## Drag of bowstern seals of an SESf flsb

This is also estimated based on test data. Two methods for predicting this drag are as follows. Based upon a series of model tests, a statistical analysis was carried out at MARIC. Putting the skirt (bow stern) drag, the drag due to the differential momentum from the bow stern seals, the wave-making drag of sidewalls, and the interference drag due to sidewalls, all into a single term 'residual drag' and then processing this by regression analysis, the following equation is obtained where Rr is...

## Effect of lcbc

The increase in ljbc can always improve the seaworthiness and the speed degradation of craft both for ACVs and SES. The relation of frontal area may be one of the reasons for reducing water drag of skirts reduction of longitudinal motion of the craft due to the increase in cushion length may be another reason for improving speed degradation. Figure 8.32 shows a speed comparison between the SR.N4 Mkl and Mk3 craft models. Keeping cushion pressure constant, the SR.N4 Mkl was stretched by 16.74 m,...

## Air jet propulsion

If we consider alternatives to a propeller to create a jet of air for propulsion or manoeuvring of an ACV, the following might be used air ducted from a mixed flow or centrifugal fans A small component of thrust is often present from the gap under stern skirts. To prevent scooping through hump speed a positive gap under stern segments is desirable anyway, usually about 10 of the segment height. The thrust developed from this air jet is where Vc is the craft speed air speed and V the air jet...

## 38 Sidewall water friction drag

Friction drag is a large proportion (up to 3CM0 for high speed craft) of total craft drag, as shown in Fig. 3.1. For this reason, calculation has to be made carefully. The main difficulty is to determine the wetted surface accurately. In general, two methods are used one is the theoretical method as described in Chapter 5 and the other is a model experimental method similar to that used in planing hull design, i.e. the wetted area can be obtained by photographs. We will introduce four different...

## Overall propulsive efficiency

If we consider the results of the calculation above and compare with open water propellers and the likely range for water jets, the following data are relevant It can be seen that water jets have a large range of possible efficiency dependent on the system component performance. Careful integration of system components into the SES hull design is necessary, aimed at optimization of the intake geometry and jet nozzle velocity, in order to achieve OPC in the range 0.65-0.7 for an SES. Once a...

## 313 ACV skirtterrain interaction drag

For an ACV which operates mainly over land, such as self-propelled air cushion platforms, it is important to accurately determine the skirt terrain interaction drag, as it is a high percentage of the total drag. The total overland drag of ACV can be written as follows gacv a + m + sp + si + sk (3.50) where i gacv is the total overland drag of ACV (N), Ra the aerodynamic profile drag (N), Rm the aerodynamic momentum drag (N), Rsp the spray (debris) momentum drag (N), Rsi the slope drag (N) and...

## 12 ACV and SES development in the UK Initial research before 1963

In 1953, Christopher Cockerell, an electronics engineer with a small commercial boatbuilding interest, began thinking about the age-old problem of decreasing the resistance to ships travel through the water. First he tried introducing air films under model boats to give a kind of lubricated surface. This was not successful and the next stages towards the evolution of the hovercraft principle are best described in his own words After I had learnt from, and found out the shortcomings of...

## Weight classification of hovercraft in MARIC

The weight classification below is used by MARIC Wl hull weight including main hull structure, superstructure, bow stern ramp structure, landing pad, air propeller ducts and their mountings, engine mountings and their strengthened structure, various bearings and mountings, etc. W2 outfit weight including anchoring equipment, rudder and their handling equipment, mast, ladder, etc., cover of windows and doors, retractable equipment for ramp, life-saving and fire-fighting system, cabin equipment,...

## 153 Ducted propellers and fans

The primary purposes for installing ducted air propulsors are to reduce diameter so as to reduce noise level for a given thrust and to provide higher thrust levels at low speeds giving greater thrust margin for acceleration through hump speed. The penalty is that of duct weight. With efficient cushion systems now making the use of heavier hull structures and diesel engines practical, this should not be a significant penalty and the advantages can be maximized. Other benefits include the...

## A theoretical method for predicting Acvses dynamic trim over calm water

We take the SES as an example to calculate the dynamic trim. The calculation method predicting the dynamic trim of an ACV on cushion is similar to that of an SES. Factors determining the dynamic trim of an SES on cushion are numerous, e.g. craft speed (Fn), LCG, configuration of sidewalls, lift fan flow rate and total pressure, cushion length beam ratio, cushion pressure length ratio and skirt clearance over the water surface. The determination of dynamic trim of a craft on cushion has to meet...

