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 ama; those used in proas are generally laterally symmetric, as the proa is designed to sail with either end forwards, while trimaran ama are one-directional and may have no axis of symmetry.
The most advanced ama 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 ama, 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.
Read more about this topic: Ama (sailing)
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