 6.5m wave. Moreover, as

14 is linearly dependent on the displacement,

E3 must be relatively smaller for the loaded

condition. An indication of the energy from the

wave impact, E3 is given in table 1.

Because 14, A44 and the integral

should be almost independent of the stability

for the same loading condition, ET should be of the same order of magnitude, regardless of

the stability of the vessel. This is not the

case, as ET is strongly dependent on the

stability.

The explanation is that a steep breaking wave

also exposes the vessel to a turning moment of

a quasi-static nature, i.e. that the moment M,

in equation (1) and consequently E2 of

equa-tion (4) is of importance. Although smaller

than M3' M2 acts over a longer period of

time. This is evident from fig. 11 which

illu-strates the capsizing.

For the breaking waves below 6.5m the impact

energy (E3) was negligible and the energy

transfer can be regarded as having been

caused by the steep wave action only.

Thus the energy content of the wave impact

alone was not large enough to capsize the

Helland-Hansen in the ballast condition in

the 5m breaking wave.

This results can only be approximinate

because the energy transfer is undetermined

when the vessel capsizes and experiences a

considerable damping for the large heeling

Wave Height

Phase Velocity

Ballast

Table 1

Energy from Wave Impact (E3)

It can be concluded that the high energy

con-tent in breaking waves is due to their large

particle velocities as well as their steepness.

The relative importance of the two components

E2 and E3 is summarised in table 2 where the

difference between the energy content for two

stability condtions are given.

Capsizing of Small Trawlers

In ref (7) results were presented of an

investigation into the behaviour in rough water and breaking waves of two inshore

fishing vessels having almost identical

principal dimensions and displacement, but with different statical stability.

GZ300

E2

E3

(m)

(tm)

(tm)

ET=E2t4E3

(tm)

Two models were used in the experiments and

their principal particulars are given in table 3. Models A and B had hulls

represen-tative of vessels built in the late 1960's

and early 1970's which are still in service

today. Model A represents an inshore trawler

built of steel with a transom stern whereas

model B represents an inshore trawler built

of steel with a cruiser-type of round stern.

Both models were used as free-running radio

controlled models; the scale of each model

was 1:15.

Loaded

E3

(tm)

Table 2

Energy components from steep wave (E2) and wave input

(E3)

for 5m breaking wave.

ET=E2+E3

(tm)

11

0.10

60

6 66

81

2 83

0.20

83 6

89

113

2

115

0.26

92 6

108

0.30

145

2

147

(m)

(m/s)

6.5

12.0

50

30

5.0

8.4

6 2

3.5

8.3

4 1

3.0

8.1

3 1

Ballast Condition

Loaded Condition

Table 3 Principal particulars of two British

trawlers corresponding to models A and B used in the experiments

Fig. 12 gives the stability curve for design

A and a comparison between calculated and

measured GZ values. A close comparison cannot

be expected at angles beyond which the deck

edge becomes immersed and for design A this occurs at about 16°. Fig. 13 gives the

stabi-lity curve for design B. The minimum stabistabi-lity

required by the current IMCO criteria is also

indicated in Figs. 12 and 13.

Model tests were carried out in breaking

waves; these were generated from waves which

corresponded to full-scale significant

heights of 3.2m with their wave length

shortened. The maximum wave height produced

by the resulting breaking waves in any given

example corresponded to 4.9m full-scale.

These waves were most realistic. A photograph of model A under test in breaking waves is

shown in Fig. 14.

Once circuling manoeuvers were attempted

model A was immediately at risk. Model A

capsized on a number of occasions, usually

when it was caught in a beam to sea position.

There were two distinct types of capsize: the

classic one in wich the hull, when balanced on

the crest of a wave, and without water on deck,

immediately lost waterplane interia and hence

stability, and the second one, where a wave

overwhelmed the bulwarks and produced a rolling

moment greater than the restoring moment

natu-rally present in the hull.

It is significant to note that the elapsed

time for each capsize was 10 to 20 seconds only

(full-scale) or about the time of two to three

roll cycles and this was occasionally shorter

when the model was kept stationary in beam seas.

Fig. 15a shows a record of the roll motion

and capsize of model A without water on deck

whereas Fig. 15b records the capsize of

model A after being overwhelmed by a breaking

wave.

All the above records were for the model

cap-sizing to leeward, when the roll angle exceeded

40°.

In contrast, model B survives circular

manoeuv-res in the breaking waves with comparative ease.

The motions were however, severe and considerable

quantities of water were shipped. On several

occasions the model survived a test period

equi-valent to about 1 hour full-size. Every attempt

Model A

Length (LOA) metres 25.91 24.36

Length (L) metres

PP 22.09 21.44

Breadth mid metres 6.86 6.71

Depth mid metres 3.35 3.35

Draught amidships metres 2.48 2.49

Draught forward metres 1.83 1.79

Draught aft metres 3.13 3.19

Trim by stern metres

(relative to datum line) 0.69 0.33

Rake of keel metres 0.61 1.07

Sheer forward metres 0.99 1.57

Sheer aft metres 0.46 0.54

Freeboard at bow metres 2.15 2.76

Displacement tonnes 167.6 160.0

Transverse GM metres 0.732 0.908

Vertical centre of gravity

VCG metres 3.153 2.58

Free surface correction

was made to bring about a capsize, but the extra

roll stiffness inherent in the hull due to the

large GM clearly contributed to its survival.