## Effect of sidewall geometric configuration on the static transverse stability

Taking the SES model 717 as an example, we calculate the static transverse stability of the craft with a specific fan speed, duct configuration and form of the seals, and Fig. 4.7 Typical transverse section of two craft models with different inner sidewall geometries, (a) Model 1, (b) model 2. Fig. 4.7 Typical transverse section of two craft models with different inner sidewall geometries, (a) Model 1, (b) model 2. Fig. 4.8 Heeling restoring moment of model craft No. 1 with different fan speeds...

## 42 Static transverse stability of SES on cushion

Calculation of static transverse stability of an SES off cushion is similar to that for a conventional catamaran, so we can solve the problem for an SES by extending the calculation method for catamarans. As shown in Fig. 4.1, the air cushion force (pc.Sc) will make a heeling moment with craft weight W, and the buoyancy of both sidewalls (yvu yv2) will create a restoring moment. This can be seen more clearly in the case of large heeling angle as shown in Fig. 4.2. While the heeling angle is...

## Ability to maintain cushionborne operation of Acvses in high waves

With respect to an ACV, since the aerodynamic momentum drag, aerodynamic profile drag and skirt drag comprise the most part of craft total drag, these drags will be significantly increased in the case of head-wind operation of craft. Under such conditions the craft will experience speed degradation not only due to the drag increase but also the light load of the air propellers, if they are fixed pitch propellers. Thus the power output and thrust will be reduced with the same propeller...

## 14 US hovercraft development Amphibious craft

The US Government has supported the development of air cushion technology primarily through its military applications. Americans like to use the aeroplane and car as passenger transport both for long and short range journeys, but have paid less attention to the development of high speed marine vessels as water transport for passengers. For this reason the development of US military hovercraft represents the main development of the US hovercraft. The US Armed Forces initially aimed to apply air...

## Effect of the cushion length beam ratio on the transverse stability

The calculated transverse stability of craft type 717 with different cushion length beam ratios is shown in Fig. 4.17. From Table 4.4, it is found that the initial transverse stability at large heeling angles will not change significantly, for several variations of SES type 717, as long as the cushion pressure is kept constant, even though with different cushion length beam ratio, weight and cushion pressure length ratio for different types of craft. It is not Fig. 4.17 Static transverse...

## 24 Static air cushion characteristics on a water surface Static hovering performance of SES on water

The various shapes of mid-sections of sidewalls are shown in Fig. 2.16 a typical one is figure (a), namely sidewalls with perpendicular inner and outer walls near the water surface. The craft total weight is supported by a combination of cushion lift and buoyancy of the sidewall, which can be expressed as where Wis the craft weight (N),pc the cushion pressure (N m2), Sc the cushion area (m2), V0 the volumetric displacement provided by each sidewall (m3) and yw the specific weight of water (N...

## Skirt height ratio for an ACV Hskfic

The effect of HJBC for an ACV is equivalent to that of HJBC for an SES. It should be noted that we are addressing the skirt height measured to the hull keel, not the upper loop attachment. The position of the upper loop attachment both vertically Fig. 11.3 Cushion pressure trends for hovercraft (hatched area for passenger hovercraft). (hull depth) and horizontally (hull sidebody width) can be adjusted to tighten or loosen the loop tension and so stiffness. If the required underkeel clearance...

## Slamming forces in waves

Slamming is normally designed by interpolation from special model tests, or from data presented in references such as the BHSRs 79 , Chapter B4 2. The slamming pressures of ACV hulls are typically 50 less than that of conventional planing hull, while those at the bow of SES sidehulls are similar. Du Cane 87 suggests that for planing boats a typical design peak pressure may be about 3.45 bar acting over a small area of 0.1-0.2 m2, based on measurements from fast patrol craft. Such pressures...

## Hovercraft seaworthiness

An ACV is able to display the special characteristics for which it is best known while running at high speed over shallow water, rapids, ice and swamp - places no other craft can go. While these 'special abilities' interest many military and civil users with particular mission requirements, such environments do not include the wind-driven waves found in an open seaway. Generally, a craft's capabilities in an open seaway will control its transit capabilities between locations where a special...

## Momentum theory and jet efficiency

Having considered the main system losses, excepting the pump, we first consider the efficiency of a jet system, before looking at the pump itself in a little more detail. Water entering the water-jet system is considered to be accelerated to the forward speed of the vessel, Vc before being accelerated through the pump and nozzle to Vs. The net thrust developed by a water jet is therefore the energy applied by the pump to the water mass is The propulsive efficiency is therefore fji TVJ 0.5m V)...

