Crab claw sail
Encyclopedia
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
.
in shape. The equal length sides are longer than the third side, with spars
along the long sides. The forward intersection of the upper spar, or yard
, 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 shunt
, 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-tightend 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.
s). 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.
Shell Boats of Vermont USA offers a kit or completed 21-ft catamaran.
"Hot Buoys" is a 65-ft trimaran with a mast-aft supported crab-claw sail. Video in external links.
"Epicure" is a 47-ft cruising proa designed, and built by Daniel Charles. It was launched in 2010 and has a few luxuries not common in traditional Polynesian crafts including a washing machine/dryer and dishwasher
Lateen
A lateen or latin-rig is a triangular sail set on a long yard mounted at an angle on the mast, and running in a fore-and-aft direction....
or Oceanic sprit
Spritsail
The spritsail is a form of three or four-sided, fore-aft sail and its rig. Unlike the gaff where the head hangs from a spar along its edge, this rig supports the leech of the sail by means of a spar or spars named a sprit...
, is a triangular sail with spars along upper and lower edges. The crab claw sail is used in many traditional Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
cultures, as can be seen by the traditional proa
Proa
A proa, also seen as prau, perahu, and prahu, is a type of multihull sailing vessel.While the word perahu and proa are generic terms meaning boat their native language, proa in Western languages has come to describe a vessel consisting of two unequal length parallel hulls...
and tepukei
Tepukei
A Tepukei is a very old Melanesian boat type, first reported by Spanish explorer Álvaro de Mendaña in 1595, on his visit to the Santa Cruz Islands...
.
Construction
The crab claw sail consists of a sail, approximately an isosceles triangleTriangle
A triangle is one of the basic shapes of geometry: a polygon with three corners or vertices and three sides or edges which are line segments. A triangle with vertices A, B, and C is denoted ....
in shape. The equal length sides are longer than the third side, with spars
Spar
In sailing, a spar is a pole of wood, metal or lightweight materials such as carbon fiber used on a sailing vessel. Spars of all types In sailing, a spar is a pole of wood, metal or lightweight materials such as carbon fiber used on a sailing vessel. Spars of all types In sailing, a spar is a...
along the long sides. The forward intersection of the upper spar, or yard
Yard (sailing)
A yard is a spar on a mast from which sails are set. It may be constructed of timber, steel, or from more modern materials, like aluminium or carbon fibre. Although some types of fore and aft rigs have yards , the term is usually used to describe the horizontal spars used with square sails...
, 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
Proa
A proa, also seen as prau, perahu, and prahu, is a type of multihull sailing vessel.While the word perahu and proa are generic terms meaning boat their native language, proa in Western languages has come to describe a vessel consisting of two unequal length parallel hulls...
.
To shunt
Shunt
Shunt may refer to:* Shunt - a hole or passage allowing fluid to move from one part of the body to another* Shunt - a device allowing electrical current to pass around a point in a circuit...
, 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-tightend 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 obviously(this discounts the use of specialised sails such as spinnakerSpinnaker
A spinnaker is a special type of sail that is designed specifically for sailing off the wind from a reaching course to a downwind, i.e. with the wind 90°–180° off the bow. The spinnaker fills with wind and balloons out in front of the boat when it is deployed, called flying. It is constructed of...
s). 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
Delta wing
The delta wing is a wing planform in the form of a triangle. It is named for its similarity in shape to the Greek uppercase letter delta .-Delta-shaped stabilizers:...
, generating vortex lift
Vortex lift
Vortex lift is a form of lift generated by delta wings operating at high angles of attack.-How it works:Vortex lift works by capturing vortices generated from the sharply swept leading edge of the wing. The vortex, formed roughly parallel to the leading edge of the wing, is trapped by the airflow...
. 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.
Crab-Sails On Other Sailboats
"Rapa Nui" is a 51-ft catamaran designed by James Wharram and owned by Hans Kaar.Shell Boats of Vermont USA offers a kit or completed 21-ft catamaran.
"Hot Buoys" is a 65-ft trimaran with a mast-aft supported crab-claw sail. Video in external links.
"Epicure" is a 47-ft cruising proa designed, and built by Daniel Charles. It was launched in 2010 and has a few luxuries not common in traditional Polynesian crafts including a washing machine/dryer and dishwasher