Information AboutProa |
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SIZE AND SAIL PLAN The Micronesian proa is found in a variety of sizes, from the small, canoe-like kor-kor (about 15 feet in length) to the medium sized tipnol (20 to 30 feet), to the tremendous walap, up to 100 feet (30 m) long. There is also a model proa, called a riwut, that is often raced by children. Proas could be paddled or sailed. The traditional sail used on the proa was the crab-claw sail. The crab-claw sail generates far more lift than the more common triangular Sloop sails used on small boats, particularly when Reaching . The sloop sail only begins to show an advantage with small angles of attack, such as encountered when Beating . This is the result of the higher aspect ratio of the sloop sail. 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 specialized sails such as Spinnaker 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 , and works by 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 Maximize Sail Power (ISBN 0071413103). 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. SAILING THE PROA When sailing in a strong wind, the crew of the proa act as ballast, providing a force to counteract the Torque of the wind acting on the sail. The weight of the crew can provide considerable torque as they move out along the akas towards the ama. A skilled crew will balance the proa so that the ama leaves the water and skims over its surface; this is called "flying the ama", and gives the proa its nickname, the "flying proa". By flying the ama, the Wetted Surface , and therefore the Drag of the proa is significantly reduced. When combined with the long, narrow shape of the vaka, and the large amount of torque the crew can apply on the akas, this gives the proa its large potential speed. HISTORY OF THE PROA The proa was, into the . There has recently been a resurgence in interest in the proa in the Marshall Islands , one of the locations the craft were traditionally built. There is also a loose group of individuals from all over the world with an interest in the proa, both from a historical perspective and from a scientific and engineering perspective. Many of these individuals with interests in proas can be found in the Amateur Yacht Research Society . MODERN DEVELOPMENTS One of the first documented Western versions of the traditional proa was built in 1898 by Commodore Ralph M. Munroe of the Biscayne Bay Yacht Club. Over the following years he built several more. They were all destroyed by the mid-1930s, when a severe hurricane leveled Munroe's bayside boatshop. Munroe used a symmetric, slightly rockered flat bottomed hull, but otherwise followed the general layout of the Micronesian proa. Munroe had to interpret the widely distributed and incorrect plan drawing from about 1748 , made during Admiral Lord Anson 's circumnavigation of the globe in search of Spanish treasure ships. It was one of several either captured or seen under sail when Anson stopped at Tinian during a dreadful Pacific crossing. The key element that the draftsman (a Lt. Piercy Brett) misinterpreted was the mast, shown fixed vertically in the center of the boat. Real proa masts were raked end to end as the vessel shunted. Munroe, however, was a talented boat designer who was able to work around the problem of the incorrect mast setup (a raking mast helps with helm balance on a traditional proa) since he didn't know any better. Only 30 feet long, his proa was capable of speeds which Munroe estimated at 18 knots, and his article in The RUDDER describes what can only be Planing on the flat hull. As this was before the advent of planing power boats, this proa was likely the first ever boat capable of planing, and this was what gave it its amazing speed in the days when boats were limited by Hull Speed . For example, a 30 foot boat that was not capable of planing would be limited by a hull speed of about 7.3 knots; Munroe's proa could reach nearly 2.5 times that speed. This accomplishment was the nautical equivalent to the X-1 breaking the sound barrier. It is not clear that traditional proas of the Pacific islanders were ever capable of planing. Munroe was building a "cheap and dirty" Sharpie hull made of two 32-foot planks, a couple bulkheads and a crossplanked bottom, and by lucky accident may have been the first sailor to plane his boat. His first iteration had an iron center fin with a half-oval profile, later replaced by a double daggerboard which must have been easier to deal with in shallow Florida waters. Robert Barnwell Roosevelt, uncle of American president Theodore Roosevelt , also built a proa at about the same time. He used it sailing somewhere out of Long Island. It was significantly different but equally creative, and at 50 feet, much longer. As far as can be determined by his 1898 article in The RUDDER, the main hull was an open 4-foot-wide sharpie/scow hull; the ama was a smaller, fully decked scow which looks like it could rock on a single aka (crossbeam); the mast was a bipod arrangement with both masts stepped to windward. A settee-type sail was suspended from the apex and was free to weathercock, much like a balanced lug sail. A balanced rudder at each end managed itself by pivoting 180 degrees when its end was the "bow", and leeboards were used. Roosevelt's short article is accompanied by photographs showing his proa, called MARY & LAMB, at rest and under sail. It is not clear if the boat predated Munroe's proa. Small proas may have been brought back to the United States in the late 1800s, but documentation is sparse. These seem to be the first two attempts to adapt the proa to American building techniques. OTHER WESTERN INTERPRETATIONS In a non-traditional variant, first seen among Western yacht racers, the "Atlantic proa" has an ama is always to the lee side to provide buoyancy for stability, rather than ballast as in a traditional proa. Because the Atlantic ama is at least as long as the main hull, this style can be thought of as a catamaran with a fat hull and a skinny hull, that shunts rather than tacking. Other modern variants of the proa place the bulk of the passenger accommodations on the ama, in an attempt to make the vaka as streamlined as possible. The terms ama and aka have been adopted by the modern Trimaran . REFERENCES EXTERNAL LINKS
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