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Hull speed or displacement speed is the speed at which the wavelength of a vessel's bow wave is equal to the waterline length of the vessel. As boat speed increases from rest, the wavelength of the bow wave increases, and usually its crest-to-trough dimension (height) increases as well. When hull speed is exceeded, a vessel in displacement mode ...
A fishing trawler, for example, always has a displacement hull for load-carrying capacity. Recreational trawlers, on the other hand, are as likely to have a semi-displacement hull. However, with the rising cost of fuel and the lower fuel consumption (though also lower speed) offered by displacement hulls, they are gaining popularity among some ...
The deep V form (between 18 and 23 degrees) is only suited to high-powered planing boats. They require more powerful engines to lift the boat onto the plane but give a faster, smoother ride in waves. Displacement chined hulls have more wetted surface area, hence more drag, than an equivalent round-hull form, for any given displacement.
Graph of power versus speed for a displacement hull, with a mark at a speed–length ratio of 1.34. For small displacement hulls, such as sailboats or rowboats, wave-making resistance is the major source of the marine vessel drag. A salient property of water waves is dispersiveness; i.e., the greater the wavelength, the faster it moves.
Planing sailing boats need a good sail area and powerboats need a high-power engine. Steps and chine ridges may also be incorporated into the design to encourage both ease of planing and stability. Most surfboards, although unpowered, are planing or semi-planing hulls. They utilize the push of the waveform more or less in combination with ...
A typical semi-displacement yacht has a wedge-shaped bow, which promotes penetrating waves, that transitions to flatter, wider surfaces aft, which promotes lifting the vessel out of the water—the "deep vee" hull, designed by Ray Hunt, found in approximately 75% of modern power boats.
Severns are constructed of fibre reinforced composite material, and their hard chine semi-displacement hull is built so that it will stay afloat with two of its five compartments flooded. For added manoeuvrability, in addition to twin engines, the Severn also has a bow thruster fitted. [ 6 ]
The result was a 22.5 metres (74 ft) semi-displacement hull displacing 22 tons, which was wider at the front than the rear, creating an almost perfect high-speed hydrodynamic shape. Having perfected the hull, Lürssen equipped the boat with three Maybach VL II V12 engines of 370 kW (500 hp) each.