An ama is an outrigger on a proa or trimaran, used to provide stability.
History
The term ama comes from the proa. The vaka is the main hull, the ama is the outrigger, and the aka or iako (Hawaiian) is the support connecting the two (not three) hulls. The term ama and aka have been widely applied to modern trimarans.
Function
On a proa, the ama may provide lift or ballast, depending on whether it is designed to be used to leeward or windward; on a trimaran it is designed primarily to provide lift. There are many shapes of amas; those used in proas are generally laterally symmetric, as the proa is designed to sail with either end forwards, while trimaran amas are one-directional and may have no axis of symmetry. The most advanced amas are composed of highly curved surfaces which generate lift when driven forward through the water, much like an airplane wing. This lift may be directed to the windward, used to counter slipping to leeward, or may be oriented vertically to counter heeling forces from the sailing rig. These highly-curved structures are much more difficult to manufacture than traditional amas, and are therefore more expensive. The Bruce foil is an example of a type of leeboard often attached to an ama to assist in producing lift.
