INTRODUCTION
Amphibious vessels must perform their operational mission regardless of the level of threat in
the theater.
Amphibious vessels must be prepared to operate anywhere, and may face weapons systems
from all over the world. Threats can generally be classified as coming from air, water, or land forces; the
electronic warfare (EW) environment; or the nuclear, biological, and chemical (NBC) environment.
Amphibious vessels are particularly at risk because their operations will bring them within close range of
threat forces.
The
air threat consists of fixed or rotary-wing aircraft. Both fixed and rotary-wing aircraft can
operate at night and use a variety of cannon, guns, bombs, and missiles to attack targets.
Precision-guided munitions let aircraft selectively attack individual targets. Surveillance
aircraft and unmanned
aerial vehicles (UAVs) are used for reconnaissance and targeting platforms. They use a variety
of photo,
infrared (IR). thermal, and electronic devices to locate potential targets. Anti-ship missiles
pose a
significant threat to amphibious vessels.
The water
threat consists primarily of other surface craft. Surface craft include every craft from
rubber boats to large combatants. Of primary concern are fast patrol boats, which operate
close to shore.
Amphibious vessels are likely targets for submarines. Many types of mines including freed
and free-floating
mines are available to threat forces. A variety of means including physical contact, magnetic
fields, or sound detonate these mines.
Land forces
present a spectrum of threats raging from individuals to large conventional forces.
The threat from artillery and short-range missile fire directed against amphibious vessel
operations is significant.
Electronic
warfare threatens amphibious vessel operations because of reliance on electronic
communications equipment. Threat forces can intercept or jam communications and can target
transmitters using direction-finding equipment linked to indirect fire delivery systems.
NBC attack
presents a variety of problems to amphibious vessel operations. Decontamination
requirements after an NBC attack may impede an amphibious vessel's
mission.
1.2 COUNTERING THE THREAT
1.2(A) The SeaCoaster
The SeaCoaster
air-assisted catamaran offered several significant advantages inherent in its design
to counter threats:
1. High speed air-assist catamaran hull form - Speed is life.
The operator can avoid trouble and get out
of trouble with the speed advantage of the SeaCoaster.
2. Air-assist cushions -
Numerous underwater shock tests by several navies have shown that an air-cushion vessel
is less susceptible than traditional hull forms to this damage mechanism. The
air-assist cushions also serve as an air void block to sound transmission into the water, lowering the
detectability of the vessel by submarines, torpedoes and mines.
3. Low draft - The vessel can go places deeper draft vessels cannot. The operator can avoid trouble and
get out of trouble with the draft advantage of the SeaCoaster. The shallower draft increases standoff
distance from mines and torpedoes that may pass underneath the vessel.
4. Excellent platform stability and ride characteristics - The design of the
SeaCoaster eliminates the SES "cobblestone effect". Other vessel motions are substantially
reduced contributing to increased crew efficiency. An alert crew is a more survivable crew.
Excellent stability contributes to an expanded envelope for helicopter operations in higher sea states.
Weapon system accuracy is enhanced by a stable platform, contribuuting to one-shot kill
capability.
5. Lower installed power to achieve speed - Lower installed power mean less
heat is generated. Less heat generated means lower thermal signature. Lower installed power means better fuel consumption
rates for a given speed.
The internationally patented SeaCoaster air-assisted catamaran is a true catamaran hull
with wave slicing fine entry sidehulls; however, it differs in that there are recesses built
into the underside of each
sidehull. When blower pressurized air is supplied to the recesses, the pressurized air cushions thus
formed support about 80 percent of displacement. There results a decrease in draft and a substantial
reduction in wetted area resistance. Propulsive power requirements are about half that of a conventional
catamaran at cruise speeds. When blower power requirements are added in the power requirements for
SeaCoaster are still only about sixty percent of that of a standard catamaran.
Figure 1
makes a comparison of a catamaran, SeaCoaster, and SES. Model test results in the form of
lift/drag values are presented in Figure 2. That figure shows the propulsive requirements of
SeaCoaster to be only about half that of other craft types.
