Ama on a contemporary Hawaiian trimaran

An ama is an outrigger on a proa or trimaran, used to provide stability. In a canoe or bangca, an outrigger is a thin, long, solid, hull used to stabilise an inherently unstable main hull. ... R. M. Munroes 1898 proa A Proa is a multihull vessel consisting of two (usually) unequal parallel hulls, superficially similar to an outrigger canoe. ... A trimaran is a multihull boat consisting of a main hull and two smaller outrigger hulls (amas), attached to the main hull with lateral struts (akas). ...

History

The term ama comes from the proa. The vaka is the main hull, the ama is the outrigger, and the aka is the support connecting the two hulls. The term ama and aka have widely been applied to modern trimarans. A hull is the body or frame of a ship or boat. ... The aka of a multihull sailboat is a member of the framework that connects the hull to the ama(s) (outrigger). ...

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 fowards, while trimaran amas are one-directional and may have no axis of symmetry. Leeward is the side of a boat away from the direction where the wind is coming (i. ... Windward is the side of a boat into which the wind is blowing. ...

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 fron the sailing rig. The Bruce foil, for example, is a type of leeboard often attached to an ama to assist in in producing lift. Lift consists of the sum of all the fluid dynamic forces on a body perpendicular to the direction of the external flow approaching that body. ... An Air France Boeing 777, a modern passenger jet. ... A Bruce foil is a variant of the leeboard, an arrangement to provide a sailing boat with resistance to sideways movement so that the sails will produce nearly forward movement under a variety of apparent wind directions. ... A leeboard is a lifting foil used by a sailboat, much like a centerboard, but located on the leeward side of the boat. ...