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Full-Displacement

Plans to embark on long-distance voyages that take the vessel offshore for more than an occasional overnight passage make the parameters of design quite clear. Likewise, when planning for mostly coastal cruising with only limited passages, yet with the security of a true passagemaker, the same set of parameters will apply. One thing is certain: A true offshore passagemaker can make coastal cruising more comfortable and enjoyable, but a boat designed only for coastal cruising is wholly inadequate for long-distance voyaging.

Among other certainties are the seakeeping characteristics and efficiency of a full-displacement hull. A full-displacement hull is designed to remain fully in the water throughout its entire range of speed. It does not climb out of its bow wave and plane on the water's surface. Requiring a fraction of the horsepower that semi-displacement or planing hulls require to attain optimum speed, a full-displacement hull in the 40- to 120-foot size will be capable of maintaining speeds in the 7- to l2-knot range while burning a minimum of fuel.

Beyond these speeds, a boat in this size range must get up on plane. With entirely different seakeeping characteristics, a planing boat requires tremendous horsepower, which leads to enormous fuel consumption as well as a disproportionate increase in noise and vibration. As the noted designer William Garden once said, "A planing hull can't carry enough fuel to get out of sight." The laws of hydrodynamics clearly recommend the full-displacement hull for efficiency and seakindliness if long-range cruising plans are on the horizon.

 

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