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 forward intersection of the spars is placed towards the bow. The sail is supported by a short mast attached near the middle of the yard, 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 crab claw is a shunting sail. It is used on a boat which has a permanent windward and leeward side, and exchanges one end for the other when coming about, such as a proa.
To tack, or switch directions across the wind, the forward corner of the sail is loosened and then transferred to the opposite end of the boat. While remaining attached to the top of the mast, the upper spar tilts to vertical and beyond as the forward corner moves past the mast and onward to the other end of the boat. Meanwhile, the mainsheet is detached and used to rotate the rearward end of the boom through a horizontal half circle. The forward corner is then re-attached at the new "forward" end of the boat and the mainsheet is re-tightened at the new "rearward" end.
The crab claw is often compared with the tacking lateen sail. While it is possible for the sails to be identical, the two rigs are not identical. The crab claw pivots around the leading edge spar, while a lateen sail on a tacking boat such as a Sunfish pivots around a mast. Many lateen sails have an arrangement to move the gooseneck on the boom; as the gooseneck moves forward, the axis of rotation moves forward as well, and the sail looks and acts more and more like a crab claw while under way.
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.
Read more about this topic: Crab Claw Sail
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