Small Waterplane Area Twin Hull: I watched a really good Super Ships that talked about SWATH ships, which stands for “Small Waterplane Area Twin Hull.” This is a type of hull that enables ships to operate in heavy seas without really rolling around much.
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”.
There’s the gist: a SWATH ship looks like a catamaran — it’s twin hull. But it doesn’t float on the water as much as it floats in the water. The “floaty part” of the hull are two bulges well under the water line. So the actual hull — presumeably the “waterplane” of the name — cuts through the water.
Here’s a better description from a company that specializes in SWATH design:
SWATH ships typically have two submarine-like lower hulls completely submerged below the water surface. Above water, a SWATH resembles a catamaran. Its haunch areas are connected to each submerged hull by one or two relatively thin vertical members, or struts. The longitudinal cross-section of each strut is somewhat streamlined to decrease wavemaking resistance.
The page linked above also contains some excellent diagrams of how it all works.
The idea is that the sea rocks much more on the surface that it does under the surface. If that doesn’t make sense, just take my word for it — the show had a great graphic that made perfect sense. While waves on the surface may be huge, the movement of the water even a few feet down is much less pronounced.
So the part of a SWATH ship that floats is 6-8 feet underneath the waves. The different is striking. They showed a SWATH ship — the Kilo Moana, a research ship from the University of Hawaii — side-by-side with a traditional ship in very rough seas.
The Kilo Moana barely rocked up and down, while the other ship heaved up over every wave, then crashed back down the other side. The Kilo Moana looked to be “cutting through” the waves, rather than going over them, because the part of the ship that actually displaced water was well below the waves.
It’s an ingenious idea. When I get that 50-foot catamaran that my wife and I are going to sail the world with, it will be a SWATH hull. I’ll save a ton of money on Dramamine.