As a result
of the successful model tests of SeaCoaster, a full size test and demonstration unit was
started in early 1995. The original design had a 13.7m length. The hull was later stretched to 19.8m to
meet a potential customer's requirement. (This slightly affected air cushion performance characteristics).
Figure 3 shows the SeaCoaster demonstator. Due to the stretch in length to 19.8 meters and minor
construction details, some modifications were required to achieve predicted performance. The most
notable modification was the addition of a mid-body foil to provide lift forward. The performance goals
were met in December 1998 when 44 knots was achieved at 30,385 kg displacement with the main
engines at 522 kW (700 HP) each. Blower power was 89 kW (120 HP).
The test results
on the 19.8m SeaCoaster, less the hydrofoil, shows lift/drag values of about 12 at
40-44 knots. When blower engine power is included, as it must be for a valid comparison to other craft
types, the lift/drag values are about 10.5. This shows that SeaCoaster, including blower power, only
requires about 60-percent of the total power compared to a catamaran or monohull. These lift/drag
values fall within the range of the model test data.
Rough water stability
was verified during tests for the Office of Naval Research. In those tests, the
maximum g-forces measured during operation at 32 knots while running into 1-2 meter head seas were
0.3g's RMS. Data for a typical planing hull in similar operating conditions show acceleration forces of
over 6-g's
The effect of the
hydrofoil at the test displacement noted was to slow the boat down by about 5
knots. However, load carrying ability and ride qualities were improved. The 19.8m demonstrator was
sold in early 1999 and then converted to a 149-passenger ferry, named "Island Rocket II." It is shown in
Figure4. During the conversion, larger CAT 3412C diesels rated at 932 kW (1,250 HP) were installed to
account for the greater displacement of the passenger ferry.
The ferry
received U.S. Coast Guard certification and went into service for Island Express Boat
Lines, Ltd., Sandusky, Ohio, in August 1999. Speed in Sea State 2/3 at 47,620 kg is 43 knots. Speed at
maximum displacement of 58,960 kg is approximately 32 knots. The lift/drag values, based on
propulsive coefficients supplied by the propellar designer and not accounting for blower power
requirements, are 9.1 at the 43-knot point, and 10.5 at 32 knots. These lift/drag values are slightly lower
than those obtained on the boat prior to installation of the cabin. More power is required simply because
of the much heavier vehicle. A new low drag hydrofoil and an optimized propellar will be installed in
early 2000. These are expected to improve full load performance significantly.
Air Ride Craft, Inc.
is starting the design of a 44m, 450-passenger SeaCoaster ferry (Figure 5)
for
the owners of the "Island Rocket II." The current propulsion power choice is two vector TF-50 gas
turbines and is expected to give speeds over 50-knots. Use of other engine typed and sizes is also being
investigated.
1.2(b) Integrated Power Systems
The
following is from the National Academy of Sciences 1997 report,Technology for the
United States Navy and Marine Corps, 2000-2035, Becoming a 21st-Century Force.The benefits
of ship electrification are as follows:
Combat systems effectiveness. Integrated electric power and
propulsion systems enable design flexibility
that in turn, will facilitate the optimization of topside arrangements for maximum combat
system effectiveness.
Survivability. The elimination of gear trains and propellar shafts, together with the flexibility and
modularity of electric systems, will enable graceful degradation and rapid reconfiguration of vital
systems, thus enhancing survivability.
Signature reduction and quieting. By eliminating
the mechanical link between the power plant and
the propulsor (i.e., propeller), electric drive will enable reduction of noise and vibration by allowing
acoustic isolation of the engine generators.
Improved operational flexibility and reliability.
With the power station concept, all power will be
supplied by a set of prime movers that provide power to propulsion, shop service, and other
designated loads. This approach will provide the flexibility to shift power between propulsion, ship
service, and other electrical loads, which cannot be done with a mechanical drive ship, and can
enable improved speed control and steaming efficiency and the elimination of less efficient
controllable pitch propellars in favor of fixed pitch types.
Increased flexibility and adaptability.
