Air-cavity ships are readyfor a wider market

ir-cavity ships (ACS) are advanced marine

vehicles that use air injection at the

wetted hull surfaces to improve a vessel’s

hydrodynamic characteristics. The concept of drag

reduction by supplying gas under the ship’s bottom

was proposed in the 19th century by the famous

scientists Froude and Laval. However, many

attempts to implement this idea in practice have

failed because this process is not as straightforward

as it seems. Deep physical understanding of multi-

phase flows is required to achieve a positive

outcome. Based on the results of systematic

research, several successful ACS’s have been

created and found practical application during the

past decade.

The position of the ACS among other ship

types is shown in Fig 1 characterising the degree

of water-hull contact. The basic type of ship

operates in a displacement mode. At sufficiently

high speed and with suitable hull lines, a boat can

glide over the water surface. Air can be injected

under the bottom, significantly reducing wetted

hull area and consequently hydrodynamic

resistance. This type of ship corresponds to the

A C S, and the phenomena of generating a gas layer

at the submerged hull surface is called artificial

cavitation or air lubrication.

A similar and more familiar concept is the

surface effect ship (SES), where air is also pumped

under the ship’s bottom. Such a vessel usually has

flexible bow and stern covers enclosing the space

between twin hulls. The next ship type after the

SES is an air cushion vehicle with no permanently

submerged parts.

Another branch of vessel types is related to

hydrofoil applications. A ship can be either

partially or fully supported. The extreme

continuation of both branches of development is

the vehicle flying near the water surface, called a

‘wing-in-ground’ effect (WIG) craft. We should

note that it is not possible to claim that some

concepts are universally better than others: all of

them have their niches, and the choice of a certain

ship type depends on the route characteristics,

available facilities, government regulations and

other factors.

The ACS concept is based on successful usage of

bottom ventilation (artificial cavitation). A gas is

supplied underneath a special profile, so that a

steady air layer is generated which separates a part

of the bottom from contact with water, therefore

reducing hydrodynamic resistance. Drag reduction

achieved on a full-scale ACS is within 15-40 per

cent, while the power spent on the cavity-

maintaining gas flow is always less than 3 per cent

of the total propulsive power of a vessel. Pressure

inside the cavity is higher than atmospheric,

providing additional support for the ship’s weight.

Although the ACS principle seems similar to an

SES, there are significant differences. First, there are

no flexible seals on an ACS. The air layer is

contained by solid hull parts, which not only

prevent air leakage from the cavity, but also

influence the air cavity characteristics. Secondly, the

Speed at Sea | February | 2003 | 13| www.speedatsea.com |

A

air cavity ships

The DK Group Netherlands’Konstantin Matveev

describes air-cavity shiptechnology which usesartificial cavitation toreduce hydrodynamic

drag and can benefit fastferries, cargo vessels,

and military craft

Fig 1: Hierarchy of fast ships basedon degree of contact with water

Fig 2: Air cavity formed under the bottom of a fast ACS with important hull parameters depicted

ACS FEATURES• air cavity ships are already produced in series

• 15-40 per cent drag reduction is achieved

• less than 3 per cent of the total ship power is

needed to support the air cavity

• low wash wake is generated due to smoothed

pressure gradients in the presence of the air

cavity

• overloads in rough seas are reduced due to a

damping effect of the air cavity

• fouling growth on the hull in warm seas is

lessened due to decreased wetted surface

• ACS is a convenient platform for effective

landing and shallow-water operations

• protected or special propulsors may be

required for ACS.

air flow rate needed to support the air cavity on an

ACS is about ten times less than that on an SES.

Therefore, an ACS is a much more economical

means of transportation.

To use the artificial cavitation effectively, a ship

bottom profile should be chosen to provide air to

cover a large bottom area at low energy expense for

air supply. There are three important components of

the bottom structure on a fast ACS: a step forming

the cavity surface, planing sidewalls (skegs), which

also protect a cavity, and a special section near the

transom that provides smooth closing of cavity

surface to the hull. The determination of geometrical

parameters of these structural components is the

main task of ACS design. An air cavity is formed in

the bottom recess by supplying gas through the

nozzles using fans.