These model experiments indicated that model A,

with a metacentric height, CM, corresponding to

0.732m full-size capsized in breaking waves of

modest severity, of a height and length which

the vessel could conceivably encounter in

service. The sequence of events during capsize

occurred very rapidly. The reason for capsize

suggested by the results of the model

experi-ments is: lack of sufficient roll stiffness.

Subsequent experiments in which either

dis-placement or GM was increased suggested that

the fault lay not so much in the hull shape

but rather in the CG position which led to a

simple deficiency in GM.

Both models experienced capsize when their

stability at rest was close to the IMCO

mi-nimum. The survival condition for model B

was obtained with the maximum righting

lever, GZ, fractionally above the IMCO value

but greater overall stability was present

due to a higher angle of vanishing stability.

Conclusions

Investigations into the effects of breaking

waves on small vessels (6,7) indicates that the stability requirements of the

Torremo-linos Conference in 1977 do not always ensure

adequate safety in beam seas, especially in

the ballast conditions in particularly steep

breaking waves of moderate height.

Large heeling angles in breaking waves have

been reported by vessels that have survived.

The investigation indicates that a limiting

heeling angle of only 400, which has been

adopted by stability regulations for larger

vessels, is far too small.

To improve the safety of smaller vessels the requirements of the GZ curve could be

strengthened ,y requiring positive GZ values

up to much larger heeling angles.

Moreover, openings where water can enter

the vessel during heeling to large angles

must alse be closed watertight.

Bulwarks are dangerous in breaking waves

as they increase the wave moment and trap

water on the deck.

The behaviour of small vessels in steep,

breaking waves has only been studied to a

limited extent. The problem is complex due

to the non-linear behaviour of the wave and

the vessels due to uncertainties of scaling.

The investigations have shown that substential

improvement in initial stability has only a

slight influence on the heeling angle.

Any improvements in initial stability might

be of limited value if positive righting

moments are not extended to large angles and

watertight integrity maintained at these

angles.

REFERENCES

Longuet-Higgins M S and Cokelet E D:

"The Deformation of Steep Surface Waves

on water: A Numerical Method of Computation".

Proc. R. Soc. London, A350, 1-26 1976.

Cokelet E D: "Breaking Waves - The Plunging

Jet and Interior Flow-Field". Proc. of the

Symposium on Mechanics of Wave-Introduced

Forces on Cylinders, University of Bristol,

1978.

Vinje T and Brevig P "Numerical Simulation

of Breaking Waves". 3rd Int. Conf. on FEM

in Water Resources, Univ. of Miss., 1980.

Kjeldsen S P and Myrhaag D: "Kinematics and

Dynamics of Breaking Waves". Norwegian VHL

Report No. STF 60A 78100.

Dahle E A and Kjaerland 0: "The Capsizing

of M/S Helland-Hansen", Trans. RINA vol.

122. 1980.

Kholodilin A N and Tovstikh E V. "The Model

Experiments for the Stability of Small Ships

on Erupting Waves" ITTC 1969.

Morrall A: "Capsizing of Small Trawlers".

t=0.31T

Deep-water wave.

Fig 1.

Development of plunging breakers from

initial sinusoidal waves (1)

t=0.51T

=0.611

t=066T

t=0 697

.4

.6

.8

1 .0 kx 1 .2

1.4

Deep-water wave (t = 0.49T)

Deep-water wave (t = 0.49T).

Figs

iind ?h.

Velocity ;Ind Jcceleration fields

under plunging breakers when the

wave Ironts 11;ive become

vertical.

k

is the wave number and

.6

kj

.

4

.2

0 . 0

1.4

1.6

1.8

2.0 kx2.2

2.4

t=

17

1.4

1.6

1.8

20k><22

_2.4

t = 0.64T

Figs 3a and 3b.

Velocity and acceleration fields

under deep water wave 0.15T after

.4

ky

.2

0 .0

-.2

Fig 4.

Closeup of the wave crests

of the

deepwater wave (1).

1.5

Total

energy

Horizontal momentum

Kinetic energy

Potential energy

0.25T

0..5T

Time

Fig 5.

Variation of the energy and horizontal

momentum (1).

2.0-1.0

1.0

ASYMMETRIC WAVE OF FINITE

HEIGHT

SL

w .

Ah

/I

A

w

T

T

C=

1'

6

=1',

X =

L"

=

L'

BOTTOM

Fig 6.

Definition of wave parameters (4).

= 0

MEL

21t

,MWI.

