Shunt (sailing)

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.

The crab claw sail consists of a sail, approximately an isosceles triangle in shape. The equal length sides are usually longer than the third side, with spars along the long sides.

The crab claw may also traditionally be 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. Spars may taper towards the leech. The structure helps the sail to spill gusts.

The crab claw characteristically widens upwards, putting more sail area higher above the ocean, where the wind is stronger and steadier. This increases the heeling moment: the sails tend to blow the watercraft over. For this reason, crabclaws are typically used on multihulls, which resist heeling more strongly.

Proa: single mast with crab claw sail. The sail is shunted; the bow becomes the stern, and the mast rake is also reversed. The vessel therefore always has the ama (and sidestay, if there is one) to windward, and has no bad tack.

Proa-like crabclaw (oddly, on a single-ended multihull with a fixed mast).

New Guinea-style crabclaw rig.

Crabclaw amalgamating mast and 'ōpe'a.

In a proa, the forward intersection of the spars 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. The lower spar, or boom, is attached at the forward intersection, but is not attached to the mast. The proa has a permanent windward and leeward side, and exchanges one end for the other when coming about.

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Wikipedia