Theory Design Air Cushion Craft 2009 Part 1 docx

Theory and Designof Air Cushion Craft Liang Yun Deputy Chief Naval Architect of the Marine Design & Research Institute of China... Air cushion theory 482.1 Introduction 482.2 Early air c

Theory and Design of Air Cushion Craft

This page intentionally left blank

Theory and Design

of Air Cushion Craft

Liang Yun

Deputy Chief Naval Architect of the Marine

Design & Research Institute of China

First published in Great Britain in 2000 by

Arnold, a member of the Hodder Headline Group,

338 Huston Road, London NW1 3BH

http://www.arnoldpublishers.com

Copublished in North, Central and South America by

John Wiley & Sons Inc., 605 Third Avenue,

London W1P9HE.

Whilst the advice and information in this book are believed to be true and accurate at the date of going to press, neither the authors nor the publisher can accept any legal responsibility or liability for any errors or omissions

that may be made.

British Library Cataloguing in Publication Data

A catalogue record for this book is available from the British Library

Library of Congress Cataloging-in-Publication Data

A catalog record for this book is available from the Library of Congress

ISBN 0 340 67650 7 ISBN 0 470 23621 3 (Wiley)

1 2 3 4 5 6 7 8 9 10 Typeset in 10/12 pt Times by J&L Composition Ltd, Filey, North Yorkshire Printed and bound in Great Britain by Redwood Books Ltd What do you think about this book? Or any other Arnold title? Please send your comments to feedback.arnold@hodder.co.uk

This book is dedicated to advancement of Air Cushion Technology, and to the specialband of researchers and engineers worldwide who have created its foundation.

This page intentionally left blank

Preface xi Acknowledgements xiii

1 Introduction to hovercraft 11.1 Hovercraft beginnings 11.2 ACV and SES development in the UK 91.3 ACV and SES development in the former USSR 221.4 US hovercraft development 251.5 ACV and SES development in China 321.6 SES and ACV developments in the 1990s 391.7 Applications for ACV/SES 411.8 The future 451.9 SES and ACV design 46

2 Air cushion theory 482.1 Introduction 482.2 Early air cushion theory developments 502.3 Practical formulae for predicting air cushion performance 552.4 Static air cushion characteristics on a water surface 662.5 Flow rate coefficient method 712.6 The 'wave pumping' concept 732.7 Calculation of cushion stability derivatives and damping

coefficients 76

3 Steady drag forces 843.1 Introduction 843.2 Classification of drag components 84

3.4 Aerodynamic profile drag 963.5 Aerodynamic momentum drag 963.6 Differential air momentum drag from leakage under bow/stern

seals 973.7 Skirt drag 98

viii Contents

3.8 Sidewall water friction drag

3.9 Sidewall wave-making drag

3.10 Hydrodynamic momentum drag due to engine cooling water

3.11 Underwater appendage drag

3.12 Total ACV and SES drag over water

3.13 ACV skirt/terrain interaction drag

3.14 Problems concerning ACV/SES take-off

3.15 Effect of various factors on drag

4 Stability

4.1 Introduction

4.2 Static transverse stability of SES on cushion

4.3 SES transverse dynamic stability

4.4 Calculation of ACV transverse stability

4.5 Factors affecting ACV transverse stability

4.6 Dynamic stability, plough-in and overturning of hovercraft

calm water5.4 Dynamic trim of ACV/SES on cushion over calm water

6 Manoeuvrability

6.1 Key ACV and SES manoeuvrability factors

6.2 Introduction to ACV control surfaces

6.3 Differential equations of motion for ACV manoeuvrability

6.4 Course stability

6.5 ACV turning performance

7 Design and analysis of ACV and SES skirts

7.1 Introduction

7.2 Development and state of the art skirt configuration

7.3 Static geometry and analysis of forces acting on skirts

7.4 Geometry and analysis of forces in double or triple bag stern

skirts7.5 Geometry and forces for other ACV skirts

7.6 Analysis of forces causing the tuck-under of skirts

7.7 Skirt bounce analysis

7.8 Spray suppression skirts

7.9 Skirt dynamic response

8 Motions in waves

8.1 Introduction

104111115115117121124130136136137152163168173185187187190197200205205207217224227232232235250258260261267270271273273