## 154 Marine propellers

Marine propeller design developed from the same momentum and blade element theories reviewed above 88 , The denser fluid in which they operate allows the blades to have much higher chord length and solidity than an air propeller. Marine propellers have total blade area which is between 50 and 120 of the disc area (solidity, or blade area ratio 0.5-1.2). The blade pressure distributions interfere with each other, reducing lift force compared to isolated aerofoil theory. Chord length at 0.7R and...

## 310 Hydrodynamic momentum drag due to engine cooling water

In general, the main engines mounted on SES have to be cooled by sea water which is ingested from Kingston valves or sea water scoops mounted at propeller brackets, via the cooling water system, then pumped out from sidewalls in a transverse direction. The hydrodynamic momentum drag due to the cooling water can be written as where Rmw is the hydrodynamic momentum drag due to the cooling water for engines (N), Vj the speed of inlet water, in general it can be taken as craft speed (m s), and Qw...

## Basic theories

Before looking at selection for a particular craft, it is useful to review the physical phenomena which apply to both air and water propellers. Our purpose is to give an insight into the efficiency with which power (torque) is translated into thrust at various speeds and to discuss the basis on which blade geometry and number are best selected. This information should allow initial sizing of propulsors without reference to design of the units themselves, allowing realistic enquiries to...

## Measures for improving resistance to ploughin and overturning

(A) Cushion and skirt air supply system 1. Keep a definite reserve on fan inflow rate and so increase the speed of fan, air gap, especially, the air supply flow rate at bow. If necessary, to increase the bow air gap and restore the running attitude to normal (in time), in the case where the craft develops a bow pitching down trim. It is very effective to provide a special air duct to supply pressured air to the cushion at the bow in order to improve the plough-in resistance. This has been...

## Equations for static geometry and force analysis of skirts

The outer and inner geometry lines of the skirt form concentric circles (i.e. the arc, B,'B2 and AiA2' are part of concentric circles), as shown in Fig. 7.19. If we take small element Aa as the investigated object, then the arc section OC,'C2' is the projection of this element on the X-Z plane. The quadrilateral C,C2B1B2 is the section plan of the bag arc in this element, which is perpendicular to the line BC. The quadrilateral C1C2E1E2 is the section plan of the finger element perpendicular to...

## Scaling conditions for wind tunnel model tests

In the case where the craft model tests are carried out in a wind tunnel to determine the aerodynamic resistance coefficients at various heading angles, the Re of jet flow at nozzles has to be equal to or in excess of the critical Re (i.e. the Re for turbulent flow). In addition, the linear velocity of any moving supporting plate in the wind tunnel has to be equivalent to the craft speed, namely V A0'5, in which Fg denotes the linear velocity of the supporting plate. Otherwise, the wind tunnel...

## 32 Classification of drag components

The method of calculating drag forces on an ACV or SES is similar to that for predicting the drag of a planing hull or a sea plane before take-off. ACVs and SES also generate spray drag, skirt friction drag and skirt inertia drag in addition to the water drag components associated with a normal ship. For this reason drag calculations are more complicated than for other marine craft. Based upon calculation methods for predicting the drag of a planing hull, the principal author and colleagues at...

## Determination of wave impact pressure [4[52

When hovercraft are running at high speed in waves, the impacting pressure due to waves at the acting centre of the waves (e.g. the maximum hydrodynamic pressure) can be written as where K is an empirical coefficient, take K3 4.6 1.9, pw is the water density (Ns2 m4) and 2 x (0.22L) when 0.22L 1 K2 1 when 0.22L J The data can also be obtained in Fig. 14.1(b). Reference 4 discussed the linear relation between the wave-impacting force acting on the bow and the craft speed, as in expression...

## Calculation method for heaving stability derivatives and damping coefficients

First of all, the profile of the skirt is assumed unchangeable in the case of deriving the air cushion stability derivatives and damping coefficient. This assumption is Fig. 2.25 Frequency response for heave motion with one degree of freedom. Fig. 2.25 Frequency response for heave motion with one degree of freedom. Fig. 2.26 Three conditions for heave motion of ACV (a) equilibrium (b) underfed (c) overfed. reasonable for a conventional medium pressure bag and finger type skirt for small...

## Calculation of the transverse stability of SES with rigid stern seal

The foregoing calculation procedure cannot be used in the case of the rigid stern seal, because the lift acting on the planing plate is so much larger than that on the flexible skirts at same heeling angles, and leads to a trim moment to change the running attitude, cushion pressure and other parameters, etc. The changing running attitude may be obtained by means of an iteration method, from which the stern plate lift and restoring moment on the craft can then be determined. Since the end of...

