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A Proa is a multihull vessel consisting of two (usually) unequal parallel hulls, superficially similar to an outrigger canoe. Found in many configurations and forms, it was developed as a sailing vessel in Micronesia (Pacific Ocean), and forms of it may be found as far as Madagascar and Sri Lanka. The word proa comes from perahu, the word for "boat" in Malay, which is similar to the Micronesian language group. *

Proa characteristics

The defining feature of a proa is that the vessel switches directions when it changes tacks (stern becomes the bow and vice versa), always keeping the same hull to windward for ballast. This operation is referred to as "shunting". The main hull, or vaka, is usually longer than the windward hull, or ama. Crossbeams called akas connect the vaka to the ama. Traditional proa hulls are aggressively asymmetrical along their length, and often curved in such a way as to produce lift to counteract the lateral forces of the wind. Modern proa hulls are often symmetrical, and will use leeboards for lateral resistance.

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, wich can be up to 100 feet (30 m) long. There is also a model proa, called a riwuit, 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 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, 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 darted like a shooting star
Lord Byron, The Island, 1823

The history of the proa is not recorded until it was first encountered by European explorers when they first explored the Micronesian islands. It is fairly certain that the proa evolved from the dugout canoe, one of the oldest watercraft and found in primative cultures across the world. The design of the proa hints at its evolution from a canoe into the world's fastest sailboat, a title it likely held for many centuries.

Adding a sail to a narrow hull like a canoe is a dangerous proposition, especially given the lack of dense materials like lead that can be used in a ballasted keel to counter the heeling moment of the sail. Attaching two dugout canoes together to form a catamaran hull will provide stability, but this is an expensive operation, as building a dugout hull is a long and labor intensive process, using nothing but fire and stone tools. The proa's simple outrigger can be produced with far less effort, and provides the needed stability to counter the force of a large sail.

The rigging of the proa also shows a high degree of elegance. By keeping the wind always to one side of the boat, the forces acting on the sail, mast, rigging and akas is always in the same direction. Where a tacking boat must have stays on both sides of the mast, with only one set under tension at a time, the layout of the proa requires stays on only one side, where they are under tension on all points of sail. Having the ama to the windward side also allows the use of materials like bamboo for the akas--the akas only need to be able to bear the weight of the ama, which is countered by the tension on the stays. Leeward akas, on the other hand, would need to bear the displacent of the ama, and cannot by assisted by tensioned rope.

There has recently been a resurgence in interest in the proa in the Marshall Islands, one of the locations the craft were traditionally built. Annual kor-kor races are held in the lagoon at Majuro, along with other events such as a children's riwut race. The kor-kors are built in traditional style out of traditional materials, though the sails are made with modern materials (often inexpensive polyethylene tarpaulins, commonly known as polytarp).

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.

Early American proas

Sailing is no name for it - flying is better. Out into the bay she skipped, boys yelling with delight on the uplifted outrigger, spray from the lee bow and steering oar riven into vapor by the speed blowing to leeward.
"A Flying Proa" R. M. Munroe, The Rudder, June 1898

There was a surge of interest in the proa in Europe and America in the late 1800s and early 1900s, which can be seen in the work of western builders like R. M. Munroe and Robert Barnwell Roosevelt (Theodore Roosevelt's uncle). The proa was, into the 20th century, one of the fastest sailing craft in existence. Indeed, the proa still forms the basis for the design of many boats involved in speed sailing.

Although there are mentions of proas in Western publications in the mid 1800s, 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, but at least two of his designs were documented in articles in The Rudder, as was one by R. B. Roosevelt. 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 builders to attempt to adapt the proa to Western building techniques.

Munroe's 1898 proa

Since Munroe had no direct experience with proas, all he had to work with was 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. The proa in the drawing was one of several either captured or seen under sail when Anson stopped at Tinian during a Pacific crossing. One key element that the draftsman (a Lt. Piercy Brett) misinterpreted was the mast, shown fixed vertically in the center of the boat. Traditional proa masts were raked end to end as the vessel shunted. A raking mast helps with helm balance by moving the center of effort of the sail fore and aft.