A. SPILLING BREAKER

PLUNGING BREAKER

C. PYRAMIDAL BREAKER

INTERACTIONS OF WAVES WITH DIFFERENT

DISPERSION DIRECTIONS D. REAR BREAKER

4---mWL mwL lAWL MW1 MWL

Fig 7.

Classification of breaking waves in

Fig 8.

Spilling breaker in deep water generated

by wave-wave interactions (4)

Fig 9.

_{Plunging breaker in deep water created}

by wave-wave interactions

(4)

CZ (w)

GZ

(in) 10

Ballast condition

20

Loaded condition

3n 40

C-Z30° - 0.40

0° = 0.26

50

300 - 0.20

70

80

90

CZ300 -

0.10

70 80

Fig 10.

Righting Moment and GZ curves for the

test

conditions of M.S. HELLAND- HANSEN

(5)

GZA (trr)

90

GZA (tit)

150

100 50

- 250

50 10 20 30 40

(metre)

Fig 11.

Capsizing

equipment

of M.S. HELLAND- HANSEN

(Ballast condition GZ30 o = 0.20m; H = 5m) (5).

,.-0 75stc 0 7S sec t .1 SO sec

23

CO 2,S) 2p 175

0.7 FOP WAVE CURVE, TPOCHOIDAL WAVE

(H 4,, L

70T) HAS BEEN ASSUMED

WITH CREST AMID SHIPS

0 0 4.--0 3 2 0 0 7

06

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HOVERCRAFT - TOWARDS THE SECOND

QUARTER CENTURY

by

G.H. Elsley, D.J. Hardy and M. Stevens

kivi

HOVERCRAFT - TOWARDS THE SECOND

QUARTER CENTURY

by

G. H. ELSLEY and D. J. HARDY

Synopsis

The paper begins with a survey of the currert scene with particular reference to B.H.C. built Hovercraft. It continues with an

inves-tigation of the power requirements of amphi-bious hovercraft and the diesel vs. gas tur-bine question.

Subsequent sections cover "future-design" and "construction". The paper concludes with some thoughts as to how hovercraft may be used and some illustrations of possible future craft.

Large Extruded I-beam

Integrally Stiffened Deck Panel

Comparison of Small and Large Deck Panels

Comparison of Fabricated and Extruded Roof

Beams "Jeff" Craft U.S.S.R. "Aist"

SR.N4 Adapted to M.C.M. Role

Outline of 750 ton ASW/Patrol Hovercraft

Bell-Halter Patrol S.E.S.

Bell-Halter Passenger Ferry

Outline of 500 ton Passenger Ferry

reS Contents

1. Cushion Pressures Cushion Length 1. _Introduction

2. Typical Main Skirt Sections at Various 2. The current scene

Stages of Development 3. Power reduction on amphibious hovercraft

3. Comparison of C.G. Vertical Accelerations of Ships, Hydrofoils and Hovercraft

4. _{The engine and structure - a simple minded}

theoretical approach

4. Seakeeping in Relation to Displacement 5. Future design

Ships and other High Speed Craft 6. Construction

5. SR.N4, BH.7 and SR.N6 Into-Wind Water Speed 7. Concluding remarks

HOVERCRAFT, TOWARDS THE SECOND QUARTER CENTURY

1. Introduction

The title of the paper was selected in order

to achieve two main objectives. It makes the

point that hovercraft have been around for about twenty-five years and are about to

enter their second quarter of a century and,

perhaps more significantly, it gives the

joint authors a considerable degree of

freedom in the choice of subject material.

The making of technical predictions is a

notoriously dangerous, but nevertheless, an

interesting and necessary activity. This

paper has been restricted to marine hovercraft

above 10 tens A.U.W., light hovercraft,

barges and overland devices being excluded.

It is perhaps of interest that one of the

first studies carried out by the then

Saunders-Roe Division of Westland Helicopters

Ltd. in 1958/59 covered a range of craft up

to 10,000 tons A.U.W. - it is humbling to

realise that in practice it took 20 years

before a craft of even 300 tons A.U.W. was

fully operational.

It should be noted that since this paper was

prepared in 1981, the AP.1-88, an 80/100 seat diesel powered passenger ferry, has

successfully entered service on two routes, one in the United Kingdom and one between

Sweden and Denmark.

The opinions expressed in this paper are

those of the authors and do not necessarily

reflect those of the British Hovercraft

Corporation.

2. The current scene

From the time when the SR.N1 first flew in

July 1959 as an unskirted "flying saucer"

propelled by air jets, two distinct types of

craft have evolved, the skirted hovercraft

which minimises its lift power by means of a

flexible stucture which "contours" the waves

Odell in theory anyway), and the sidewall

hovercraft which has rigid side walls which

cleave the waves, together with a skirt (or

seals as our American cousins call them)

at the bow and the stern. With one notable

exception, all skirted hovercraft have been

treated as amphibious vehicles and have been

propelled by air propulsors while sidewall

craft are, by their very nature, marine craft

and have been propelled by marine propellers

or water jets.

