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Trimaran

Single hull sail boats are conventionally equipped with a large, heavy keel extending downward from the bottom of the hull. A keel is required to counteract the tendency of a boat to roll (heel over) excessively about the longitudinal axis of the boat when it encounters heavy seas and/or stiff winds. The large weight of the heel, concentrated at a posiion well below the hull of the vessel, lowers the center of gravity of the vessel. This provides a static righting force counteracting the tendency of the vessel to roll.

Typically, the keel of a sailboat has a broad flat shape, as viewed from the side of the vessel. The large surface area of the keel provides a substantial resisting force to rapid movement through water in a direction perpendicular to the surface. Thus, the keel provides a dynamic resisting force counteracting the tendency of the sailboat to rapidly heel over.

Trimarans avoid the requirement for a large and heavy keel by using auxiliary hulls or floats, one on either side of the main hull of the sailboat. The auxiliary hulls or floats are usually attached to the main hull by means of connecting crossarms transversely disposed between the main hull and each float. Typically, the floats are of an elongated shape, and in parallel alignment with the main hull of the sailboat, at equal distance on either side of the main hull. The transverse cross-sectional shape of both the main hull and the floats are so configured as to provide a hydrodynamically streamlined body, which may move longitudinally on the surface of water with a minimum amount of drag.

Auxiliary hulls or floats eliminate the requirement for a large and heavy keel as follows. The upwardly directed buoyancy force on a float, multiplied times the relatively long moment arm comprised by the distance between the float and roll axis of the main hull, provides a large torque ending to counteract excessive roll of the main hull in either a clockwise or counterclockwise direction.

The distance between the outer lateral edges of the floats of a trimaran is referred to as the beam of the vessel. A large beam typical for a trimaran is advantageous for sailing purposes, as has been described. However, the large beam can be disadvantageous when the boat is docking in crowded areas, or being transported by trailer. Accordingly, it would be advantageous to provide means for varying the beam of a trimaran.

The trimaran achieves stability from two outrigger pontoons at each side of the main hull. The monohulls have the disadvantage of not only being heavier because of the weight of the ballast but has only one large hull that is harder to push through the water. The lighter un-ballasted trimaran with three narrow hulls has less resistance through the water. In addition, the trimaran has more stability and can carry more sail area per pound of displacement.

In general catamarans are much more popular and therefore more available than trimarans. Because of this trimarans are normally more expensive because production levels are not high enough to reduce cost through mass production and volume sales.

The inherent disadvantage of the trimaran is a beam roughly 2/3 the length. A 27' trimaran would therefore be 18' wide. Over the years numerous devices have been developed to reduce the beam for transporting the boat on a trailer or for marina berthing in a slip narrower then the fully-extended beam. The most common way to reduce the beam for trailering is to fold the outriggers down. Folding the outriggers down can be done in the water however; It is not suitable to leave the boat in the water for an extended time with the outriggers folded down. Yet another design involves swinging the outriggers aft and into the side of main hull to reduce the beam enough for marina storage. Unfortunately, swinging the outriggers aft can result in numerous disadvantages. In swinging the outriggers aft, the overall length increases which changes the fore and aft pitch of the boat, affecting stability.

Trimaran sailboats conventionally include one or more masts mounted on the main hull with shrouds often fixed at or near the rail of the side hulls. Such sailboats normally have very good sailing properties, particularly for ocean racing, since a great stiffness relative to weight is obtained with the three-hull configuration. A great initial stiffness gives a smaller inclination, and thus better comfort. Further, such a construction allows each hull to be designed without requiring that it remain stable, allowing use of narrow hulls with low resistance. The bottoms of the side hulls are conventionally positioned vertically above the main hull bottom so that when the sailboat is upright with the wind from the stern, or in light winds, the boat sails on the main hull alone, thereby providing low resistance. Since no ballast is necessary, the trimaran also can be made unsinkable.

While trimarans have many advantages, there is one significant drawback associated with conventional trimarans. A conventional trimaran reaches its greatest stability at 20 degree to 35 degree inclination, and at larger inclinations the stability steadily decreases down to zero at about 80 degree inclination. Thus, trimarans can capsize due to the impact of wind or waves, and are not self-righting, so that once capsized they remain upside down.

The use of trimaran hulls is relatively well known for sailing craft. However, such hulls have seldom, if ever, been used for power boats. In particular, such hulls have not been used for boats which have sufficient power to pull water-skiers at a high rate of speed across the surface of the water. Also, skier pulling power boats do not generally have sufficient space for other uses such as fishing. Still further, boats which are suitable for pulling water-skiers at high speed across the surface of the water do not generally have sufficient accommodations for several people to sleep on them. A single boat structure which would have all of the above advantages and which would be relatively lightweight and trailerable, would be highly desirable.

Sponsons have been incorporated as part of power boat hulls to reduce, somewhat, high speed drag by reducing (at high speed only) the amount of hull surface which contacts the water. Such hulls have, however, had too much drag for efficient sailing operation (the entire hull contacts the water at low or sailing speeds) and are relatively heavy and not conveniently shaped for carrying on a trailer.



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