SWATH (Small Waterplane Twin Hull)

The buoyancy of a SWATH ship is provided by two submarine hulls connected to the upper platform by twin narrow struts from each of the submarine hulls. This mature technology used by the military and for deep-sea research ships. Until now, it has not been available in a private yacht. Very simply the hull form reduces the upward forces on the vessel as the wave passes through. The biggest advantage comes in a beam sea because the technology significantly reduces the vessel’s vertical movement and totally eliminates the quick jarring movements. 

The idea of SWATH was taken from the principle of submarines at periscope depht, which has been shown to have minimal or no motion, placing most of the ship's displacement under the waves.

The Small Waterplane Area Twin Hull (SWATH) is a twin-hull ship design that minimizes hull volume in the surface area of the sea. By minimizing hull volume in the sea's surface, where wave energy is located, the vessel becomes very stable, even in high seas and at high speeds. The bulk of the displacement necessary to keep the ship afloat is located beneath the waves, where it is less affected by wave action, as wave excitation drops exponentially with depth.

Advantages are: (1) ability to deliver big-ship platform steadiness and ride quality in a smaller vessel and (2) ability to sustain a high proportion of its normal cruising speed in rough head seas. SWATH ships typically have two submarine-like lower hulls completely submerged below the water surface. Above water, a SWATH resembles a catamaran.

However, the purpose for a balanced Small Waterplane Twin Hull ship design is NOT to minimize ship motions at the expense of speed-power or payload capabilities. During the design process, if the total amount of strut waterplane area is decreased, the transverse spacing between the hulls must be increased to regain adequate transverse stability to resist heeling over moments as a result of wind or movement of all passengers to one side of the ship.

Adequate clearance to the underside of the connecting structure is also essential to allow the Small Waterplane Twin Hull ship to ride over surface waves that are typically present in coastal waters. This type of operating mode results in the smallest vertical motions and is called platforming. For rough seas, with wave heights exceeding the amount of cross-structure clearance, SWATH ships are designed to have a sufficiently short heave period to provide inherent contouring behavior at low speeds.


A little history,

1938 - Frederick G. Creed, a Canadian, presents his idea for a small-waterplane-area twin-hull aircraft carrier to the British Admiralty. Several years later Creed is permitted to show it to the U.S. Navy, but they do not pursue the concept.

1946 - Frederick Creed is awarded a British patent.

1959 - U.S. Navy activity in moderately high speed "semi-submerged ships" begins with H. Boericke proposing the streamlined "shark form" monohull1, for which he was awarded a patent in 1962.

1965 - Alan McClure of Houston proposes the MOHOLE platform, with a configuration similar to Blair's, as a mobile drilling rig.

1967 - Dr. Reuven Leopold of Litton Industries presents to the U.S. Navy his moderately high speed TRISEC concept2, for which he was awarded a patent in June, 1969.

1968 - The 40m long, low speed small-waterplane-area twin-hull vessel Duplus3 is launched by the Boele Shipyard in the Netherlands. Designer of the 1200-ton Duplus is a Dutch naval constructor, J. J. Stenger, who based his design for a self-propelled oil exploration support vessel on the fact that submarines lying at periscope depth experience little wave-induced motion.

1968 - an M.I.T. student proposes a streamlined version of the MOHOLE platform for a class project and carries out model tests on the design, which he called a semi-submerged catamaran.

1968 - Dr. Tom Lang of the Naval Underseas Center (NUC) in San Diego begins intensive development of his concept for a "high speed ship with semi- submerged hulls", for which he was awarded a U.S. patent in 1971. A key element of the concept is the provision of movable horizontal fins located aft of the vessel's center of gravity to stabilize vessel trim and pitch motions at higher speeds.

1970 - Mitsui Engineering & Shipbuilding Co., in Tokyo, begins basic research on the "semi-submerged catamaran", or SSC.

1972 - Construction begins on the 190-ton SWATH workboat SSP KAIMALINO for NUC after 18 months of research by engineers at NUC and nearly 2.5 years of design and confirmatory model testing4. Launching occurs in March, 1973.

1973 The acronym "SWATH" is coined by U. S. Navy technocrats who promote its use, rather than "semi-submerged" ship or catamaran, to distinguish this concept from conventional catamarans.

1979 - Mitsui Engineering & Shipbuilding completes the world's first commercial SWATH ferry, the 26.5 knot MESA 80 (aka, SEAGULL), with a capacity of 446 passengers5.

1991 - The first of a class of 4 SWATH acoustic Surveillance ships designed by the U.S. Navy, the 71.5 m long victorious, is delivered to the Military Sealift Command.

1992 - Finnyards delivers the first SWATH cruiseliner, the 131 m long Radisson Diamond, to Diamond Cruise Ltd.

1993 - The existence of the world's first so-called "stealth" ship, the U.S. Navy's 50 m long Sea Shadow, an A-Frame SWATH ship built by Lockheed Missiles and Aerospace Co., is declassified and disclosed publicly, leading to a cover article in the July '93 issue of Popular Mechanics.

REFERENCES
1. Boericke, H., Jr., "Unusual Displacement Hull Forms for High Speed," International Shipbuilding Progress, Vol. 6 (1959).
2. Leopold, R., "A New Hull Form for High-Speed Volume-Limited Displacement-Type Ships," Paper No. 8, Society of Naval Architects and Marine Engineers Spring Mtg. (May 1969).
3. Stenger, T.T., "The Trident Stabilized Vessel Concept for Offshore Drilling and Construction Operations," Offshore Technology Conference, Paper OTC 1138 (1969).
4. Lang, T.G.; Hightower, J.D.; Strickland, A.T.; "Design and Development of the 190- Ton Stable Semi-Submerged Platform (SSP)," ASME Paper No. 73-WA/OCT-2 (Nov. 1973).
5. "Mesa 80: Mitsui's Semi-Submersible Catamaran as a Fast Ferry," The Motor Ship, (July 1980).
6. "America's Invisible Warship," Abe Dane, Popular Mechanics, (July 1993).

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