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Hull shapes on motor boats
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Which one's for you? BoatsExplained.com explains the differences.

ACK in the early nineteen hundreds, when auxiliary power was just starting to catch on for sailing yachts, designers worked on a simple rule of thumb: one horse-power per ton. A 35 footer would probably have a 6hp petrol motor, while a 45 footer might warrant a 12. For big yachts, the ratio was often even lower.

By comparison, a 30 foot family cruiser of the 1990s, weighing about 4 tons and with an 18 or 20hp diesel, seems grossly overpowered. To our grandfathers, the power to weight ratios of 100-200hp per ton which are commonplace amongst modern motor boats would have been quite outrageous - as well as technically impossible.
Throughout this century, hull design and engine developments have gone hand in hand: the reason we 'need' such powerful engines is because we have hulls capable of using them, and the reason we have the hulls is because the engines are available to drive them.
Back at the turn of the last century, pretty well all boats were what we would now regard as 'displacement' types. They sat in the water rather than on it, supported by the upward thrust of buoyancy. Builders and naval architects alike had come up with all sorts of theories about what constituted a fast or easily-driven hull, but the flavour of the month was the 'wave line theory'. This suggested that a graph of the areas of a boat's immersed cross sections should follow a smooth curve, starting from nothing at the bow, filling out to a maximum around the middle, and fading away to nothing at the stern. There was no hydrodynamic justification for the wave line theory: it's just a quasi-scientific description of the hull shape that happens to work reasonably well at typical sailing boat speeds. Nevertheless, it was a major influence on the design of early motor boats. Designers also realised that they had to keep wetted area to a rninimum in order to reduce friction, and that they needed to make their boats as narrow as possible in order to minimise the amount of energy that would be wasted in breaking waves.
Racing rowing boats are still built according to these principles; they are very slim, to reduce the size of the waves they produce, so they also have to be very long in order to support all the sweating and grunting muscle power that will drive them through the water, but their wetted surface area is kept to a minimum by making them almost semi-circular in cross section. Anyone who's Towed such boats will know that stability isn't exactly their strong point; without the oars acting like a tight-rope walker'sbalansing pole,they simply fall over. To overcome this. problem, the earliest racing motor boats were given very flat stems. It increased their wetted area, but at least it kept them upright.
 
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