## Transverse and longitudinal metacentric heights

These are very important criteria to characterize the stability of craft, particularly in the case of craft with large cushion beam and at small heeling angles therefore, in general, designers always take this as the parameter for characterizing the transverse and longitudinal stability. According to 52 , they can be written as follows. For ACVs For British ACVs, ref. 19 recommended where hog, hvg denote the dimensionless transverse and longitudinal metacentric heights of an ACV hovering on a...

## Twodimensional bag and finger type bow skirt and stern planing rigid seal

As shown in Fig. 1.35, these are typical seals mounted on the Chinese inland river SES, which have characteristics of fine take-off capability, stable drag and low sensitivity to LCG shifting, but poor seaworthiness due to the larger heaving stiffness of the stern-fixed planing surface than that of the bow bag finger type of skirts to stop plough-in in following waves. Thanks to the small waves of inland rivers, these types of seals have been developed but they are not really suitable to be...

## Strouhal number Sr

Strouhal's number is closely related to the elastic aerodynamic characteristics of the skirt fingers. It characterizes the ratio of inertia force due to the air pressure and the elastic modulus of skirt materials and can be expressed as where vis the velocity of air flow (m s), a the velocity of sound in skirts or other structural materials (m s), co the vibration frequency of skirts (1 s) and any linear length. In the case where the skirt material of the full-scale craft has the same...

## Vibration damping

Mounting of main and auxiliary engines by vibration dampers should be considered. After making efforts to reduce noise level, such as reducing the noise level of gearboxes and hydraulic pipelines, vibration isolation for the main engines, packing the volute of centrifugal fans with isolation material, etc. the cabin noise level can be reduced to under 70 dBA (Table 10.5). After making efforts to reduce noise, the cabin noise of SES version 717 also dropped more than 10 dBA to 79 dBA. If the bow...

## 139 Skirt configuration design

Skirt design begins by determining the various parameters and geometric relationships of the skirt sections at side bow and stern, such as XA, Xc, aE, YA Yc YE, etc., as shown in Figs 13.16 and 13.17. Fig. 13.16 Geometric features of skirts, (a) Skirt of bag and finger type (b) Skirt of twin bag type. Fig. 13.16 Geometric features of skirts, (a) Skirt of bag and finger type (b) Skirt of twin bag type. Fig. 13.17 Some geometric features of skirts. Fig. 13.17 Some geometric features of skirts....

## 63 Differential equations of motion for ACV manoeuvrability

To identify the necessary forces which a control surface may have to apply, we need to solve the equations of motion for a given ACV design, the relevant manoeuvre and environmental condition. The manoeuvrability of an ACV is different from that of conventional ships in that the cushion air blows from the side skirt to provide the transverse force during manoeuvring. Therefore from the point of view of hydrodynamics, we should not investigate the manoeuvrability by means of forming differential...

## Calculation of transverse stability for an ACV

It is very complicated to calculate the transverse stability of an ACV hovering over water because of the deformation of the water surface. The suggested analysis procedure is therefore to investigate the transverse stability of an ACV hovering on a rigid surface, followed by the corrections necessary due to the water surface deformation, obtained with the aid of model experiments. The coordinate system and the basic assumptions can be written as follows 1. Since the ACV is supported on a rigid...

## Use of flat lift fan and duct system characteristics

The fan air duct characteristic curves have to be as flat as possible, for instance, the air ducts of air inlet outlet should be as large as possible to reduce the inflow and outflow velocity, which had not been possible to satisfy on SES version 7203. In addition, the parallel operation of multiple fans can also flatten the fan characteristic, e.g. there are two double inlet fans operating in parallel on SES version 719G and 713 but only a single inlet fan on SES version 7203, which is more...

## 147 Hovercraft vibration The importance and complexity of hovercraft vibration

Hovercraft vibration is a complicated problem, for the following reasons. Vibration with a severe and superharmonic excitation source The installed power is high for ACV SES even though the displacement is small, hence the specific power is as high as 15-60 kW t and with a high harmonic exciting force. For instance, on ACVs with turbine propulsion, the speed of a gas turbine is about 10 000 r.p.m. and the speed of air propeller about 1000 r.p.m. and lift fan about 500-1500 r.p.m. There are also...

## ACV fan air outlet system

At the fan air outlet, the air flow velocity and dynamic pressure are highest. In addition the air flow will be very turbulent unless a downstream volute and guide vanes are fitted. For this reason, it must be designed carefully. Outflow has to be led with no air blockage due to sudden changes of cross-sectional area and diffused gently to the craft air duct system. Air blockage sometimes may occur to the craft, because of a large amount of pipeline arranged in the plenum chamber, but it is...