Historically amphibious craft have generally

come out of an aircraft stable while all

non-amphibious craft have had a boat-building

lineage, with the possible exception of the

SES 100 A and B which were however strongly

influenced by the USN requirements.

The "highlights" of the technical achievement

of these "first generation" craft from which we must advance are shown in Table 1.

One of the interesting features of the table is the relatively high cushion pressure and

power/weight ratio of JEFF

B. This is

lar-gely due to the design constraints imposed

by the need to dock the craft in landing

ships. Figure 1, which plots cushion

pres-sure Pc against cushion length L,

illu-strates how JEFF B virtually fits the side-wall line and appears to be quite divorced

from other large amphibious hovercraft.

Using Froude Scaling, and for a given

geo-metry, one would expect Pc to scale as L

(i.e. as weight to the one third, W1/3).

Figure 1 shows that there is no such

cor-relation and in fact the cushion pressure of

the larger amphibious craft (excluding the

LCAC, a JEFF B derivative) is substantially

constant. This arises from the fact that the

craft were all designed to carry car and

passenger payloads over comparatively short

ranges with, except for the N.500,

accommo-dation all on one deck. In this context, it

is interesting to note that the initial

API-88 tank tests were conducted on an AR.N4

Mk3 model loaded to a scale 600 tons which

arose from tests at scaled weights inter-mediate between 300 and 600 tons aimed at

high enduance variants. Similary high

endurance variants of BH.7 will have higher

cushion pressures.

It is likely that future large civil amphi-bious hovercraft will be double decked over

at least part of their plan area and will

have greater ranges. Thus we can expect that

a 200 ft. cushion length civil craft will

have a cushion pressure in the range 100 to

150 lb/ft2. As will be seen from the points

added to the right hand side of the figure, this implies craft A.U.W.'s of 600 to 900

tons. Military variants are likely to have

even higher cushion pressures.

Sidewall craft can be expected to have

significantly higher cushion pressures at

the larger sizes if current trends continue.

This arises from the philosophy of using

welded marine type structures and diesel

engines coupled to marine propulsion. Thus,

althought the cushion pressures are high,

the resulting disposable load/A.U.W. frac-tions are likely to be less as shown in

Table 1.

Some details of current, i.e. in-servicing,

B.H.C. craft are given in Table 2 which

shows data obtained from the "Worst Intended

Enviromental Conditions" specified in the

craft "Type Operating Manuals" in the form

of wind strengths and wave heights associated

with critical length seas.

It should be noted that these limitations

are applicable to normal fare paying

pas-sengers operation, military hovercraft can,

and have operated safely, well outside them.

For example, the 10-tons SR.N6 stationed in

the Falkland Island in 1967 was operated in

waves of 3.7 metres and swell of 4.5 to 5.5

metres.

During these and other operations, the SR.N6

has also demonstrated a good surf-crossing

capability, having no difficulty in beaching

through a 1 to 2 metres surf.

Similary, the production clearance trials of

the SR.N6 Mk5 military craft included

sorties in gale force conditions in the Solent, including wind speeds of 32-38 knots

gusting to 45 knots with observed wave

heights of up to 3 metres.

Apart from the natural tendency for seakeeping

ability to improve with size (increase of

both skirt depth relative to wave height and cushion length relative to wave length being

advantageous), the skirt characteristics

themselves have a considerable effect. This

has been illustrated by the development of a

more responsive, low pressure ratio, skirt

for SR.N4 Mk3, which has operated very

satisfactorily in 5-7 metres significant

height seas and 40-50 kt. winds during rough

weather trials. Adequate control was

demon-strated even with simulated failures on 1 or

2 engines or pylon, fin and propeller pitch

The combination of improved (deeper bow)

skirt and increased craft length has

resul-ted in reduced pitch motion and notably

better ride comfort. This improved behaviour

is associated with the graded stiffness of a

bag/finger system which provides rapid skirt

response but low craft response in small

waves whilst affording protection from

structural impacts in larger seas. Such

protection is not provided in the simplest

loop/segment skirts. B.H.C. skirt development

over the years is shown in Figure 2.

TABLE 1

TABLE 2

B.H.C. SERIES TABLE ROUGH WEATHER OPERATING LIMITATIONS

As indication of the vertical acceleration

levels (r.m.s. g) measured at the C.G. on

SR.N4 Mk3 is given in Figure 3. At a given

wave height, these measurements range from

about 0.05 to 0.15 r.m.s. g, depending on

wave length/form and craft speed. Also shown

on the diagram are corresponding measures

for similar size hydrofoil craft and D.P.B.'s.

o Clearance to 4.0 metrea in anticipated These are all for paying passenger operation, military limitations can be such higher

