Performance
The crab-claw sail is something of an enigma. It has been demonstrated to produce very large amounts of lift when reaching, and overall seems superior to any other simple sail plan (this discounts the use of specialised sails such as spinnakers). C. A. Marchaj, a researcher who has experimented extensively with both modern rigs for racing sailboats and traditional sailing rigs from around the world, has done wind tunnel testing of scale models of crab-claw rigs. One popular but disputed theory is that the crab-claw wing works like a delta wing, generating vortex lift. Since the crab claw does not lie symmetric to the airflow, like an aircraft delta wing, but rather lies with the lower spar nearly parallel to the water, the airflow is not symmetric.
This can clearly be seen in Marchaj's wind tunnel photos published in Sail Performance: Techniques to Maximise Sail Power (ISBN 0-07-141310-3). The vortex on the top spar of the sail is much larger, covering most of the sail area, while the lower vortex is very small and stays close to the spar. Marchaj attributes the large lifting power of the sail to lift generated by the vortices, while others attribute the power to a favourable mix of aspect ratio, camber and (lack of) twist at this point of sail.
Read more about this topic: Crab Claw Sail
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