High Speed Sealift
There is an increasing need for surface ships that can transit oceans with greater speed, i.e. in the range of forty to fifty knots, and with high stability because of the commercial requirements for rapid and safe ocean transits of perishable cargoes, high cost capital goods, military strategic sealift cargoes, cargoes whose dimensions and density cannot be accepted for air freight, and other time-sensitive freight, particularly in light of the increasing worldwide acceptance of "just-in-time" inventory and stocking practices.
Today's container ships are tending towards greater size, for reduced cargo ton-mile costs, carrying up to 25,000 tons of containerized cargo at a time. This necessitates their visiting a number of ports on both sides of an ocean crossing to load and unload cargo. This is time-consuming and means that the largest ships can only undertake a relatively small number of ocean crossings per year, thus limiting the available financial turnover on their considerable investment cost.
A much faster--but smaller--ship, operating at between 40 and 50 knots, can undertake a transatlantic roundtrip each week between only one port on each side of the ocean crossing. Although carrying only up to 10,000 tons of cargo, this smaller, faster ship could transport about 60% more cargo per year than the larger ship, with each container being subject to a much more controlled collection and delivery system using more disciplined intermodal techniques because at each port the ship is fully unloaded and reloaded. Thus the time taken from pick-up to delivery of each container (door-to-door) could be significantly reduced. For this service a cost premium may be charged, such as is presently charged for airfreight, lying somewhere between the current sea and airfreight tariffs. This premium, together with the much greater cargo turnover on each ship, more than compensates for the increased fuel consumption required for operating at over twice the speed of most current larger container ships.
It is impracticable to achieve such an increase in speed by the traditional method of making such container ships very large because, as their length is increased to raise their threshold speed according to Froude's laws, their cargo payload and stability are eroded. Serious questions also arise over the ability of propellers to deliver the necessary power due to their performance being degraded by the onset of cavitation, their impractical size and the problems of optimizing blade pitch at intermediate speeds, which could necessitate very complex gearboxes.
The monohull fast ship (MFS) develops hydrodynamic lift above a certain threshold speed as a result of the presence of high pressure under the aft part of the hull and also in the upper surfaces of the inlet pipes for the waterjets. Such a hull reduces the residuary resistance of the hull in water. Therefore, power and fuel requirements are decreased. Since hydrodynamic lift increases as the square of the velocity, a lifting hull allows higher speeds to be achieved than a traditional hull which tends to "squat" or sink at speeds above a Froude number of 0.42 or a speed length ratio of 1.4. Working boats utilizing the MFS form are now being used at sea or in many of the world's harbor approaches. This hull form has also up to now been considered limited to certain size fast pilot boats, police launches, rescue launches and fast lifeboats, custom launches, patrol boats, and even motor yachts and fast fishing boats which range in size from 16 to 200 feet (from 2 to about 600 tons). For their size, these boats are much heavier and sturdier than the planing boats. In the speed range of 5 to 25 knots, they have a much smoother ride. They also use much less power for their size at speed length ratios lower than 3.0 than does the planing hull, and they are very maneuverable. Although it has generally been claimed by leading naval architects that the practical use of this type of hull is limited to quite small craft, such a hull has been used for a 600 ton yacht. However, it has never been contemplated for commercial or military ships of over 2,000 tons.
Different interpretations of fast displacement hulls differ in both hull-form and operational aspects. One "double-ended" boat with a lifting stern combines the alleged superior seakeeping qualities of the pointed or "canoe" stern with the lifting qualities necessary to prevent such a boat from "squatting" at more than "a moderate speed", although such speed is not defined in any respect.
Whatever the capability of such a hull to generate hydrodynamic lift at the stern, such a stern is specifically unsuitable for ships of greater than 600 tons displacement and an operational speed such as 40-50 knots for the present invention due to the fact that a wide transom stern (which Troyer specifically excludes in his teaching) is a fundamental requirement for the efficient installation of waterjets as taught by the present invention as discussed hereinafter. Furthermore, at the speeds for which the present invention is intended (viz: a speed length ratio of 1.4 to 3.0) a greater area of lift is required than is obtainable from the Troyer boat without recourse to excessive beam and associated increase in drag. The stern has, characteristically, a rounded or pointed plan-form, a chine or sharp angle at the conjunction of the bottom portion and sides below the waterline; and angles of deadrise at the stern which are greater than 10 degrees. In these important features it diverges from the design features set out for the present invention as discussed below.
Another "displacement-type hull" is intended to overcome "the rapid increase in wave generating drag attendant with increased speed", placing the relevant speed as a Froude Number of between 0.6 and 1.20. The major feature of this design is a high speed displacement hull in which a substantial portion of length comprises a parallel midbody of constant and full section.