31

PARAMETER VALUE CRAFT

a) An.phibious Craft

Max AUW tns 300 Super 4

Max disposable load/

A.U.E. fraction 44% Jeff B, 'Gus' Max speed kts 75 Super 4, Jeff B Max Eig wave height ft 21.5 Super 4 (as civil

craft) Max gradient capability 2.7.5 SR 55 Max cushion pressure psf 220 Jeff B Max pouer/wt ratio shp/tn 150 Jell B Min power/al ratio shp/tn

b) Sidewall Craft

50 Super 4

Max AUW tns 115 Bell Bolter 110 Max disposable load/

A.U.W. fraction 25% 51218 Max speed Its 92 SES 110B

'Jan cushion

pressure psi 105 SES 100B Max power/wt ratio shp/tn 140 SES 100B Min power/wt ratio shp/tn 32 BELL HALTER 5E110

MAXIMUM WEIGHT

tons

WIND (kts) SIGNIFICANT WAVE HEIGHT MEAN GUST METRES % MEAN SKIRT DEPTH

SR.N5 6.7 30 40 1.2 100

SR.N6 (Sing, Prop Craft) 9-14 30 40 1.5 123

SR.N6 (Twin Prop Craft) 17 30 40 1.8 128

BH.7 50-55 40 - 2.0 131

SR.N4 14k.2 200 35 45 2.4 87

It will be noted that the fully submerged incidence-controlled hydrofoil system craft

are expected to give the lowest acceleration

in slight to mederate seas. The hovercraft

of SR.N4 size gives higher accelerations in

the smaller seas, but is better in the largest

seas in which the hydrofoil craft may reach

a limitation associated with water-jet intake

venting, hull cresting or foil broaching (in

relatively short seas) beyond which it has

to operate in a boating mode. The hydrofoil

craft can then become difficult to drive

in-to larger seas and rolling motion can become

large with a consequent drastic worsening of

crew comfort.

Typical waterspeed performances for

displace-ment ships, F.P.B.'s and hydrofoils (with

fully submerged incidence-controlled foils)

are compared in figure 4 with the achievable performance of the SR.N4 Mk2 and Mk3

hover-craft.

Typical Eastern English Channel wave height

occurrence data is indicated and it will be

seen that the hovercraft are almost twice as

fast as the ships in average conditions. A

progressive improvement in rough weather

capability has been achieved by the SR.N4

series craft, the operating limit being 2.4 m for Mk2 and 3.5 m for Mk3 (Super-4) with a

planned increase to 4.0 m. The associated

occurrence of worse conditions (causing

operational cancellation) is reduced from 6%

to 3% as shown in figure 4.

Figure 5 illustrates the expected waterspeed

performance on the most adverse into-wind

and sea heading, for the range of B.H.C.

craft. Performance in beam wind conditions

is significantly better than into wind and

for this reason, in commercial service, operation into wind and sea in the rougher

conditions is usally avoided. In the range

of wave heights considered it will be noted

that the craft speed does not fall very far

below hump speed.

Hence larger craft, with correspondingly

higher hump speeds, will be able to maintain relatively high speeds, 30 knots or so, in

adequate conditions. This is considerably

better than all but the largest ship.

3. Power reduction on amphibious hovercraft

Previous papers have presented figures much

like that shown in figure 6. This shows a

progressive reduction of installed poser per

ton of all up weight with time for Cowes

built amphibious hovercraft. Wrapped up in

this plot are some design improvements but

also some considerable variation in weight

and performance and in fact, the curve is

essentially a series of discrete points.

Clearly to make the hovercraft more competitive

with other forms of transport it is desirable

to reduce the installed power on any given

design to allow a level as possible consistent

with attempting to retain some performance edge. This then reduces both the acquisition

costs and the running costs.

Lift power

The SR.N4 Mk3 operates with a cushion flow

coefficient CQ af about 0.0075 where CQ

is defined as.

c

-Q. 11

IC) Pc

Where Q volume flow per second

= air density

Pc - cushion pressure

S = cushion area

This represents a considerable reduction on

earlier amphibious hovercraft and it is

unlikely to be reduced below about 0.006 in

the near future. Coupling

CQ with an atmosphere to bag

ef-ficiency of 0.65 and a bag to cushion

pres-sure ratio of 1 : 1 leads to lift horse power/

ton values of about 1.2p½ or from 9.6 to 12.0 for cushion pressures of from 64 to 100

Simple sums can show the likely magnitude of

momentum and profile drag terms and with

some effort it is possible to derive

wave-making drag - or at least establish it from

published curves. The major unknown drag

terms are those which we at B.H.C. have attributed to wetting and the natural waves.

Clearly there may be some possibility of

reducing all of these, expect the

wave-making, with some design effort.

If we consider the SR.N4 Mk3 at its design

all up weight the installed power is such

that it can readily negotiate the humps

and achieve over 70 knots in calm conditions

and still maintain speeds of 40-50 knots in

quite adverse conditions. However, its static gradient capability is only about 1

in 16 or 0.06 although higher gradients can

be negotiated if taken at speed. Hence it

appears that irrespective of any drag

reduction which can be made, the installed

propulsive power will be designed around

the requirements for the craft to come

ashore. Thus taking 0.06 as a limit and

assuming propulsors delivering say 6 lbs of

thrust per horsepower, 22.4 horsepower per

ton will be required of the static gradient

requirement. With some allowance for

trans-mission efficiency and control, it is then clear that an installed value of propulsion

horsepower per ton of at least 25 is likely

to be required for all future amphibious

hovercraft.