Contents ix

8.2 Transverse motions of SES in beam seas (coupled roll and heave)

8.3 Longitudinal SES motions in waves

8.4 Longitudinal motions of an ACV in regular waves

8.5 Motion of ACV and SES in short-crested waves

8.6 Plough-in of SES in following waves

8.7 Factors affecting the seaworthiness of ACV/SES

9 Model experiments and scaling laws

9.1 Introduction

9.2 Scaling criteria for hovercraft models during static hovering tests

9.3 Scaling criteria for tests of hovercraft over water

9.4 Summary scaling criteria for hovercraft research, design and tests

10 Design methodology and performance estimation

10.1 Design methodology

10.2 Stability requirements and standards

10.3 Requirements for damaged stability

10.4 Requirements for seaworthiness

10.5 Requirements for habitability

10.6 Requirements for manoeuvrability

10.7 Obstacle clearance capability

11 Determination of principal dimensions of ACV/SES

11.1 The design process

11.2 Role parameters

11.3 Initial weight estimate

11.4 First approximation of ACV displacement (all-up weight),

and estimation of weight in various groups

11.5 Parameter checks for ACV/SES during design

11.6 Determination of hovercraft principal dimensions

12 Lift system design

12.1 Introduction

12.2 Determination of air flow rate, pressure and lift system power

12.3 Design of fan air inlet/outlet systems

12.4 Lift fan selection and design

13 Skirt design

13.1 Introduction

13.2 Skirt damage patterns

13.3 Skirt failure modes

13.4 Skirt loading

13.5 Contact forces

13.6 Selection of skirt material

13.7 Selection of skirt joints

13.8 Assembly and manufacturing technology for skirts

13.9 Skirt configuration design

279 294 308 322 324 328 342 342 343 348 352 353 353 355 363 364 365 374 376 377 377 378 379 384 397 399 405 405 407 413 420 433 433 433 435 437 441 442 447 449 451

x Contents

14 Structural design 45814.1 ACV and SES structural design features 45814.2 External forces on hull - introduction to the strength

calculation of craft 46114.3 Brief introduction to the structural calculation used in

MARIC 465

14.4 Calculation methods for strength in the former Soviet Union 46714.5 Safety factors 47314.6 Considerations for thickness of plates in hull structural design 47414.7 Hovercraft vibration 476

15 Propulsion system design

15.7 Surface contact propulsion

16 Power unit selection

16.1 Introduction

16.2 Powering estimation

16.3 Diesel engines

16.4 Gas turbines

16.5 General design requirements

16.6 Machinery space layout

16.7 Systems and controls

16.8 Operation and maintenance

References

Index

487487507515520536564574577577585588596604606607607

612 618

It is 39 years since sea trials of the first hovercraft Hovercraft are a new means oftransportation, and so machinery, equipment and structural materials have had to beadapted for successful use in their special operating environment, which differs fromthat in aviation and for other marine vessels

A somewhat difficult technical and economic path has been negotiated by the opers of hovercraft technology to date Currently about 2000 craft are in operation forcommercial water transportation, recreation, utility purposes and military applica-tions around the world They have taken a key role for a number of military missions,and provide utility transportation in a number of applications which are quite unique.Hovercraft in China have developed from prototype tests in the 1960s, to practicaluse as ferries and military craft More than 60 hovercraft types have been constructed

devel-or impdevel-orted fdevel-or operation in China This book has been written to summarize theexperience in air cushion technology in China and abroad to date, with the aim ofimproving understanding of air cushion technology

Due to the relatively quick development of the cushion technology relative to otherwater transportation, the theories and design methods applied to hovercraft designand operations are continuing to develop at present For instance various quasi-statictheories of the air jet cushion were derived in the 1960s, but once the flexible skirt wasdeveloped, the hydrodynamic and aerodynamic forces acting on hovercraft changed

so significantly that these earlier theories and formulae could not continue to serve inpractice