## Air duct calculation for ACVs

The arrangements of ACV fans and air ducts differ greatly and dependent on the passenger cabin arrangement, machinery bay, transmission shaft system, fans and propellers. The calculation of air duct head loss and the flow distribution have to be analysed for the specific craft design. A typical example can be seen in Fig. 12.1. This figure shows that one fan blows the pressure air via the pressure chamber into the bags (such air ducts have been used on Chinese ACVs 711-IIA, 716-11, 7206 and...

## 11 Hovercraft beginnings

Since the 1970s, with the price of fuel becoming an important component of operating costs, transport efficiency has become a significant factor guiding concept development. During the last century, the service speed of many transport concepts has dramatically increased, taking advantage of the rapid development 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...

## Sidewall thickness ratio Bswflc

BSJBC influences the transverse stability, seaworthiness, speed and general arrangement. Therefore, it is a very important parameter for an SES. In the case of craft with medium speed (Fr 0.7-0.9), it is suggested thicker sidewalls are adopted. Start with BSJBC at 0.08-0.125 as an average to start, assuming that the main engines will be housed in midships flared areas if diesel power is used. The designer should concentrate on a few of these parameters at any one time. For ACVs, IJBC, pjlc and...

## 71 Introduction

Early in the development of ACVs, before the flexible skirt had been thought necessary, powerful lift engines were used to obtain a hovering gap of 50-150 mm under the hull hard structure. High-pressure peripheral air jets were used at that time to provide this vertical obstacle clearance over land and water. These craft had sufficient amphibious capability and vertical obstacle clearance to prove the air cushion concept, but they often encountered terrain with variation in surface elevation...

## 36 Differentia air momentum drag from leakage under bowstern seals

According to momentum theory this drag can be written as Ru. pa < j> h,BcP - (f> h2BcP)P - Wa (3.8) where Ra is the air momentum drag from differential leakage under bow stern skirts, < f> the discharge coefficient of air leakage (in general we take < f> 0.5-0.6), hx the bow air leakage clearance, i.e. the vertical distance between the lower tip of bow skirt seal and the corresponding inner water-line, (m), h2 the stern air leakage clearance (m), a the declined angle between the...

## B A Kolezaev method [19

Kolezaev defined the residual drag of sidewalls as a function of craft weight where R is the residual drag of sidewalls (N), Kh the coefficient of sidewall residual drag, obtained from Fig. 3.30, and Fthe craft weight (N). Fig. 3.29 Wave-making drag coefficient of slender sidewalls with the parabolic water planes. 39 Fig. 3.29 Wave-making drag coefficient of slender sidewalls with the parabolic water planes. 39 Fig. 3.30 Residual drag coefficient of sidewall as a function of LJBC and Froude...

## 162 Powering estimation

The first stage in sizing a craft is to make an overall weight estimate, as described in Chapter 11. Initial estimates for the known components (payload for example) are grossed up by use of relations for the unknown components based on previous experience. It is also necessary to investigate the craft overall dimensions and cushion parameters (Pc, Qc) at least in a preliminary way before trying to estimate powering. A choice between diesel or gas turbine power may be left open at the initial...

## Design and analysis methods for skirts

So far there are no systematic and complete analytical design methods for skirts, so we introduce some design considerations on skirts and the determination of some skirt parameters for the reader's reference, as follows. Determination of height of the bow stern skirts We can determine the height of the bow stern skirts according to the requirements for seaworthiness. With respect to coastal hovercraft the slope of bow stern skirt can be obtained from arc tan 0.4-0.55 (13.2) where Hskh and Hsks...

## SES propulsion

SES use water screws (open propellers) or water jets for propulsion, see Fig. 15.2. A subcavitating fixed pitch propeller is the simplest installation, relatively light and inexpensive. This has limitations though. Subcavitating propellers are very efficient at low speeds. Installation under the sidehulls of an SES often requires considerable shaft angles and possibly a vee gearbox to the engine. Fig. 15.1 Air propulsion types, (a) Pylon mounted open propellors on Hoverlloyd SRN-4 at Calais....

## 13 ACV and SES development in the former USSR

The former USSR has carried out ACV and SES research since the beginning of the 1960s. More than two hundred sidewall passenger hovercraft have been built since then, and over two hundred amphibious ACVs for military missions and passenger Fig. 1.21 USSR air cushion oil exploration platform model BU-75-VP. Fig. 1.21 USSR air cushion oil exploration platform model BU-75-VP. Fig. 1.22 Sormovich Aist large amphibious assault ACV. Fig. 1.22 Sormovich Aist large amphibious assault ACV. Fig. 1.23...

