Hulls for ships are, viewed from the shape of the hull under the water line, generally classified into three categories: a displacement type hull for a low speed range, a semi-planing type hull for a medium speed range and a planing type hull for a high speed range.
The displacement type hull is suitable for a low speed travelling. The ship of this type is travelled primarily under a hydrostatic pressure with a hull bottom submerged relatively deep under a water line. In an attempt to reduce frictional resistance and wave making resistance the hull bottom often is given a streamline shape, and a lateral crosssectional area under the water line as measured on a still water is maximal at the middle of the hull and is gradually decreased from midship toward the end of the stern. The hull bottom is curved upward from the middle of the hull toward the bow and stern, respectively. When the displacement type hull is travelled at high speeds, it is subjected to a dynamic water pressure tending to lift the front half portion of the hull upwardly, while at the same time it is subjected to a dynamic water pressure tending to pull the rear half portion of the hull downwardly. The hull, therefore, has a greater attack angle with the bow lifted upwardly and the stern pulled downwardly, increasing water resistance to the hull and making it very difficult to effect high speed travelling. To avoid such a situation attempt is made to, for example, move crew-members toward the bow of the ship, thereby shifting the center of gravity of the ship. This, however, provides no essential settlement to this problem.
The planing type hull is suitable for high speed travelling. The hull of this type has, in an attempt to provide a suitable attack angle during the high speed travelling period to the hull and effectively support a hull bottom by a dynamic water pressure, a substantially planar, wide planing surface at the bottom and a substantially vertical planar transom at the rear end of the stern. The lateral cross-sectional area under the water line of the hull is maintained substantially constant at the rear half portion of the hull. When the ship of this type is travelled at low speeds, water flowing along each side wall of the hull is turned inward behind the transom, creating an eddy current. The eddy current imparts a resistance tending pull back the hull rearwardly. For this reason, the planing type hull is subjected to a relatively large travelling resistance during the low speed travelling period, as compared with the displacement type hull, resulting in a prominently greater propulsion loss.
The semi-planing type hull is suitable for a medium speed range, i.e., a speed range intermediate between the displacement type hull and the planing type hull. The lateral cross-sectional area under the water line of the hull is slightly decreased from midship to the rear end of a stern where it shows a value intermediate between the displacement type hull and the planing type hull. In other words, the semi-planing type hull shows a poor performance in the low speed range as compared with the displacement type hull and in the high speed range as compared with the planing type hull.
As will be evident, an optimum designing speed range is determined dependent upon the type of hulls. In the case of the displacement type hull, a speed/length ratio (speed/.sqroot.water line length) is restricted to a range of below 1.5 kn/ft1/2; in the case of the semi-planing type hull, a range of 1.5 to 3.5 kn/ft1/2; and in the case of the planing type full, a range of above 2.5 km/ft1/2. If, therefore, the speed-length ratio is off the optimal range, a poor performance results.
The planing hull does not yield the solution to designing large fast ships. However, if the speed categories in relation to waterline length are examined, the semi-planing hull appears to offer attractive opportunities for fast sealift ships over a continuum of sizes of semi-planing hulls, small to very large. The monohull fast sealift (MFS) hull or semi-planing monohull (SPMH) design is the hull form which is widely used today in smaller semi-planing ships because it offers the possibility of using waterline lengths approaching that of displacement hulls and maximum speeds approaching that of planing hulls.
An advantage of a waterjet propulsion system in the semi-planing hull is its ability to deliver large amounts of power at high propulsive efficiency at speeds of over 30 knots and yet decelerate the ship to a stop very quickly. The system also largely eliminates the major problems of propeller vibration, noise and cavitation. A principal advantage of the integrated MFS hull or SPMH and waterjet system is that the shape and lift characteristics of the hull are ideal for the intakes and propulsive efficiency of the waterjet system, while the accelerated flow at the intakes also produces higher pressure and greater lift to reduce drag on the hull even further.
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