Small-waterplane-area twin hull

A Small Waterplane Area Twin Hull, better known by the acronym SWATH, is a twin-hull ship design that minimizes hull cross section area at the sea's surface. Minimizing the ship's volume near the surface area of the sea, where wave energy is located, maximizes a vessel's stability, 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. Wave excitation drops exponentially as depth increases (deeply submerged submarines are normally not affected by wave action at all). Placing the majority of a ship's displacement under the waves is similar in concept to creating a ship that rides atop twin submarines.

The twin-hull design provides a stable platform and large, broad decks. The main disadvantages of SWATH watercraft are that they are more expensive than conventional catamarans or mono-hulls, require a complex control system, have a deeper draft, and have higher maintenance requirements. The design of SWATH vessels is also considerably more complex due to the structural complexities inherent to the design. Additionally, SWATH vessels cannot operate in planing or semi-planing modes and as such gain no drag reduction when operating at speeds normally associated with such modes. This marginally limits SWATH speed when compared to equivalent catamaran vessels.

The SWATH form was invented by Canadian Frederick G. Creed, who presented his idea in 1938, and was later awarded a British patent for it in 1946. The first full-scale SWATH vessel to be built and put into productive service was MV Duplus, a diving support vessel built in the Netherlands in 1968. In the 1970s, several units were built in different countries (including SSC Kaimalino by the US Navy, and an 80-ft ferry in Japan). Since the 1980s, oceanographic research vessels, pilot tenders, yachts and other craft are more routinely being built with the SWATH hull type.

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