The important physical properties of cavitating

flow aimed at reducing drag can be illustrated using

a simple example of the flow behind a wedge

attached to a horizontal wall in the presence of

gravity, as shown in Fig 4. A characteristic feature of

cavity 1 is the formation of a pulsating re-entrant jet

in the tail part of the cavity, while the cavity

boundary close to the wedge remains stable. This

flow is similar to usual cavitation and ventilation

with a positive cavitation number in the absence of

a horizontal wall.

Shape 2 is associated with a flow mode when no

re-entrant jet is present, and the tail of the cavity

attaches smoothly to the plate. In this case, the

cavity-maintaining gas flow, as well as the cavitation

drag, is theoretically equal to zero. Pressure inside

the cavity exceeds that in the undisturbed flow,

making the cavitation number negative.

The peculiarity of shape 3 is that in theory the

cavity pierces the plate at its aft end (as shown by

the dashed line). During tests, strong pulsations are

observed all over the cavity in this case, as in over-

ventilated flows with positive cavitation numbers.

This regime is realised at high gas consumption.

The formation of an unclosed cavity 4 is also

possible under certain conditions; however, the

power needed for air injection is too high to make

this regime attractive for practical drag reduction.

Thus, the flow mode that produces cavity 2 is

the most promising. As shown by calculations and

verified in experiments, the cavity length in this

case scales as the flow velocity squared. Cavity

geometrical characteristics, and a cavitation number

corresponding to this most favorable situation, are

called the limiting parameters. Successful ACS’s are

designed to operate in this regime.

The idea of drag reduction by air lubrication is

also applicable to relatively slow vessels, such as

tankers and cargo ships. However, due to the

stability limit on cavity dimensions, a different

arrangement of air cavities must be employed. If the

ship length is large and its speed is not sufficiently

high, an entire bottom of the vessel cannot be

covered by a single cavity. This explains unsuccessful

attempts to reduce drag by supplying gas through

only a single nozzle in low speed regimes. Several air

cavities (up to 7-8) must be created on a slow ACS

operating in a displacement mode.

When a ship is moving in a semi-displacement

regime, a significant portion of hydrodynamic

resistance is of the wave nature. In this case,

artificial cavitation is not as effective as in the case

of slow and planing ships. However, the presence of

a compressible air cavity decreases pressure

gradients at the ship hull. This effect leads to the

wave drag reduction and lower wash wake

generated by a ship. A total increase of the efficiency

of ACS moving in a semi-displacement regime

should reach 15-25 per cent in comparison with

conventional vessels.

Systematic research on air cavity applications for

ship resistance reduction was started at the Krylov

Shipbuilding Research Institute in St Petersburg,

Russia, in the 1960s. The most significant

contribution to this field was made by Anatoly

Akimovich Butuzov. It was established that the

apparatus of the theory of developed cavitating

flows was suitable for determination of the major

hydrodynamic characteristics of the ships with air

lubrication. Successful laboratory tests were

followed by implementation of the air cavity concept

on the full-scale river cargo ships and barges. Those

trials demonstrated significant reduction of the

power (up to 30 per cent) needed for vessel motion

in optimal speed regimes.

In the early 1970s, the first high speed full-

scale ACS was build on the initiative of Ivan

Ivanovich Matveev at the Central Hydrofoil Design

Bureau, a world-leading company in hydrofoil and

WIG technologies, based in Nizhniy Novgorod,

Russia. The speed increment achieved on that boat

was up to 27 per cent in comparison with an

analogous boat without the air cavity system.

Energy expense for air supply was below 3 per cent

of the total power.

