Small Waterplane Area Twin Hull

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For the definition of the word "swath", see the Wikitionary article

A SWATH ship resembles a catamaran. The twin hulls (blue) remain completely submerged.

The Small Waterplane Area Twin Hull (SWATH) is a hull form used for ships that require a ship of a certain size to handle equally well as a much larger ship, particularly in rough seas. Particularly in terms of its roll characteristics, a SWATH vessel "thinks it is a much larger ship". An added benefit is a high proportion of deck area for their displacement — in other words, large without being heavy. 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. It was first used in the 1960s and 1970s as an evolution of catamaran design for use as oceanographic research vessels or submarine rescue ships.

Catamarans provide large, broad decks, but have much higher water resistance than monohulls of comparable size. To reduce some that resistance (the part that generates waves), as much displacement volume as possible is moved to the lower hull and the waterline cross-section is narrowed sharply, creating the distinctive pair of bulbous hulls below the waterline and the narrow struts supporting the upper hull. This design means that the ship's floatation runs under the waves, like a submarine (the smooth ride of a sub was the inspiration for the design). The result is that a fairly small ship can run very steady in rough seas. A 50-meter ship can operate at near full power in nearly any direction in waves as high as 12 meters.

[edit] Examples of SWATH ships

CCGS Frederick G. Creed, a Canadian Coast Guard survey ship.
Sea Shadow, a US Navy experimental stealth ship.
Asia Star, formerly theRadisson Diamond, a 350 person all balcony luxury cruise ship with 20295 tonnes gross tonnage.
R/V Kilo Moana, a University of Hawaii reasearch vessel.
Cloud X, a passenger ferry running between Florida and the Bahamas.
Nekton Rorqual/Swacat 80, an 80 foot cruise ship operated by Nekton Diving Cruises.
Cetus and Perseus, pilot vessels for the Netherlands Loodswezen.
Döse, Dunen, pilot vessels for the German Brotherhood of Elbe - Pilots.
Elbe, pilot station vessel for the German Brotherhood of Elbe - Pilots.
Planet Type 751, weapons + sonar research ship of the German Navy.