The crab claw is a type of sailing rig developed originally by the
Polynesians. It is popular with traditional peoples throughout the
Pacific Basin,
South America, and
Western Africa. It consists of a flat triangular sail stretched between upper and lower
booms. A short
mast supports the upper
boom. Think of a
lateen sail with its
foot fastened to a
boom. The intersection of the booms is at the
tack, and fastened near the
bow of the boat.
It shares a key disadvantage with the lateen sail, in that the mast overlaps the sail, making tacking difficult. That, however, is outweighed by the simplicity and efficiency of the rig. According to wind tunnel tests performed by CA Marchaj, and published in his book Sail Performance, the primitive crab claw is the most efficient sailing rig, beating out the more traditional Bermuda rig on all points of sail with the exception of close hauled, and even there it loses by a tiny amount. On a broad reach, however, the crab claw is absolutely dominant, with twice the driving force of an equivalent Bermuda rig.
However, in practice, a well-managed crab claw can beat out a Bermuda rig even there, because the center of effort is much lower than a Bermuda rig of equivalent area. As a result, it produces less healing moment. As a result, it can be pushed harder and produce a greater effective drive.