Integrating electric power and propulsion systems will provide
flexibility in servicing other loads such as environmental controls (air conditioning) and launch and
recovery systems, electric armor, high-power advanced electronics, and electric weapons.
More space available.
Eliminating gears and shafts from the propulsion system will make more
space available for other users. Also, the prime mover will no longer be tied to the propellar shaft
line, and the power sources can be distributed throughout the ship as necessary.
Reduced manning.
Digital control and automation, which will be an integral part of ship
electrification, will reduce the requirements for human operators (in machinery spaces, for example).
Reduced logistics.
Common power and propulsion modules can be used across the fleet.
Reduced costs.
Commercial technology appears likely to be available for many of the system
elements.
Life-cycle cost and fuel-consumption savings.
Overall fuel efficiency can be improved if a ship is
able to operate at varying speeds over a substantial part of its operational profile, and variable speed
propulsion will be enabled by electric drive."
Figure 6 shows the advantage of integrated
power systems over separate drives for propulsion and
ships service electrical generation. This provides the operator with flexibility in power lineups for best
efficiency Progress is being made in the development of suitable generators and motors for IPS vessels.
Kaman Electromagnetics has successfully commercialized very compact, lightweight 450 kilowatt and
750 kilowatt permanent magnet motors. Newport News Shipbuilding and General Dynamics are
working on 3-4 MW prototype motors and larger motors.
On
the generator side, Brush Electrical Machines has supplied a 58-ton, 21 MV unit for the U.S.
Navy's IPS program. Honeywell Aerospace and Engines Division is supplying a high speed, direct drive
3.2 MW alternator to the University of Texas Center for Electromechanics for the Federal Railroad
Administration Advanced Locomotive Propulsion System. The unit is designated the MegaGen 3200
and is designed to connect directly to a Vericor TF-40/TF-50 gas turbine operating at 12,000-15,000
RPM. The alternator unit weighs only 1,000 kilograms and dimensions are only 1.4 meters in length and
a diameter of only 0.55 meters. Coupled with the TF-50 gas turbine, the 3.2 MW power package weighs
only 1,675
kilograms (Figure 7)
1.2(c) Signature Reduction
The
first step in providing increased survivability for a vessel is to focus on the vessel's
signature. There are four areas to consider:
Radar cross section (RCS)
Infrared radiation
Airborne noise
Underwater noise
The
hull form of the SeaCoaster lends itself readily to RCS reduction measures. The broad beam
and large deck area allow the use of slanted surfaces without significant impact on volume or
arrangement issues. There are plenty of radar absorbent materials now on the market for application on
vessel surfaces and equipment to accomplish spot reductions in RCS. Proper selection and installation of
topside equipment will contribute to lower radar signatures.
As previously
mentioned, the efficiency of the SeaCoaster hull form allows for less power to be
installed to achieve desired speeds. Less power installed means less heat generated reducing the vessel's
self-generated infrared signature. In the case of the amphibious support craft concept discussed in this
paper, the exhaust from the gas turbines is ducted to locations between the two hulls of the SeaCoaster.
This action will shield the hot exhaust from direct observation while it is mixing with ambient air. To
take care of sun radiation effects, numerous low emissive solar paints are now available. The U.S. Navy
is currently testing a solar paint that is expected to lower a vessel's inside temperature by eight degrees
Celsius. An additional measure is to install thermal insulation on the inside of the vessel. Finally, a water
mist system can be employed to provide a barrier between the vessel's outside skin and infrared sensors.
Engine noise
is the primary source or airborne noise. The SeaCoaster needs less power to achieve
speed and one of the benefits is less noise generated. The use of IPS allows for the placement of engines
in locations where acoustical and damping materials can be more effectively applied and isolate the
engines from noise paths to the hull.
The air cushions of the
SeaCoaster act as a masking system to prevent the transmission of noise
from the vessel into the water. The use of air cushions means less of the hull is contact with the water
and also improves the flow of water around the hull.