Total installed power

Summing the simple lift and propulsive power

terms suggests that, depending on cushion

power, the installed power per ton will

always have to be at least 35 and for

"denser" craft perhaps 40. Drag reduction,

particularly through skirt development could

then lead to the interesting anomaly that

more power is required for the landing phase

than for cruise.

IG HP ,

wD = K- 2240 "

WG HP (W-4-fDt

K- k(1- K)

lc 2240

which can also be interpreted as the payload

ratio for craft of equal weight or the power

ratio.

Examination of the equation shows that

neg-lecting any possible structure weight increase

i.e. K = 0, the top will always be greater

than the bottom if W + fdt > f t

Since current values for fp are about 0.36,

fc about 0.5 and w at least 4, it can be

seen that some 28 hours are required for the

diesel to break even.

Assuming K = 0.45, a good value

we then have, assuming = 45 and t = 5

WD

= 1.20 i.e. the diesel powered craft

is about 20% heavier.

WD

Again if k = 0.05w(7.; = 1.31 i.e. the diesel

powered, marine structure craft is about 30%

heavier, and if k = 0.1 ve41 = 1.44.

In this latter case the diesel/marine type

structure craft actually burns more fuel than the gas turbine/aircraft type structure craft.

Clearly as HP/ton values improve towards our

"theoretical" limit of 35, the discrepancy

will lessen as it will if disposable load/all

up weight ratios improve. Conversely,

improve-ments in gas turbine specific fuel consumption

will work the other way.

33

Propulsive power writing

WGD

4. The engine and structure problem - a simple

minded theoretical approach

Much has been written about the relative

merits of the gas turbine engine and the

aircraft type structure vs. the diesel

engine and the marine type structure and as

you are all aware, B.H.C. having long been

wedded tot the first approach have now

designed a craft, currently under

construc-tion, using the second approach. Until the

accountants were consulted there was, and

still is for that matter, some justification

for the first approach.

Assume that the payload of a gas turbine

powered craft can be written

1G HP ViGt

2240

and for the diesel powered craft with a

heavier structure

fD HP

-WDP = WDD

_{2240 WD}

Where W = weight in tons

t = time in hours

f = specific fuel consumption

w = weight penalty in lab/HP of diesel

relative to the gas turbine

k = increase in structure weight as

a decimal suffices

G = Gas turbine D Diesel etc.

GP = Gas turbine payload

GD = Gas turbine disposable load WGP = WGD

J-12 WD k (WD-WDD

2240 ViD

Fortunately for the competitiveness of the amphibious hovercraft, this is not the whole

story. Diesel engines at least, in the lower

horsepower ranges, tend to be very much

cheaper than gas turbines both to buy and to

maintain. Similarly, marine type structures

tend to be more robust and more durable as

well as cheaper. Thus, at least for smaller

hovercraft, perhaps up to 100 tons, it may

be possible to justify the APL-88 approach on economic grounds which is really what

makes a transport vehicle a succes.

5. Future design

5.1 Design phisolophy

In order to make future civil or military

craft viable in an ever more competitive

world, it is clear that we must

significant-ly reduce both first cost and operating

costs, the latter particularly with respect

fo fuel consumption.

The options open to the designer in respect

of reducing first cost are:

to greatly simplify the construction to use low cost engines, transmission and

propulsers/fans

to use marine rather than aircraft type

equipment

to increase the density of the craft (i.e.

to use a higher cushion pressure) in order

to reduce the structural size

Similarly, to reduce the direct operating costs

(D.O.C.) we must lower:

fuel consumption per payload ton mile

maintenance costs

crew costs

At present, fuel costs is the dominant

factor with respect to civil craft having a

high utilization, for military craft the

dominant factors are probably depreciation

and crew costs; lower fuel consumption is

important to enhance operational capability

(payload/range). As discussed above, the

specific installed power has been falling

steadily with time for amphibious craft and

will, in the authors view, continue to do so

at least if considered as cruise power.

However, gains in the payload/A.U.W. ratio

or speed in a given sea state for

given-power are also effective in reducing the

fuel consumption per payload ton mile and

these form part of striking the right design

balance.

Reduced specific power is being achieved by

slowly but surely improving the efficiencies

of toe various component parts of the craft.

i.e. propulsive system, lift system and

aerodynamic drag, but by far the biggest

incentive is to reduce the rough water drag

of the craft by skirt and/or sidewall

development and by reducing craft motion

(response).

The design philosophy is also likely to be

affected by size and role. We have already

shown that higher cushion pressures are desirable on larger craft, this will enable

the designer to use two or even three decks,

developing the theme started by the Sedam

N500, and thus make the craft structurally

more efficient, at least with respect to

those carrying low density cargoes such as

passengers and cars.