Munroe, however, was a talented boat designer who was able to work around the problems with the drawings, and his adaptations can be seen in successive proas. Rather than the deep, asymmetric hull of a traditional proa, Munroe's hulls were flat bottomed, with keels or centerboards for lateral resistance. His first iteration had an iron center fin with a half-oval profile. Rather than the traditional crab-claw sail's spars which meet at the front, Munroe's sails used what could be described as a triangular lugsail or spritsail with a boom, similar to the modern lateen sail with a shorter upper spar.

Munroe's first proa was only 29 feet long, yet was capable of speeds which Munroe estimated at 18 knots. 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 one of the first boats capable of planing, which helped give it its amazing speed in the days when most boats were limited by hull speed. For example, a 30 foot boat that was not capable of planing would have 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, though the long, slender hull would have a much higher speed/length ratio than other contemporary designs. 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.

Roosevelt's Mary & Lamb

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 of Roosevelt's proa was an open 4-foot-wide scow hull; the ama was a smaller, fully decked scow which looks like it could rock on a single aka. The mast was a bipod arrangement with both masts stepped to windward, with a boomed, balanced lugsail suspended from the apex. 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 1898 proa.

Munroe's 1900 Proa

Since Munroe wasn't aware of the raking mast, his 1900 model used two daggerboards set fore and aft of the mast, which would allow adjustment of the center of lateral resistance to provide helm balance. From the drawings, it appears the mast is higher as well, allowing a larger sail. The sail design also changed, with the upper spar now being slightly longer than the upper edge of the sail, and projecting past the apex slightly to allow the apex to be attached to the hull. The sail was loose footed, with the boom attached to the upper spar near the sail apex, and to the clew of the sail. His article in a 1900 issue of The Rudder included more details on the construction of his second proa. A 1948 book of sailboat plans published by The Rudder includes the following specifications for the 1900 proa:

  • Length overall 30 ft.
  • Beam (of main hull) 2 ft. 6 in.
  • Draft of hull about 5 in.
  • Draft with boards down 2 ft. 5 in.
  • Sail area 240 sq. ft.

From the drawings, the distance from the center of the main hull to the center of the aka is about 12 feet.

Other Western Interpretations

Western designers often feel the need to tinker with the proa. They are attracted by the minimalist nature and amazing speeds that proas are capable of (they may still be the fastest sailboats per dollar spent for the home builder) but they often want the proa to do more; adding cabins, different sailing rigs, and bidirectional rudders are common changes made.

Lee pods

The terms ama and aka have been adopted by the modern trimaran. Since trimarans are generally designed to sail with one ama out of the water, the are similar to an Atlantic proa, with the bouyant leeward ama providing the bulk of the stability for the long, relatively thin main hull. Some modern proa designers have returned the favor, and borrowed trimaran design elements for use in proas. Trimarans often have main hulls that are very narrow at the waterline, that then flare out and extend over a significant portion of the akas. This topheavy design is only practical in a multihull, and it has been adapted by some proa designers. One notable example are the designs of Russell Brown, a boating fittings maker who designed and built his first proa, Jzero, in the mid 1970s. He has created a number of proa designs, all of which follow the same theme.

One of the design elements that Brown used, and a number of other designers have copied, is the lee pod. The akas extend past the main hull and out to the lee side, and provide support for a cabin extending to the lee of the main hull. This is similar to the platform extending to the lee on some Micronesian proas. The lee pod serves two purposes--it can be used for bunk space or storage, and it provides additional boyancy on the lee side to prevent a capsize should the boat heel too far. Crew can also be moved onto the lee pod to provide additional heeling force in light winds, allowing the ama to lift under circumstances when it would not otherwise. The Jzero also used water ballast in the ama to allow the righting moment to be significantly increased if needed. While Brown's proa was designed to be a cruising yacht, not a speed sailing boat, the 36 foot Jzero is capable of speeds of up to 21 knots.