Waterjet propulsion systems which substantially reduce the cavitation and vibration problem of propeller drives are known. To date they have not been perceived as useful for propelling larger ships, particularly at high speeds, and are deemed generally too inefficient because they require high pressure at the water inlet in the aft part of the submerged hull, rather than low pressure which generally exists at that portion of traditional large displacement hulls.
In the 1960's & 1970's, technologies for small passenger ferries were concentrated on hydrofoils and air-cushioned vehicles (ACVs) such as hovercraft and surface effect ships (SESs). In the 1980's, fast catamarans were developed and achieved much greater market acceptance than others. Today, most of the new fast passenger ferries being built are catamarans. The fast passenger vessel technologies are now being applied to larger fast car ferries. In 1990's more ships were being built and the majority have been "Wave Piercers" but there were also fast catamarans and mono-hulls. There are various types of these vessels based on the variety of technologies. The wide range in types and sizes was a clear indication that the industry is a long way from maturity, which is further demonstrated by significant variations in selection of hull materials, propulsion machinery, etc. Technologies are being developed and improved dramatically, which enable new designs capable of 55-60 knots. It is too early to accurately predict the design types and features that will predominate but the proponents of Wave Piercers and other types of fast catamarans are very confident. With the establishment of the viability of fast car ferries, there were clear possibilities to adapt their technologies to fast freight vessels.
In 1989 Japan initiated a new development program, Techno-Super Liner (TSL), which aimed at developing a vessel capable of carrying 1000 tons of cargo at 50 knots over a distance of 500 miles. The immediate challenge lies in proving commercial viability of fast freight vessels. Japanese shipbuilders started testing an experimental high-speed freighter in mid-1994. One of the jet-propelled ships, the Hisho, or flight, can cruise at 50 knots, skimming over the water rather than plowing through it. The 1,590-ton, 70meter-long catamaran can skip over waves up to 6 meters high. Conventional passenger hydrofoils with comparable speed capabilities have to slow to a crawl in seas half as rough. The Hisho was built by Mitsubishi Heavy industries Ltd. and Mitsui Engineering and Shipping Ltd. in Nagasaki. A similar experimental hydrofoil, the TSL, was built in Kobe by a consortium made up of Kawasaki Heavy Industries Ltd., Ishikawajima-Harima Heavy Industries Co., Sumitomo Heavy, NKK Corp. and Hitachi Zosen Corp.
Not to be outdone by Japan's plan to dominate the future market of high speed cargo transportation with Techno-Super-Liners, Australian government commissioned a study on developing a high speed catamaran that would out-perform TSL in the cargo capacity and the range. International Catamaran Designs Pty, reviewed the development of a design for a fast container ship based on the successful and proven fast Wave-Piercer car ferries, which would be capable of carrying 3,000-ton cargo with a service speed of 50 knots over the range of 3,000 nm. The study recommended a cargo handling system for loading/unloading 20, 40, 48 ft containers that would enable loading the 3,000 ton cargo within 2-3 hours.
These ships are enabled by evolving hullform technologies such as are in use today in the commercial ferry sector. The Australian Navy's HMAS Jervis Bay, a catamaran, is cited as an example of such a ship. HMAS Jervis Bay is utilised to transport troops and their vehicles as part of Australia's amphibious lift capability. HMAS Jervis Bay is one of a series of high speed ships built by Hobart shipbuilder International Catamarans Australia (INCAT) and is the first vessel of her type to be operated by any navy worldwide. In her role as a fast sea-lift ship, Jervis Bay can transport up to 500 fully equipped troops, together with their vehicles and equipment, to ranges of up to 1000 nautical miles at speeds of more than 40 knots. The boat's maximum range is approximately 1,500 nautical miles, at speeds of more than 40 knots, and the four diesel engines --7,080 kilowatts each -- can drive the catamaran up to speeds of 45 knots. In contrast, the fastest US Navy amphibious ship can reach only 24 knots. Under charter by the Royal Australian Navy, Jervis Bay is used to trial and evaluate the suitability of high-speed, multi-hull technology for future maritime projects.
The hull form is known as a wave-piercing catamaran - it has a "notched" bow profile to reduce reserve buoyancy forward. This allows the ship to cut through waves rather than riding up over them, thus avoiding pitching motion. The bow arrangement incorporates a centerline v-section that is entirely above the design waterline, but which provides progressive reserve buoyancy and at the same time reduces slamming in heavy seas. Additional dynamic control of pitch and roll is provided by a retractable T-foil mounted in the v-section and stern trim tabs (transom extensions). Welded aluminum construction is used throughout the ship's structure, with some steel reinforcement in the vehicle deck areas, using longitudinal stiffeners supported by transverse web frames and bulkheads. The superstructure is mounted resiliently above the vehicle deck. This allows for lower noise in the accommodation spaces. Furthermore, because the superstructure is not an integral part of the hull structure, the configuration of the upperworks of the ship can be substantially altered to suit different missions without costly and time-consuming re-design of the main hull.