## Contents

Introduction to hovercraft 1 1.1 Hovercraft beginnings 1 1.2 ACV and SES development in the UK 9 1.3 ACV and SES development in the former USSR 22 1.4 US hovercraft development 25 1.5 ACV and SES development in China 32 1.6 SES and ACV developments in the 1990s 39 1.7 Applications for ACV SES 41 2.2 Early air cushion theory developments 50 2.3 Practical formulae for predicting air cushion performance 55 2.4 Static air cushion characteristics on a water surface 66 2.5 Flow rate coefficient...

## 155 Water jets

Water jets for marine propulsion have a similar background to marine screw propellers. Reference 113 gives a comprehensive description and bibliography for waterjet history. The British Navy carried out parallel trials of a water-jet powered and a water screw driven vessel, the Waterwitch vs the Viper, in 1863. At that time pumps used as water jets were found to be less efficient than screw propellers and so the British Royal Navy concentrated on water screw development as an alternative to...

## Theory of thin peripheral jet air cushion hovering on a rigid surface

This theory was used on early ACVs with rigid jet nozzles over ground for determining the air cushion performance. It assumes that The nozzles are infinitely thin, therefore the air flow is jetted uniformly perpendicular to the centre line of the jet. The air flow jetted from nozzles is non-viscous and incompressible. The air flow jetted from nozzles will not combine with media around the air jet (induced flows are not treated). The cushion is supported on a rigid surface. The transverse...

## The A A West single wall theory [10

Fig. 2.2 Cross-section of SES with plenum chamber cushion. 1 lift fan, 2 lift engine, 3 propulsion engine and propeller, 4 bow seal, 5 air cushion plenum chamber, 6 rigid surface, 7 sidewall, 8 stern seal. Fig. 2.2 Cross-section of SES with plenum chamber cushion. 1 lift fan, 2 lift engine, 3 propulsion engine and propeller, 4 bow seal, 5 air cushion plenum chamber, 6 rigid surface, 7 sidewall, 8 stern seal. Fig. 2.3 Skirt configurations (a) rigid peripheral jet (b) inflated bag with short...

## Payload fraction 116 and W7 combined

At the initial stage of hovercraft design, it is difficult to separate the components of 1 6 and Wl and so a combined estimate is given here. The designer will usually prepare a more refined estimate at the detailed design stage. where AT4 is a coefficient. Typical values are 0.15 for small ACV (AUW < 10 000 kg) 0.16 for medium ACV (AUW < 30 000 kg) 0.20 for large ACV 0.20 for small SES (AUW < 60 000 kg) 0.25 for large SES An initial cross-check can be made here against the designer's...

## 33 Air cushion wavemaking drag J

Wave-making drag generated by a pressure distribution is a classical theme of hydrodynamics, since a ship's hull is generally represented by a surface consisting of a varying potential function which applies positive pressure in the forebody and suction pressure around the stern 8,17 . The equivalent problem for a hovercraft was addressed by Newman and Poole 18 , who derived a calculation method for predicting the wave-making drag. They simplified the air cushion to an equivalent rectangular...

## Amphibious ACV skirt configurations

We will start with the evolution of Chinese and British skirt configurations, because most of the skirt types applied world-wide to ACVs or SES to date are similar to these (see Figs 7.1 and 7.2). Initially the flexible skirt appeared as a type of extension jet nozzle. The skirt of SR.N1 is shown in Fig. 7.1 and its plan configuration in Fig. 7.2. Designers attempted to gain the benefit of the peripheral jet air curtain to seal the air cushion and enhance the hovering efficiency, while...

## Theory and Design of Air Cushion Craft

Deputy Chief Naval Architect of the Marine Design & Research Institute of China Shell International Exploration and Production Ho Ian A member of the Hodder Headline Group LONDON Copublished in North, Central and South America by John Wiley & Sons Inc., New York Toronto First published in Great Britain in 2000 by Arnold, a member of the Hodder Headline Group, 338 Euston Road, London NW1 3BH Copublished in North, Central and South America by John Wiley & Sons Inc., 605 Third Avenue, New...

## Surface effect ship development

The US Navy were also interested to develop the SES as a military combat ship. They met with several setbacks during the development of these air cushion vehicles, which can be divided into three stages, as outlined below. In 1963, the US naval aviation development centre constructed a test craft, model XR-1A (Fig. 1.25), which was rather successful. For this reason, under the suggestion Fig. 1.25 Early US SES test craft XR-1A. Fig. 1.25 Early US SES test craft XR-1A. Fig. 1.26 US water-jet...