Sail rigs

One of the issues Western designers have with the proa is the need to manipulate the sail when shunting. Even Munroe's early sails discarded the curved yards of the traditional crabclaw for the more familiar straight yards of the lateen and lug sails. Munroe's designs likely lacked the tilting mast because he was unaware of it, but many designers since have use a fixed mast, and provided some other way of adjusting the center of effort--which should lead the center of lateral resistance, which in a proa is usually right in the center. The Jzero, for example, used a sloop rig and ran up a jib on whichever end was the current "bow". Other designs use two masts, or a rotationg jib/main combination on a large boom.

One of the more practical rigs was invented by Euell Gibbons around 1950 for a small, single handed proa. This rig was a loose footed lateen sail hung from a centered mast. The sail was symmetric across the yard, and to shunt, what was previously the top end of the yard was lowered and became the bottom end, reversing the direction of the sail. Proa enthusiast Gary Dierking modified this design further, using a curved yard and a sprit perpenduclar to the yard. This allows a greater control of the sail shape than the traditional Gibbons rig, while retaining the simple shunting method, and is often referred to as the Gibbons/Dierking rig.

Foils

While a proa is fairly efficient at minimizing the amount of wave drag and maximizing stability, there is at least one way to go even further. The use of underwater foils to provide lift or downforce has been a popular idea recently in cutting-edge yacht building, and the proa is not immune to this influence.

The Bruce foil is a foil that provides a lateral resistance with zero heeling moment by placing the foil to the windward side, angled so the direction of the force passes through the center of effort of the sail. Since proas already have an outriger to the windward side, a simple angled foil mounted on the ama becomes a Bruce foil, making the already stable proa even more stable. Bruce foils are often combined with inclined rigs, which results in a total cancellation of heeling forces. Inclinced rigs are also well suited to the proa, as the direction of incline remains constant during shunting.

Another use of foils is to provide lift, turning the boat into a hydrofoil. Hydrofoils require signficant speeds to work, but once the hull is lifted out of the water, the drag is signficantly reduced. Many speed sailing designs have been based on a proa type configuration equipped with lifting foils.

Variations on the theme

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, to reduce wave drag, this style can also be thought of as an asymmetric catamaran, that shunts rather than tacking. The first Atlantic proa was the Cheers, designed in 1968 by boat designer Dick Newick for the 1968 OSTAR solo translatlanic race, in which it placed third. Newkirk's designs are primarily trimarans, and the Atlantic proa's buoyant outrigger follows naturally from a conversion of a trimaran from a tacking to a shunting vessel.

Other proa designers blur the lines between Atlantic and Pacific style proas. The Harryproa from New Zealand uses a long, thin hull to lee, and a short, fat hull, containing the cabin, to windward. This would normally be more like an Atlantic proa, but the rig is on the lee hull, leaving it technically a Pacific design. This and other similarr proas place the bulk of the passenger accommodations on the ama, in an attempt to make the vaka as streamlined as possible, and put much of the mass in the lee side to provide a greater righting moment.

Perhaps the most exreme variants of the proa are the ones designed for pure speed. These often completely discard symmetry, and are designed to only sail well, sometimes at all, one direction realtive to the wind. These "one way" proas, such as world record speed holding The Yellow Pages Endeavor, or YPE. While the YPE is often called a trimaran, it would be more correct to call it a Pacific proa, because two of the planing/hydrofoil hulls are in line. This design has been considered by others as well, such as the Monomaran designs by "The 40 knot sailboat" author Bernard Smith, and has been called a 3-point proa by some, a reference to the 3 point hulls used in hydroplanes. A previous record holding design, the Crossbow, was a proa/trimaran hybrid. Eqippped with sliding akas, the windward ama would be extended out to the full extent of the akas, with the lee aka tucked up in against the hull, turning it into a one-way Pacific proa layout under sail.

References

External links

Multihulls

Proa

 

This article is licensed under the GNU Free Documentation License. It uses material from the "Proa".

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