The idea of using catamarans caught the attention of the US Navy. The US Naval Warfare Development Command, working in conjunction with the Naval Special Warfare Command, initiated a program to explore the possibilities of including catamarans in intra-theater operations to respond to the littoral warfare operations around the world. As of late 2000 the Navy was considering whether an Advanced Concept Technology Demonstration was warranted, using sophisticated second-generation high-speed catamarans now entering the commercial market. The basic requirements are for a craft that can handle 400 tons of cargo, travel 1,200 nautical miles without refueling and achieve a speed greater than 40 knots.
If the Navy needs to "kick down the door" and make a statement, then current amphibious assault forces are definitely the way to go. However, sometimes, the Navy wants to go into a situation without causing political problems. The catamaran may be able to help do just that, since it is a very cost effective way of quickly moving troops and material. The catamaran's advantages are intriguing. The craft only requires a crew of approximately 25 people, and only a couple of days each year for maintenance and shipyard work. Although it voraciously gobbles 125,000 gallons of fuel per hour, the pay back is tremendous. In addition to its ability to quickly move a large amount of troops, the catamaran can hold up to 15 tanks. With modifications, it could possibly serve as a launch platform for the rigid-hulled inflatable boats used by Special Forces. An articulated ramp attachment eliminates the need for much of the port services required by larger ships.
The US Navy's annual Global War Game in August 2000 at the Naval War College in Newport RI incorporated a number of vessels, termed Theater Support Ships, that were modeled after high speed catamarans. Within a 24-hour period, a fast catamaran could threaten 2,000 nautical miles of coastline.
International Catamaran has conducted studies on vessels with design goals of 3,000 ton cargo, 50 knot service speed, and a range of 3,000 nm. In the future, a vessel with 5,000-10,000 ton cargo at 40 knots over the range of 35,000 nm may be feasible. A "Cargo Express Concept" has been studied with 1,500 ton cargo at 45 knots over 2,000 nm.
Surface Effect Vessel (SEV)
Surface Effect Vessel (SEV) are hybrid surface effect ships that incorporate rigid catamaran-like sidehulls and bow and stern seals to create a plenum pressurized by air. The result is a craft which is 80 percent supported by pressurized air and 20 percent supported by buoyancy. When the plenum is pressurized (on-cushion), the wetted surface of the sidehulls is reduced, reducing drag and allowing high speeds.
Litton Ingalls Shipbuilding has designed (concept design) a transoceanic SEV capable of average transit speeds of 70-75 knots with payloads of 5,000 short tons over a range of 8,700 nautical miles. Ingalls is also designing a smaller coastal SEV (container/RO/RO) as a possible contender for the Army Theater Logistic Vessel (TLV) requirement, and a fast ferry SEV (RO/PAX) that may have intra-theater or riverine warfare applications. The transoceanic and coastal SEV designs are gas turbine powered, water jet propelled and have shallow drafts. They can transport both track and wheeled vehicles as well as containers and will be self-sustaining (i.e., can offload itself) vessels that can rapidly offload in damaged or austere ports, or directly across a beach without the aid of JLOTS. The speed and payload of the vessel would provide the JFC with the capability to strategically maneuver forces into positions of operational advantage on a global basis. The vessel's shallow draft, coupled with its other features would make it useful for maneuvering forces within the theater. It could provide the JFC with the element of surprise and keep the adversary "off balance" because his entire shoreline would be vulnerable to attack.
The need for high speed, heavy payload capable, long distance transportation has led to the investigation of alternate ship designs capable of performing highspeed sealift. Several options are being explored including hydrofoils, multi-hulls, air cushion vehicles, and semi-planing hulls. A proposed commercial example of the latter variety is thought to be capable of 40+ knots with a payload of over 8,000 long tons. Appraisals of the technical feasibility of developing a 50-kt displacement-hull type ship capable of delivering a 12,000 LT payload over a distance of 9000 nm concluded that this ship will have a length of 1500 ft, a displacement of 64,000 LT, a draft of 30 ft, a length/beam ratio of 12.6 and a transport efficiency factor nearly twice that of any existing 50-kt ship. Further, it will require nearly 600,000 hp, which is substantially larger than that in any existing marine vessel.
|Join the GlobalSecurity.org mailing list|