Small Waterplane Area Twin Hull (SWATH)
The Small Waterplane Area Twin Hull (SWATH) is a hull form used for vessels that require a ship of a certain size to handle in rough seas as well as a much larger vessel. 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 of 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 flotation runs mostly 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.
A SWATH vessel consists of two parallel torpedo like hulls attached to which are two or more streamlined struts which pierce the water surface and support an above water platform. The US Navy commissioned the construction of a SWATH ship called the ‘Kaimalino’ to prove the theory as part of their ship research programme. The Kaimalino has been operating successfully in the rough seas off the Hawaiian islands since 1975.
Swath designs are more expensive to produce than conventional twin hulled vessels. The advantage to such a design is that a significant proportion of the hull remains below the surface of the sea. For this reason wave contact is reduced where it may only act on the thin leg areas. Wave drag is a major component of the total drag on a vessels hull.
For this reason the hull is more stable, being less prone to pitching and rolling and requires less power for propulsion than conventional designs.A Swath design wastes proportionally less energy climbing wave peaks and accelerating down troughs. The passage is smoother. The below surface hull (in normal operating conditions) is subject to laminar (or pipe) friction, which is more predictable.
Why is a SWATH vessel so stable?
The key is the two submarine pontoons that can ride completely submerged beneath the surface of the water (unlike a catamaran where the pontoons only ride on the surface of the water). If you are a diver, you would recognise that even on a rough day, the water is calmer just a few feet below the choppy surface. That is precisely where the pontoons which provide the buoyancy for a SWATH are located, hence the submerged pontoons act as stabilizers giving you the feeling you’re on a vessel up to eight times its size!.