Post on 28-Mar-2015
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