The theory of air cushion performance has therefore changed significantly since the1960s On one hand a lot of technical references and some technical summaries andhandbooks with respect to air cushion technology are available to translate the phys-ical phenomena but on the other, owing to different research methods, objects andmeans, there are many different methods which suggest how to deal with such theo-ries So far no finalized rules and regulations for hovercraft construction can be stated

In addition regulatory documents concerned with stability, seaworthiness and the culation methods determining the static and dynamic deformation have not reachedpublic literature

cal-The aim in writing this book has been to summarize the technical experience, both

in China and abroad, to systematically describe the theory and design of hovercraft,and endeavour to connect the theories with practice in order to solve practical prob-lems in hovercraft design

xii Preface

There are three parts to this book The first chapter gives a general introduction tohovercraft, which introduces briefly the classification of hovercraft, and the develop-ment and civil and military applications of the hovercraft in China and abroad in thelast three decades The second part, from Chapters 2 to 9, systematically describesACV and SES theory - primarily the hydrodynamics and aerodynamics of cushionsystems The third part, from Chapters 11 to 16, describes the design methods of ACVand SES, including the design criteria and standards for craft performance, lift systemdesign, skirt design, hull structure design, and methods for determining the principaldimensions of craft

The principles for material presented in this book are to describe the features of aircushion technology, and give sufficient design information to allow the reader to pre-pare a basic project design Engineering subjects which are similar to those for con-ventional ships are not covered here, being available to the student in existing navalarchitecture or marine engineering texts Thus, stability here covers only the calcula-tion method for stability of ACV and SES on cushion, and not stability of hovercraftwhile floating off cushion

With respect to the design of machinery and propulsion systems of ACV and SES,for instance, air or water propeller design, water-jet propulsion installation andmachinery installtion in hovercraft, which is rather different from that on conven-tional ships, these are covered in summary in the last chapters

The intent is to guide the reader on how to perform machinery and systems tion within ACV or SES overall design Detail design of these systems requires sup-port of specialists in turbo-machinery, piping design, etc who will normally beincluded in the project team The student is referred to specialists in these fields forinterface engineering advice, or to the marine or aeronautical engineering department

selec-at his college or university

The intended audience for this book are teachers and students, both at uate and postgraduate level in universities, and engineers, technicians and operatorswho are involved in ACV/SES research, design, construction and operation or wish towork in this field

undergrad-During the writing of this book, the authors have had the help and support fromsenior engineers and researchers of MARIC and used research results and theoriesfrom many sources, such as the references listed at the end of this book, and theywould like to express sincere thanks to those authors for their inspiration Meanwhilethe authors also would like heartily to thank Professor IS Dong of the Chinese NavalEngineering Academy for his help and revision suggestions for this book

Hovercraft and component manufacturers throughout the world have kindly plied data and many of the photos Our thanks for their continuing support andadvice

sup-Alan Bliault and Liang Yun

August 1999

The authors wish to thank all organizations and individuals who have assisted in thepreparation of this book by supplying key data and illustrations These include thefollowing: ABS Hovercraft, British Hovercraft Corporation, Dowty, Griffon Hover-craft, Hoffman Propeller, Hovermarine International, KaMeWa, KHD Deutz,Kvaerner, Marine Design and Research Institute of China (MARIC), MJP Waterjets,Mitsui Shipbuilding Corporation, MTU Motoren, Rolls Royce, the US Navy, andmany persons too numerous to name individually Thank you all sincerely

Publications of the China Society of Naval Architects and Marine Engineers(CSNAME), the Society of Naval Architects and Marine Engineers (USA), the RoyalInstitution of Naval Architects (UK), and the Canadian Aeronautics and Space Insti-tute document that core research by engineers and scientists on ACV and SES whichhas been an essential foundation resource for our work We trust that this innovativematerial has been repressented acceptably in this book

The tremendous assistance of colleagues at MARIC, as well as assistance and ration of experts, professors, and students at the Harbin Shipbuilding EngineeringInstitute, Wu Han Water Transportation Engineering University, Naval EngineeringAcademy of China, and other shipyards and users in China, is gratefully acknowl-edged as the driving force behind the publication of this book

inspi-Sincere thanks goes to our two families over the long period of preparation, whichhas spanned most of the last decade

Finally, the staff at Arnold have given tremendous support to see the task through.Many thanks for your unending patience!