Mr. Hulick,
in his paper, Design Guide for Small Craft Detectability Characteristics, estimated that a
vessel can reduce its RCS by 40 dB or one-eighth of a non-RCS vessel. He also notes that a vessel that
utilizes an air masking system should be able to reduce its underwater noise signature by 25-34 dB or
one-fourth that of a non-masked vessel
1.2(d) Self Defense Systems
Affordable
command and control systems are now available for the smallest of vessels. This in in
large part due to the great increase in the computing power of commercial-off-the-shelf (COTS)
equipment. Software code is readily ported among open architecture platforms avoiding the costs of
re-inventing the code. Through the use of multi-function consoles and automation, less numbers of
personnel are required to monitor systems and situations and decide upon appropriate action. With
regard to sensors and weapons, for the two concepts discussed in this paper, the emphasis is on
affordability and mission requirements. Both concepts are support vessels and have limited self-defense
systems. The following are provided in each concept:
(The following is based on EDO Corporation's Integrated Combat System Components Brochure).
Combat Information Subsystem
Multifunction consoles
Electronic surveillance measures (electronic and laser)
Low probability of intercept surveillance radar (Scout/ Pilot type)
IFF
Weapon interface to stabilized weapon platform with 30mm gun, Stinger and Hellfire missile launcher
Fire control to surveillance radar and electro-optical sensors
Helo control
Data link operation
Navigation Subsystem
Navigation radars
Electronic charts
GPS
Underwater log (doppler)
Gyro
Bridge indicators
Bridge multifunction consoles
Attitude and heading reference system
Environmental sensors
Fathometer
Sonar Subsystem
Multifunction console
CMAS-36/39 multi-putpose sonar (retractable small sonar) for:
- Underwater surveillance
- Mine detection/mine avoidance
- Navigation aid (obstacle avoidance)
Underwater telephone
Communication Subsystem
UHF, HF and VHF radios
INMARSAT
Remote radio control
Internal communications system
General marine distress system
2. AMPHIBIOUS SUPPORT CRAFT (ASC)
2.1 ASC PLATFORM REQUIREMENTS
The following are the postulated ASC requirements:
1. Self deployable.
2. Range and cruise speed at full load displacement - 2,000 nautical miles at an average speed of 28
knots in sea state 4.
3. Range and cruise speed at overload displacement - 3,000 nautical miles at an average speed of 24
knots in sea state 4.
4. Maximum speed - 50 knots in sea state 4.
5. Payload capacity (other than fuel) - 50 tons
6. Large open deck for multi-helicopter operations in Sea State-4. Capability for one V-22/H-60
landing
spot and one landing spot for a second H-60 helicopter. Limited aviation services.
7. 11 meter RIB boat stern deployment system.
8. Deck area of 7,000 square feet to handle low-density payloads such as troops and their equipment.
9. Design characteristics that enhance survivability within vicinity of shore areas.
10. Manning - 15 or less personnel.
2.2 ASC OBJECTIVES
The
ASC is a flexible platform that should be able to perform the following missions within its
platform size and cost:
1. Verticle replenishments to forces ashore
2. Information and communications connectivity of forces ashore to over-the-horizon (OTH) assets
3. Support of mine countermeasure (MCM) operations
4. Intercept of small, high speed surface threats with lethal and non-lethal engagements
5. Escort for landing forces
6. Emergency hospital, medical evacuation and triage support services
2.3 ASC CONCEPTS
With
the above objectives and ASC platform requirements as guidance, a 700-ton design with an
overall length of 67 meters was developed for the ASC role. Perspective views are shown in
Figure8.
Principal characteristics are listed in Table 1.
Weight estimates for the craft are in Table 2.
The
ASC achieves, at full load displacement (FLD) of 700 long tons, a speed of 50 knots in sea
state 4. Maximum speed trials (550 long ton displacement) is a speed of 60 knots in sea state 4. In an
overload displacement (OLD) condition of 800 long tons, the ASC can achieve a maximum speed of 40
knots. Range at FLD with a payload (exclusive of fuel) of 50 long tons is 2,000 nautical miles at an
average speed of 28 knots and 1,300 nautical miles at an average speed of 50 knots.