Operational requirements will, of course, in

many cases dictate the form of the craft and

reduce the choices before the designer.

Assault landing craft will inevitably be

amphibious and may, as with the L.C.A.C.,

have artificial size constraints placed upon

it by other equipment (e.g. the associated

ship). M.C.M. craft are also likely to be

amphibious as their low vulnerability

springs from this feature. On the other

hand, it is possible that a lower speed

sidewall ferry would be more economical for

certain civil roles providing other restraints

such as depth of water en route and at the

terminals permit. As size increases then

this will also affect the way the craft is

constructed and its machinery arrangements

Modular construction may become even more

necessary and the use of high grade materials

demanded, at least for the main load carrying

members, if good payload/weight factors are

to be achieved. The development of flexible

structures to match the growth in size will

demand a sustained effort.

Perhaps the best overall piece of design

philosophy to apply to all future designs

is the old aircraft designers adage:

"Simplicate and add more lightness".

6. Construction

The key action in reducing structural cost

is to reduce the number of components,

es-pecially fastenings. The obvious choices open

to the designer are:

to mould the structure in composites

(g.r.p.) using as few components as

possible

to weld the structure from light alloy sheet and extrusions

At third approach may well be attractive

where high performance craft are demanded

and this is:

(c) pre-fabricate the largest possible

sub-assemblies (modules), probably by

bon-ding, using high grade materials

Both (a) and (b) have been tried and give

cheap but relatively heavy structures. In

addition, the cost of the mould for large

g.r.p. structures can be quite considerable,

making a small production run expensive.

In the rest of this section we would like to

give you some idea of the advantages and

dis-advantages of the type (b) approach and an

indication why the third (type c) approach,

at least in port, may well be used for future

craft.

6.1 Extruded vs. fabricated components

Extrusions have been used in hovercraft from

their inception but they have tended to be

small in cross section and made of high

grade hard to extrude alloys. One of the

more advantageous developments in recent

years is the availability of really large

comparatively thin extrusions in moderate

strenght alloys. Two such extrusions are

shown in figures 7 and 8. The first is a large I-beam 500 mm (19.7"), deep extruded

in an RE30 type alloy which may be suitable

for a floor beam or for a deckhead beam on

very large craft. The second is a stiffened

deck plank 450 mm (17.7) wide having a

minimum thickness of 2.5 mm (0.1"). This is

similar to the deck used on the Sedan N500,

the planks being joined by automatic seam

welding along the mating edges. Prior art

can be detected in the construction of the

hulls of various U.S. hydrofoil craft,

par-ticularly the AG(EH)-1 delivered to the U.S. Navy in 1969 (ref. 1). Note the integral

weld backing gulley in the extrusion shown.

Why are these extrusions so important?

Figure 10 shows a hypothetical comparision

between a 16" deep fabricated roof beam as

used on the SR.N4 compared with a similar

depth and strength extrusion. It will be seen

that the extruded version is only about one

fifth as expensive per unit length but twice

the weight.

With respect to the construction of the deck,

the extrusion shown in figure 8 weights 2.4 lb/ft2 and is almost as light as one can

get for this width of extrusion. Its cost is

less than £3 per sq. ft.

The equivalent sandwich panel design, made in

8 ft x 4 ft. modules, would weigh only 1.2 to

1.8 lb per ft2 depending on the degree of re-finement involved but would cost at least

£10-£15 per ft2, again a factor of up to five in

cost.

It should be noted that the extrusions can

be obtained in long lengths, up to 72 ft.

(22m) if transport permits (B.R. Engineering

take such lengths into the Derby works by

rail). This means that advantage can be

taken of the fully heat treated condition

when loaded axially as the heat affected zone due to the machine welding of the

longitudinal joints is comparatively small.

Transverse joints are, of course, another

matter, these will weaken the structure

un-less suitably reinforced. A very similar but

lighter extrusion can be used to form

bulk-heads.

The third approach, the production of large

bonded subassemblies is an extension of the

B.H.C. current techniques and is well worth

considering where performance is at a premium

as it can save considerable weight over some of the more lightly loaded areas such as the

roof, passenger decks or plenum decks. The

SR.N4 and the BH.7 is an 8' x 4' panel,

al-though we also produced 4' x 16' panels

stiffened with top hat stringers. With the

advent of large autoclaves the size capacity

has already been more than doubled and

panels as large as 8' x 32' may be considered

for the future. Such panels cut down the

number of fasteners required quite

signifi-cantly. For instance an 8' x 32' panel will

require only 50% of the fasteners of 8, 8' and 4' panels even when bolted to an 8' x 4'

grid (fig. 9).

One advantage of the sandwich panel

construc-tion is that it gives a smooth surface on

both sides and by increasing the depth of

panel, allows frame spacing to be increased,

thus on military craft, a deep buoyancy tank

can be used to house equipment and

operatio-nal rooms, making very efficient use of the

structure. On smaller craft, the buoyancy

tank can be used for the carriage of fuel

although this only occupies a comparatively

small area.