This page intentionally left blank

Introduction to hovercraft

1.1 Hovercraft beginnings

Transport is driven by speed Since the 1970s, with the price of fuel becoming animportant component of operating costs, transport efficiency has become a significantfactor guiding concept development During the last century, the service speed ofmany transport concepts has dramatically increased, taking advantage of the rapiddevelopment of internal combustion engines Aeroplane flying speed has increased by

a factor of 10, and the automobile by a factor of three In contrast, the highest mercial ship speeds have increased by less than a factor of two, to a service speed ofabout 40 knots

com-Some planing craft and fast naval vessels reached this speed in the 1920s They wereable to do this because payload was not a key requirement, so that most of the carry-ing capacity could be devoted to power plant and fuel Hydrodynamic resistance wasthe prime factor limiting their performance A displacement ship moving at highspeed through the water causes wavemaking drag in proportion to the square of itsspeed This limits the maximum speed for which a ship may be designed, due to prac-tical limitations for installed power It is possible, however, to design ship forms usingthe surface planing principle to reduce wavemaking at higher speeds Many planingboat designs have been built, though the power required for high speed has limitedtheir size Their application has mostly been for fast pleasure and racing craft, and formilitary vessels such as fast patrol boats

Planing vessels demonstrated the potential for increased speed, but slammingcaused by wave encounter in a seaway still created problems for crews, passengers andthe vessels themselves, due to high vertical accelerations Two possibilities to avoidslamming are either to isolate the hull from contact with the water surface, or sub-merge it as completely as possible under the water to reduce surface wave induceddrag Hydrofoils, air lubricated craft, amphibious hovercraft (ACV), surface effectships (SES) and wing in ground effect machines (WIG and PARWIG) arose from thefirst idea, while the latter concept produced the small waterplane thin hull vessel(SWATH) and, more recently, thin water plane area high speed catamarans Fig 1.1shows a classification of high speed marine vehicle types

ACV and SES - the subject of this book - developed from the idea to design a craftwhich is supported by a pressurized air 'cushion' By this means the hard structure is

1

2 Introduction to hovercraft

Fast marine craft

Primary support Vessel classification Vessel subclassification

Stepped planning hull Captured air bubble craft Hydrokeel

Fig 1.1 Classification of high-performance marine vehicles.

just far enough away from the water surface to reduce the surface interference, waterdrag and wavemaking, while at the same time close enough to trap the pressurized airbetween the ground and the lifted body Under these circumstances the pressure gen-erated is many times greater than the increased pressure under a free aerofoil, whilethe drag of the lifted body is much reduced compared to a planing surface

The idea to take advantage of an air cushion to reduce the water drag of a marinecraft has actually been established for over one hundred years [210] [211] In GreatBritain, Sir John I Thornycroft worked on the idea to create a thin layer of air overthe wetted surface of a ship, and was awarded a UK patent in 1877 He developed

a number of captured air bubble hull forms with cavities and steps in the bottomand model tested them as alternatives to conventional displacement torpedo boats,which his company built for the British Navy at the time No full scale vessels werebuilt to translate the idea into practice, though the model testing did give favourableresults

A patent for air lubrication to a more conventional hull form was awarded to Gustav deLaval, a Swedish engineer, in 1882 A ship was built based on the proposals,

Hovercraft beginnings 3

but Laval's experiments were not successful The air lubrication created a

turbulent mixture of air bubbles and water around the hull, rather than a consistent

layer of air to isolate the hull surface, and so drag was not reduced

Air lubrication has been pursued at various times since these early experiments by

engineers and scientists In practice it has been found that it is very difficult to create

a consistent drag reducing air film on the wetted surface of a normal displacement

hull On the contrary sometimes an additional turbulent layer is added, increasing the

water friction drag A more substantial 'captured air bubble' is needed

In 1925, D K Warner used the captured air bubble principle to win a boat race in

Connecticut, USA He used a sidewall craft with planing bow and stern seals A little

later, the Finnish engineer Toivio Kaario developed and built prototypes of both the

plenum chamber craft and the first ram wing craft (Fig 1.2)