## 83 Longitudinal SES motions in waves

In section 8.1, we introduced the seaworthiness studies of hovercraft by experts from various countries. The investigations concentrated mainly on the longitudinal (pitch) motions of ACV SES in waves, as in refs 11, 67 to 69 71, 73 etc. These research papers present differing approaches and analytical methodology. In this section, the various nonlinear factors associated with motion of a craft running in waves will be discussed. Skirt contact with the water makes the forces acting on the craft...

## 16 SES and ACV developments in the 1990s

In 1984 85 a shipbuilder in Norway, Brodrene Aa, teamed up with a firm of Naval Architects, Cirrus, to design a large passenger SES, after being impressed with the performance of the US Navy's test craft SES200 when it performed a series of demonstrations in Europe for NATO. Their concept was a GRP hulled development similar in concept to the BH-110, with catamaran hulls and diesel engine power. Propulsion was by propellers. Passenger capacity was 264. This craft performed very well, reaching...

## Technical issues to take into account for lift fan design and manufacture

Choice of impeller speed and diameter It is very important to select the optimum speed and diameter of impellers. From the point of view of craft general arrangement, the impeller diameter should be decreased for higher craft design speeds, to minimize frontal area. However, the decrease of diameter will need to be compensated by an increase in the number of fans to produce the same airflow volume and their speed will have to be increased so as to support the required pressure head. For this...

## Vertical acceleration

Passenger craft have to satisfy the requirements stipulated by the IMO and the International Standards Organization (ISO), which are based on a large amount of test and statistical information. Figure 10.8 shows the vertical acceleration data for Fig. 10.8 Vertical acceleration of ACV SES in waves. various hovercraft (models) collected by Peter Mantle of the USA 4 and stipulates approximately the permitted vertical acceleration for SESs operating within 30 minutes and 1 day. Figure 10.9 shows...

## References

The History of Air Cushion Vehicles. Kalerghi McLeavy Publications, 1963. 2. King HF. Aeromarine Origins. Putnam & Co. Ltd, 1966. 3. Wheeler RL. From River to Sea - the Marine Heritage of Sam Saunders. Cross Publishing, Newport, Isle of Wight, 1993, ISBN 1 873295 05 7 (contains extensive summary of SRN and BHC series ACY design development). 4. Mantle PJ. A Technical Summary of Air Cushion Craft Development, David W. Taylor Naval Ship Research and Development Center, Report 80...

## Modularized design of centrifugal fans

The industrial centrifugal fan models 4-73, 4-72, etc. have been widely used in China and a large amount of experimental results and data have been obtained which verify Fig. 12.14(a) Aerodynamics characteristics of Chinese centrifugal fan model 4-73, its configuration and streamlined blade offsets. Fig. 12.14(a) Aerodynamics characteristics of Chinese centrifugal fan model 4-73, its configuration and streamlined blade offsets. Fig. 12.14(b) Chinese fan model 4-73 geometric data. Fig. 12.14(b)...

## Equivalent cushion beam method

SES with thin sidewalls create very little wave-making drag, owing to their high length beam ratio, which may be up to 3CM0. To simplify calculations this drag may be included in the wave-making drag due to the air cushion and calculated altogether, i.e. take a equivalent cushion beam Bc to replace the cushion beam Bc for calculating the total wave drag. Thus equation (3.1) may be rewritten as where Rw is the sum of wave-making drag due to the cushion and sidewalls, Cw the coefficient of...

## 15 ACV and SES development in China

The Harbin Shipbuilding Engineering Institute (HSEI) started to develop a new kind of water transport concept - the hovercraft with plenum chamber type air cushion -in 1957, and completed the first model craft in China with a length of 1.8 metres. The model was constructed in both wood and aluminium alloy, and used an aviation type electric motor for lift power. Because of the lack of high speed towing tank facilities at that time the towing model experiments were carried out in a natural lake...

## C

Tulin two-dimensional supercavitating section - TMB 1954 Posdunine supercavitating section 1945 5-term Johnson J 3-term Johnson 2-term Circular arc Comparison of 2-term and TMB modified sections Comparison of 2-term and TMB modified sections Hydronautics, 1961 Fig. 15.26 b Examples of supercavitating blade sections. Johnson 5 term section Johnson 3 term section Newton Rader section Tulin section Sulzer-Hydro ventilated partially submerged propeller installed on Mekat SES Fig. 15.26 b cont....

## Cushion pressure ratio Hq Euler number

In order to correctly simulate air leakage from an air cushion and external aerodynamics of full-scale craft, as well as the spray caused by the cushion acting on the water surface, the scaling criteria of cushion pressure ratio Hq, the Euler number, can be written as where pc is the cushion pressure N m2 , a the air density Ns2 m4 and Fa the velocity of air leakage, m s . According to Table 9.1, V.d can be written as If Fa denotes the velocity of the craft model, then expression 9.2 is similar...