To sum up, the construction of future craft

will be undoubtedly be influenced very

strongly by the need to reduce the first

cost of the craft. In this respect, we see a

far greater use of welded construction but

this will be combined to a certain extent, with the use of very large fully heat

treated extrusions and possibly large composite components. Many military craft

will be dense highly powered craft featuring

fully welded construction. On the other

hand, larger turbine powered civil craft,

may make rather greater use of high grade

structure in order to maximise the load

carrying potential for a given power usage

thus minimising fuel consumption.

7. Concluding remarks

After 25 years of development we have seen

that the Hovercraft, both amphibious and

sidewall, have reached the stage of providing

a reliable and safe means of transportation.

In the military field their use in the West

has been confined to small and medium size

but with the forthcoming production of the

L.C.A.C. for the U.S. Navy this is likely to

change. Progress throughout the period, as

with all human endeavours, has been uneven

with some ideas introduced before their time

and others perhaps not soon enough. As the

authors see it, the next period will be one

of consolidation with new, more economic and

versatile craft taking over from the earlier

types, and for this reason we predict that

there will be an upward trend to hovercraft

production in the next decade in military,

pars-military (e.g. "coastguard") and civil

roles.

The amphibious hovercraft is undoubtedly by

far the best vessel for assault landing and

logistic supply work, operating as it does, over shallow water and possible minefields

from and to almost any beach. Instead of

taking several days to ship supplies to the

Continent, as demonstrated by a recent

exercise, all types of equipment could be shipped from the east coast of Britain to

the Continent in a matter of hours, in some

cases to areas well inland if the main river

accesses are employed.

It is considered that European Air Cushion

Landing Craft should be larger and longer

ranged than the American L.C.A.C. and would

operate from shore to shore, rather than

from ship to shore. These craft would be

rugged, of welded construction and _highly

powered with a radius of action of at least

500 n.m. and would be significantly bigger

than either the JEFF craft Figure 11, or the

Russian Aist, Figure 12.

Equally unique in its effectiveness is the

M.C.M. hovercraft, being immune to

under-water shock when in the cushoin-borne mode

and having very low magnetic, acoustic and

pressure signatures. With the advent of high

speed search sonars, these craft will be

many times more effective than the best

current surface vessel with the added

advantage of being able to move from one

operational zone to another far more quickly.

Figure 13 shows a version of the SR.N4 Mk2

adapted to the Mine Counter Measures Role.

Large craft, of say 750 tons, could make

excellent ASW vessels capable of outrunning the fastest submarines. The craft shown in

figure 14 would have an overall length of

about 250 ft., a beam of 90 ft. resulting in

a cushion loading of about 125 lb/ft'.

Bell Halter have also looked at the longer

term feasibility of the larger sidewall

craft as a low cost replacement of current frigates. An artists impression of a 1,000

ton patrol craft is shown in Figure 15.

On the civil front, the Bell Halter type of

craft will undoubtedly make a name for

it-self in the passenger ferry and offshore supply vessel roles as well as in the

offshore patrol, search and rescue and other

para-military roles.

However, it will become progressively less

economic as speed is pushed up and will

probably be normally restricted to maximum speeds of between 40 and 50 knots. A

pos-sible passenger ferry of about 200 tons

A.U.W., capable of carrying 400 passengers

at a cruise speed of 35 knots is shown in

Fiat/re 16.

It is extremely difficult to make an

ef-ficient car/passenger ferry due to the low

specific density of the payload. For this

reason the larger and more advanced

amphi-bious ferries will almost certainly be

double or even triple decked, passengers

probably being restricted to the upper

deck if anyting like a reasonable car/

passanger ratio is to be obtained.

Figure 17 shows a hypothetical craft, based

on the Super-4 but stretched by a further 55

ft. and fitted with ducted propellers aft

and bleed air thrusters forward. This craft

would carry 144 cars and 600 passengers. The

variable height mezzanine deck in the centre

not only facilitates easy loading of cars

but also allows coaches or lorries to be

carried in place of cars if the traffic

demands.

Although it would appear that th '1,000' ton

hovercraft will almost certainly make its

debut in the period under review, the authors

have refrained from speculating on the

multi-thousand ton "Merchantman" or naval S.E.S.,

since the immense productivity of even a 1,000

ton craft will produce marketing and

operatio-nal problems which we have hardly started to

grasp. Route lengths of between 100 and 1,000

n.m. will become a reality in the civil field

while military craft will be capable of

mis-sions extending over several days.

Other advances such as the use of very thin

sidewalls, artificial stabilisation and more responsive skirts will undoubtedly develop

which together with even more efficient

marine propulsion units and power plants,

will ensure a continuous improvement in

effectiveness. Whatever these developments are, we feel sure that the next 25 years

will be a interesting and rewarding period

when the hovercraft will challenge the supremacy of the ship on an increasing

number of ferry routes and for very many

naval and military roles.

Ref. 1 - AIAA Paper no. 74-330

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