Crab claw sails

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Deep sea boat with outrigger and crab claw sails from the Santa Cruz Islands in the Ethnological Museum Berlin-Dahlem . When turning or jibing, the boat travels in the opposite direction: You only have to put the sail from the bow to the aft in the abutment there, the support mast and sail stand have to be folded down.
Modern Proa replica with all-round leash to facilitate "shunting"

The crab claw sail (also known as the delta sail ) comes from Polynesia . It has a triangular shape and is used on proas or outrigger canoes . The lift is not generated by a wing profile, as is the case with conventional sail types , around which the flow should be as laminar as possible. The flow against the crab claw sail is at the tip of the triangle, a tip vortex forms on each of the legs, in which the flow is so fast that a negative pressure is created on this side (delta wing). In this way, 1.7 times more lift is achieved with the same sail area.

The crab-claw-like shape of the sail has the same efficiency as a rectangular area measured over the outer edges, which was recognized in wind tunnel tests. Presumably, with the indented shape of the sail, an excessive increase in pressure is avoided under side winds (“flow separation”) and thus the usefulness is increased. The "ornamental hanger" attached to one side of the sail is used to control turbulence.

Basically, the sail type was developed for outrigger boats that maintain their position to the wind when turning or jibing, so that the outrigger always remains on the windward side, so that there is no permanently defined "bow and stern side". The tip of the sail, which is flexibly suspended from the mast, is attached to the front end of the boat - in the direction of travel - and, after turning, to the other end (so-called shunting). But there are modern attempts to turn the sails overhead, so as to conventional boats contact .

Web links

Commons : Crab Catcher Sails  - Collection of images, videos and audio files