Research and development activity at the Central

14 | Speed at Sea | February | 2003 | www.speedatsea.com |

air cavity ships

Fig 3: Schematic view of the bottom of a fast ACS

Serna-class landing craft have a maximum speed of 30 knots

Hydrofoil Design Bureau resulted in creating several

vessel types that have been produced in series in

recent years. Linda craft (displacement 24.6 tonnes,

speed 30 knots) are used for passenger transportation

in inland waters. The Serna landing craft (full-load

displacement 100 tonnes, maximum speed 30 knots)

is able to deliver 45-tonne vehicles and discharge

them over a ramp. A recent ACS, called Mercury (100

tonnes displacement, top speed 55 knots), is a sea-

going patrol boat capable of safe sailing in Sea State

5. As well as these mid-size vessels, runabouts using

artificial cavitation are also built in Russia. Exhaust

gases are sometimes utilised as the cavity-

maintaining gas on small boats, which simplifies the

ACS structure and increases operating efficiency.

Despite Russian organisations’ profound

knowledge and experience of ACS technology, it

seems that they cannot penetrate world marke t s .

Military ACS craft developed in Russia are of interest

to defence companies in Nato countries, but

technical collaboration is not possible for political

reasons. Lack of capital, limited marketing efforts,

and Russian R&D centres’ limited experience of

designing large ropax fast ferries (a would-be

primary market for ACS technology) make it difficult

to expect that Russian ACS’s will find wide

application abroad in the near future.

Potential benefits of air injection under ship

hulls without flexible seals have always been of

interest to the shipbuilding community worldwide.

However, until the last decade development

attempts were not serious enough to achieve

convincing results. In recent years, R&D activity in

this field was significantly increased in Europe,

USA, Japan, Korea and Australia. Because of the

commercial nature of these projects, reliable data is

not yet available to judge for certain the progress in

air cavity technology.

Perhaps the most comprehensive efforts have

been made so far by The Netherlands-based DK

Group. In collaboration with world leaders in

marine innovations, such as research institute

MARIN and design office Nevesbu, this company

has undertaken an extensive study of the potential

application of artificial cavitation both to fast ferries

and cargo vessels. The research programme involves

laboratory and tank testing of ship models where

air is delivered to the specially profiled bottoms.

Test results demonstrate a great potential for future

air cavity ships. Theoretical and numerical analyses

of multi-phase flows and hull structure

optimisation are aimed at creating effective

approaches to ACS design.

A characteristic ACS feature is that pockets of

air and bubbles periodically escape from the cavity

end and shed downstream, especially during

pitching motions in rough seas. Conventional

propulsors, such as propellers and waterjets, lose

their efficiency significantly if air is present in the

incident flow. To avoid this negative effect, special

deflectors are applied on cargo vessels equipped

with air cavity systems. In the case of a fast ACS,

supercavitating and surface propellers can tolerate

air presence in the water flow.

There are two new developments aimed at

improving the efficiency of propulsion systems when

air is present in water flow. The first is the

Ventilated Wing Jet, developed by the DK Group,

which had previously been involved (but is no

longer) with development of the Hydro Air Drive

propulsor. The rotor in a Ventilated Wing Jet unit is

located inside a close-fitting protective duct and

operates half-submerged at cruising speed similar to

a surface propeller. At low speeds, the water flow

rate can be increased and sufficient thrust is

produced. The Ventilated Wing Jet remains aerated

at all times, so no fall-off in performance is observed

when air is entrained in the incoming flow.

The second concept, the Ventilated Waterjet, is

being developed at the Krylov Institute. Air cavities

are formed on the suction sides of the blades of the

Ventilated Waterjet and connected to the

atmosphere. The thrust is produced mostly by the

pressure sides of the blades. Hence, the Ventilated

Waterjet is not sensitive to the presence of air

bubbles in the flow.