## Design loads for craft structure overall bending and torsion

The loads acting on the craft structure during the calculation of overall bending and torsion can be determined using the maximum inertial load coefficient measured at the craft's CG. The inertial load coefficient operating in waves can be obtained from prototype or experimental results of models in various operation modes and various modes of overall deformations. The loads acting on locations other than the CG can be determined as follows tj 1 px O, - xg x - xg p2 yx y lp2 p2 x2 - xg x - xg...

## Open loop and segment Fig 713d

This skirt is used on smaller craft, generally below 5 t. The loop is at pc and so this is the most efficient skirt form. The geometry of the loop can be varied to give the right combination of responsiveness for a seaway and skirt shift mechanisms can be used to enhance manoeuvring. Tapered geometry is possible, though not normally used. A swept-back bow skirt is necessary, as it is not practical to design a stable bulbous bow geometry as for a responsive bag and finger skirt. This is not a...

## Air cushion adiabatic stiffness coefficient Cb

The coefficient which characterizes the adiabatic stiffness of air cushion can be expressed by where Vc is the cushion volume m , pa the atmospheric pressure N m , pc the cushion pressure N m and y the adiabatic constant for air m2 N . This coefficient characterizes the adiabatic change of cushion air in compression due to the heaving, pitch and roll motion of hovercraft running at high speed, which affects heaving stiffness and damping. The cushion pressure denotes the excess pressure of...

## Nondimensional characteristic curves of fans and air ducts

The Reynolds number for model air ducts is rather small because the dimensions of the model decrease with the dimensional ratio A. This will influence the air duct friction drag. However the air friction drag losses are usually small the main losses of air pressure in air ducts are due to the curved or sharp bends in the ducts and sudden expansion of air flow into the cushion. Thus the air pressure loss in ducts can be considered independent of Re. Calculation of losses is similar to analysis...

## 45 Factors affecting ACV transverse stability

Based on the equations mentioned above, one can discuss the effect of the various parameters on the static transverse stability of an ACV. However, the errors of calculation are rather large since no account is taken of the deformation of skirts caused by the Fig. 4.33 Typical static transverse stability curve for ACV. Fig. 4.33 Typical static transverse stability curve for ACV. change of cushion pressure of the craft in heeling and the effectiveness of the air cushion blown from the nozzle of...

## Platforming analysis

The first mode is platforming, i.e. the cushion pressure and the vertical position of the wet deck remain constant, then the vertical acceleration will also be constant. This is the ideal operating attitude of craft and what the designer requires. However, one has to regulate the lift power and lift inflow rate to keep the cushion pressure constant. This condition is also the one which will absorb the greatest volume of air therefore we will make an analysis of this case. When the craft moves...

## 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, are designed to minimize the lift power. Due to the lack of air leakage at the craft sides, lift power can be reduced significantly compared with an ACV. Also, it is possible to install conventional water propellers or waterjet propulsion,...

## Winginground 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 seaplane. An aeroplane wing operated close to the ground generates lift at the pressurized surface of the wings which is increased significantly due to the surface effect. The...

## Concept development from the early 60s to the early 70s

The results of research trials with SR.N1 indicated that a truly competitive commercial hover ferry would probably need to be 125 to 150 tons in weight and some four times the length and breadth of the SR.N1 manned model, in order to cope with 4 to 6 feet seas. A jump from 4 to 125 tons represented such a major engineering step that it was decided by Saunders-Roe to approach this in three stages over a 7 years programme. 207 The first stage was implemented with the 27 ton SR.N2, which was used...

## Deck area and cabin volume

The ACV and SES both give spacious deck area and cabin volume. These vessels need to be large relative to their displacement, to keep cushion pressure realistic. They are therefore suited to applications where volume is the most important parameter Table 1.7 The footprint pressure of various forms of transport Table 1.7 The footprint pressure of various forms of transport The configuration of transport forms passenger and car ferries, fast military logistics vessels, utility vehicles and, at...

## Market development from the beginning of the 80s to the present

Although air cushion technology had advanced significantly by the end of the 70s, there were still difficulties to overcome in order for hovercraft to compete fully with Fig. 1.18 Hovermarine HM-221 SES fireboat on trials before delivery to port of Tacoma. Fig. 1.18 Hovermarine HM-221 SES fireboat on trials before delivery to port of Tacoma. other transport systems such as hydrofoils, high-speed monohull passenger craft, high speed catamarans and long range buses and trains where appropriate....