Car Automotive

Archive for July 20th, 2009

The waterline length refers to the part of the kayak hull that meets the water’s surface. If the kayak has a long sloping bow and/or stern, its waterline length will be shorter than its overall length. Waterline length is a critical element in establishing the maximum speed of the kayak. The greater the waterline length, the greater its potential for a top-end speed.Kayaks are displacement hulls—that is, they move through the water rather than skim across its surface as a planing boat does. The top speed of a displacement hull is determined by the waves it creates. At maximum speed, the hull creates a wave at the bow and at the stern with a trough in the middle. Since a wave of a given distance between crests travels at a particular speed, the hull can only be as fast as the wave it creates, known as the wave of translation. Increases in speed are possible but only with dramatic increases in the energy applied to propelling the hull forward.Waterline beam: This refers to the maximum width of the hull area that meets the water’s surface. Since most kayaks have rounded or sloped sides, the waterline beam is usually less than the overall maximum beam. The waterline beam also has an impact on the efficiency in converting effort into speed. Narrower beamed boats have less distance to push the water aside and therefore create less resistance to forward motion.The beam also has an impact on stability: The wider the boat, the more stable it will feel when you’re sitting with the kayak on an even keel.Draft: The vertical distance between the surface of the water and the deepest part of the hull.Prismatic Coefficient: The prism refers to a shape that has a cross-section identical to the widest part of the immersed hull and a length equal to the kayak’s waterline length. The prismatic coefficient, or PC, is the ratio of the displacement of the kayak to the volume of the prism. A kayak that displaces 45% of the volume of the prism has a PC of 0.45. The PC is useful as a measurement of how fine or full the ends of the kayak are.A kayak with a high PC has more volume in its ends. This increases the wetted surface, creating more drag from friction between the hull and the water. This is a significant factor in the efficiency of the hull at low speeds. At high speeds, the full ends create a long wave of translation, so a kayak with a higher PC has a potential for a higher top speed (for the application of a given amount of effort).