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Kayak

The traditional kayak has a covered deck and one or more cockpits, each seating one paddler. The cockpit is sometimes covered by a spray deck that prevents the entry of water from waves or spray, differentiating the craft from a canoe. The spray deck makes it possible for suitably skilled kayakers to roll the kayak: that is, to capsize and right it without it filling with water or ejecting the paddler.
Native people made many types of boats for different purposes. The Aleut baidarka was made in double or triple cockpit designs, for hunting and transporting passengers or goods. An umiak is a large open sea canoe, ranging from 17 to 30 feet (5.2 to 9.1 m), made with seal skins and wood. It is considered a kayak although it was originally paddled with single-bladed paddles, and typically had more than one paddler.
Contemporary traditional-style kayaks trace their origins primarily to the native boats of Alaska, northern Canada, and Southwest Greenland. Wooden kayaks and fabric kayaks on wooden frames dominated the market up until the 1950s, when fiberglass boats were first introduced in the US, and inflatable rubberized fabric boats were first introduced in Europe. Rotomolded plastic kayaks first appeared in 1973, and most kayaks today are made from roto-molded polyethylene resins. The development of plastic and rubberized inflatable kayaks arguably initiated the development of freestyle kayaking as we see it today, since these boats could be made smaller, stronger and more resilient than fiberglass boats.
If the displacement of a kayak is not enough to support the passenger(s) and gear, it will sink. If the displacement is excessive, the kayak will float too high, catch the wind and waves uncomfortably, and handle poorly; it will probably also be bigger and heavier and than it needs to be. Being excessively big will create more drag, and the kayak will move more slowly and take more effort. Rolling is easier in lower-displacement kayaks. On the other hand, a higher deck will keep the paddler(s) dryer and make self-rescue and coming through surf easier. Many paddlers who use a sit-in kayak feel more secure in a kayak with a weight capacity substantially more than their own weight. Maximum volume in a sit-in kayak is helped by a wide hull with high sides. But paddling ease is helped by lower sides where the paddler sits and a narrower width.
Length alone does not fully predict a kayak's maneuverability: a second design element is rocker, i.e. its lengthwise curvature. A heavily rockered boat curves more, shortening its effective waterline. For example, an 18-foot (5.5 m) kayak with no rocker is in the water from end to end. In contrast, the bow and stern of a rockered boat are out of the water, shortening its lengthwise waterline to only 16 ft (4.9 m). Rocker is generally most evident at the ends, and in moderation improves handling. Similarly, although a rockered whitewater boat may only be a few feet shorter than a typical recreational kayak, its waterline is far shorter and its maneuverability far greater. When surfing, a heavily rockered boat is less likely to lock into the wave as the bow and stern are still above water. A boat with less rocker cuts into the wave and makes it harder to turn while surfing.
A chine typically increases secondary stability by effectively widening the beam of the boat when it heels (tips). A V-shaped hull tends to travel straight (track) well, but makes turning harder. V-shaped hulls also have the greatest secondary stability. Conversely, flat-bottomed hulls are easy to turn, but harder to direct in a constant direction. A round-bottomed boat has minimal wetter area, and thus minimizes drag; however, it may be so unstable that it will not remain upright when floating empty, and needs continual effort to keep it upright. In a skin-on-frame kayak, chine placement may be constrained by the need to avoid the bones of the pelvis.
A chine typically increases secondary stability by effectively widening the beam of the boat when it heels (tips). A V-shaped hull tends to travel straight (track) well, but makes turning harder. V-shaped hulls also have the greatest secondary stability. Conversely, flat-bottomed hulls are easy to turn, but harder to direct in a constant direction. A round-bottomed boat has minimal wetter area, and thus minimizes drag; however, it may be so unstable that it will not remain upright when floating empty, and needs continual effort to keep it upright. In a skin-on-frame kayak, chine placement may be constrained by the need to avoid the bones of the pelvis.
Strip-built designs are similar in shape to rigid fiberglass kayaks but are generally both lighter and tougher. Like their fiberglass counterparts the shape and size of the boat determines performance and optimal uses. The hull and deck are built with thin strips of lightweight wood, often cedar, pine or Redwood. The strips are edge-glued together around a form, stapled or clamped in place, and allowed to dry. Structural strength comes from a layer of fiberglass cloth and epoxy resin, layered inside and outside the hull. Strip–built kayaks are sold commercially by a few companies, priced USD 4,000 and up. An experienced woodworker can build one for about USD 400 in 200 hours, though the exact cost and time depend on the builder's skill, the materials and the size and design. As a second kayak project, or for the serious builder with some woodworking expertise, a strip–built boat can be an impressive piece of work. Kits with pre-cut and milled wood strips are commercially available.