To investigate thin film air lubrication, some experiments were carried out in the

towing tank of MARIC in Shanghai, China by the author and his colleagues in 1968,

but the tests verified the earlier results of Laval and others Based on these results they

confirmed that a significant air gap was necessary to separate the ship hull fully from

the water surface This needed a concave or tunnel hull form

In the mid 1950s in the UK, Christopher Cockerell developed the idea for high

pressure air jet curtains to provide a much greater air gap This invention provided

sufficient potential for a prospective new vehicle technology that the British and later

the US government committed large funds to develop ACV and SES China and the

USSR also supported major programmes with similar goals over the same period

Air cushion supported vehicles could only be successfully developed using suitable

light materials for the hull and engines Initial prototypes used much experience from

aircraft design and manufacture to achieve the necessary power to weight ratio

Experience from amphibious aeroplanes or flying boats was particularly valuable

since normal aircraft materials are not generally designed to resist corrosion when

Fig 1.2 Finnish ACV constructed by Toivio Kaario in 1935.

4 Introduction to hovercraft

immersed in salt water an important design parameter for marine vehicles.Additionally, it suggested a number of alternatives to the basic principle of pumping airinto a cavity under a hull, using a modified wing form instead, to achieve vehicles withspeeds closer to that of aircraft Several vehicle concepts have developed from this work

Amphibious hovercraft (or ACV)

The amphibious hovercraft (Fig 1.3) is supported totally by its air cushion, with anair curtain (high pressure jet) or a flexible skirt system around its periphery to seal thecushion air These craft possess a shallow draft (or a negative draft of the hull struc-ture itself) and amphibious characteristics They are either passive (being towed byother equipment) or active, i.e propelled by air propellers or fans Some 'hybrid' crafthave used surface stroking, balloon wheels, outboard motors and water jets to achievedifferent utility requirements

Fig 1.3 First Chinese medium-size amphibious hovercraft model 722-1.

Sidewall hovercraft (or SES)

This concept (Figs 1.4 and 1.5) reduces the flexible skirt to a seal at the bow and stern

of a marine (non-amphibious) craft, using walls or hulls like a catamaran at the sides.The walls or hulls at both sides of the craft, and the bow/stern seal installation, aredesigned to minimize the lift power

Due to the lack of air leakage at the craft sides, lift power can be reduced significantlycompared with an ACV Also, it is possible to install conventional water propellers orwaterjet propulsion, with rather smaller machinery space requirements compared to thatfor air propellers or fans used on ACVs This more compact machinery arrangement,combined with the possibility for higher cushion pressure supporting higher specific pay-load, has made a transition to larger size much easier for this concept than for the ACV

Hovercraft beginnings 5

Fig 1.4 Chinese passenger sidewall hovercraft model 719-1

Fig 1.5 First Chinese passenger sidewall hovercraft type, Jin Sah River.

Wing-in-ground effect (WIG) and power augmented ram wing

(PARWIG) craft

These craft are rather different from the ACV or SES They are more like low flying

aircraft, and use ground proximity to increase lift on the specially shaped wing The

craft are supported by dynamic lift rather than a static cushion

The WIG (Fig 1.6) initially floats on the water and its take-off is similar to a

sea-plane An aeroplane wing operated close to the ground generates lift at the

pressur-ized surface of the wings which is increased significantly due to the surface effect The

aero-hydrodynamic characteristics of a WIG are therefore a significant optimization

of the design of a seaplane to improve payload

The PARWIG shown in Fig 1.7 differs from a WIG by the different location of

lift fans, in which the lift fans (or bow thrusters) are located at the bow and beyond

the air cushion; consequently a large amount of air can be directly injected into the