3 COMBAT LOGISTICS VESSEL (CLV)
3.1 CLV PLATFORM REQUIREMENTS
The following are the postulated CLV requirements:
1. Self deployable.
2. Range and cruise speed at full load displacement - 4,500 nautical miles at an average speed of 30
knots in sea state 4.
3. Range and cruise speed at overload displacement - 4,500 nautical miles at an average speed of 24
knots in sea state 4.
4. Maximum speed - 45 knots.
5. Deck area of 10,500 square feet to handle 24 M1 main battle tanks or 25(50 double stacked) 20-foot
ISO containers
6. Payload capacity - 2000 tons (with some degradation in range).
7. Design characteristics that enhance survivability within vicinity of shore areas.
8. Manning - 15 or less personnel.
3.2 CLV OBJECTIVES
The
CLVis a flexible platform that should have certain attributes and be able to perform the following
missions within its platform size and cost:
1. Shallow draft
2. Provide both bow and stern ramps for RO/RO cargo.
3. Provide bow thruster(s) to assist in beaching and beach extraction
4. Conduct rapid transfers
5. Provide worldwide transport of general and vehicular cargo
6. Provide intrtheater line haul of large quantities of cargo and equipment
7. Perform tactical resupply missions to remote underdeveloped coastlines and inland waterways
8. Discharge and/or backload of sealift vessels, such as Large, Medium Speed, Roll-on/Roll-off Ships
(LMSRs)
9. Transport cargo from ship to shore in logistics-over-the-shore (LOTS) operations including those
in
remote areas with unimproved beaches
10. Utility transport for unit deployment and relocation
11. Capable of handling wheeled and tracked vehicles, including Main Battle Tanks, dozers,
container
handling equipment, etc.
3.3 CLV CONCEPT
With
the above objectives and CLV platform requirements as guidance, a 4,200-ton design with an
overall length of 110 meters was developed for the CLV role. Perspective views are shown in
Figure9.
Principal characteristics are listed in Table 3.
Weight estimates for the craft are in Table 4.
The
CLV achieves, at full load displacement (FLD) of 4,200 long tons, a speed of 33 knots in sea
state 4. Maximum speed trials (2,400 long ton displacement) is a speed of 45 knots in sea state 4. In an
overload displacement (OLD) condition of 5,000 long tons, the CLV can achieve a maximum speed of 27
knots. Range at FLD with a payload (exclusive of fuel) of 725 long tons is 4,500 nautical miles at an
average speed of 32 knots. It should be noted that the beam of the vessel were increased by 6 meters, a
twenty-percent gain in performance would be expected due to a reduction in cushion pressures.
4. Conclusions
The
SeaCoaster hull form offers numerous advantages in the amphibious vessel application.
Among them are:
lower susceptibility to underwater shock
less sound transmitted by the hull into the water
improved platform for personnel efficiency and weapons employment
expanded helicopter operation envelope in higher sea states
In conjunction with IPS, the SeaCoaster is able to obtain endurance ranges at speeds for a given fuel
load that may be difficult to achieve in other vessel types.
|
|
5. References
1. TRADOC, 'Field Manual No. 55-50. Army Water Transport Operations,' Headquarters, Department
of the Army, Washington, DC, 30 September 1993.
2. 'Technology for the United States Navy and Marine Corps, 2000-2035, Becoming a 21st-Century
Force, Volume 2; Technology,' National Academy of Sciences, Washington, DC, 1997.
3. 'Technology for the United States Navy and Marine Corps, 2000-2035, Becoming a 21st-Century
Force, Volume 2; Technology,'
4. Hulick, T.P., 'Design Guide for Small Craft Detectability Characteristics,' Naval Engineers Journal,
American Society of Naval Engineers, Alexandria, Virginia, May 1987.
5. Hulick, T.P.
6. Hulick, T.P.
7. EDO Corporation Brochure, 'EDO Integrated Combat System Components', Chesapeake, Virginia, 1997
|
|
Figures and Tables
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Table 1
Table 2
Table 3
Table 4
Home Page
|
|
|
|
|
|
|