Trim tab

Although this word has been formed from catamaran, it is misleading if it suggests that because a catamaran is a two-hulled boat a trimaran is three-hulled. The trimaran, like the Proa, has a single hull which is too narrow to be stable without the assistance of a Sponson or an Outrigger. But whereas the proa has only one sponson, a trimaran has two. In most tris, only the leeward sponson is in contact with the water, the other being lifted clear by the slight heel of the boat.