Other companies developing air assisted

Speed at Sea | February | 2003 | 15| www.speedatsea.com |

Fig 4: Ventilated flow behind a wedge

DK Group’s models of an ACS ropax (below) and bulker being tank-tested at MARIN

platforms utilise somewhat different approaches for

hull drag reduction. For instance, Harley

Shipbuilding in the USA and SES Europe in Norway

are promoting the ‘air lifted vessel’ (ALV), which is

essentially a catamaran with an air cushion similar

to that of an SES but the hulls have planing sections

in front of the bottom recess and no flexible seals

are used. Fast Cat Boatworks’ 30m version,

PurrSeaverance, has received its USCG certification

and is operating in Florida [see News].

Another characteristic feature of an ALV is the

longitudinal keels on the sides of the cavity that

prevent air from escaping. The reported resistance

reduction is around 30 per cent, however, it take s

10-20 per cent of the total ship power to achieve

this benefit. Also, the depth of a bottom recess is

about 25 per cent of the total draft (on an ACS it is

10-15 per cent), which decreases options for cargo

and machinery arrangements. Since only one

principal configuration of ALV has always been

presented for the past several years, this scheme is

probably not adaptable for a wide range of vessel

types. The higher power needed for support of the

air cavity on an ALV in comparison with an ACS

demonstrates that the ALV bottom structure is not

yet optimised.

Another company that develops air assisted

marine vehicles without flexible seals is Air Ride

Craft (USA). DK Group no longer holds any interest

in the company’s patents. Air Ride Craft’s concept is

even further from the artificial cavitation philosophy

and closer to air cushion technology.

The lift supporting the vessel is predominantly

generated by compressed air located in a deep

bottom recess. Hull sections do not significantly

affect properties of the air cushion. Since, during

motion, the air cavity does not close smoothly to the

hull in this case, energy consumption for air supply

is much greater than on an ACS. Therefore, the ACS

principle has significant advantages from an

economical standpoint over the ALV and Air Ride

Craft concepts. However, as we indicated before, one

hydrodynamic arrangement cannot be universally

better than the others. The choice of a certain design

is always affected by many factors peculiar to

particular situations.

A rather popular concept related to air

lubrication is drag reduction achieved by using

micro-bubbles. It is certainly easier from the

technical side to create a bubbly flow instead of

large stable air cavities, but the overall effectiveness

of this idea is questionable. Air bubble motion

usually includes a random component, and some

bubbles may stick to the hull. These effects can even

augment the effective roughness of a hull surface

and lead to an increase in drag.

H o w e v e r, in the case of nearly vertical submerged

hull sections, stable air cavities cannot be created,

and the bubbly flow may be the only available option

for drag reduction. If the flow velocity is high

enough and the hull surface is covered by a non-

wetting coating, then a certain drag reduction effect

can be achieved. Several Japanese and US

organisations are involved in studying this problem.

As well as commercial companies, government

agencies are showing interest in developing drag

reduction technologies involving air supply to the

wetted hull surfaces. Supercavitation phenomena

have been investigated with government support in

both Russia and the USA for a long time, with the

purpose of creating ultra fast underwater

projectiles. The Office of Naval Research has

recently solicited proposals for the design of the

High-Speed Drag Reduction Experiment that would

certainly involve some form of air addition to the

water flow. One of the current Small Business

Innovation Research topics is dedicated to the study

of ship hulls with captured air plenums that would

lead to less drag and greater operating efficiency.

The growing efforts of private companies and

government agencies to advance artificial cavitation

technology, and an existence of successful

prototypes and even serially built ships with air

lubrication, raise a hope that the air-cavity ship

concept will find worldwide applications for various

vessel types in the next several years. S@S

Konstantin Matveev specialises in advanced marine

vehicle R&D. He graduated from Moscow Institute of

Physics & Technology with a MSc in applied physics,

and is a mechanical engineering PhD candidate at

California Institute of Technology. He has long

experience of hydrofoil, wing-in-ground, and air-

cavity ship technology.

16 | Speed at Sea | February | 2003 | www.speedatsea.com |

air cavity ships

Fuel consumption per tonne-mile of payload for existing monohull ropax fast ferries and projected ACS analogs

Linda series passenger vessels operate at up to 30 knots on inland waters

55-knot Mercury - class patrol cra f t