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.
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 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 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.
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. 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. 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 twist at this point of sail.
External links Video "Hot Buoys" Self-Tacking Crab-Claw Trimaran
Polynesian Voyaging Society Wind Tunnel results on effects of crabclaw
sail orientation - apex position and consistent camber
References ^ "Die pazifische proa". 2007. Retrieved 2009-09-24.
"The link starts the video at 6m56s where a shunting maneuver starts. Another shunt
occurs at 9m33s." ^ Michael Schacht, Proafile.com. "Proafile
Rig Options - Crab Claw". Retrieved 2009-09-24. ^ Marchaj, C. A. Sail Performance: Techniques
to Maximise Sail Power. ISBN 0-07-141310-3. ^ "Delta Sail in A "Wind Tunnel" Experiences
from B.J. Slotboom".