Crab claw sail

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Micronesian proa with crab claw sail

The crab claw sail or, as it is sometimes known, Oceanic lateen or Oceanic sprit, is a triangular sail with spars along upper and lower edges. The crab claw sail is used in many traditional Pacific Ocean cultures, as can be seen by the traditional proa and tepukei.

[edit] Construction

The crab claw sail consists of a sail, approximately an isosceles triangle in shape. The equal length sides are longer than the third side, with spars along the long sides. The intersection of the yards is placed towards the bow. The sail is supported by a short mast attached near the middle of the upper spar, and the forward corner is attached to the hull

While it is often compared with the lateen sail, the crab claw differs in a number of ways. First, it always has spars along two edges of the sail instead of just one, which results in a different shape. Second, it is carried with the lower spar at an angle, so that airflow across the sail always encounters a curved surface. The modern lateen sail as used on the Sunfish, for example, carries the lower spar horizontally, leaving the foot of the sail flat.

The crab claw is also traditionally constructed with curved spars, giving the edges of the sail along the spars a convex shape, while the leech of the sail is often quite concave to keep it stiff on the trailing edge. These features give it its distinct, claw-like shape. Modern crab claws generally have straighter spars and a less convex leech, which gives more sail area for a given length of spar.

[edit] Performance

In his book Sail Performance, C. A. Marchaj records the results of wind tunnel testing of a number of primitive and modern sailing rigs, including a number of different configurations of crab claw sails. While the upwind performance of the crab claw was less than modern high aspect ratio sails, the reaching and running performance was markedly superior, and Marchaj rated the crab claw sail as the top overall performer.