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An experimental investigation for resistance reduction on displacement-type ships by parabolization of hull form at waterline Tan, Beng-Yeow

Abstract

Hull optimization for minimum total resistance exhibits great interests among ship hydrodynamic research community for the attractive benefits it brings along in terms of fuel consumption, power requirement, payload capacity, cruising speed, traveling range, endurance and operational cost effectiveness. Even minute improvement can translate into significant savings throughout the design life of the vessel. Ship total hydrodynamic resistance mainly consists of frictional resistance, form resistance and wave resistance. Considerable improvement in frictional resistance is less likely to be attained through hull form modification. With form resistance being a fraction of frictional resistance and that wave resistance dominates as speed is increased, it becomes clear to turn toward minimizing wave resistance for hull form optimization studies. Hull parabolization was originally tested successfully on a tanker. The present investigation was initiated as a feasibility study to expand the idea of hull form parabolization for wave resistance reduction on to a small craft. The objective is to attain beneficial wave resistance reduction over moderate to relatively higher operating speed range between 0.30 ≤ Fn[sub L] ≤ 0.40. The parabolization was done by continuously extending the hull form at waterline and replacing the conventional parallel middle body section with parabolic side bulbs that function as 3-D wave maker. The bulbs modify the pressure field in the vicinity and generate stronger shoulder wave system to interact with bow and stern wave systems. Beneficial reduction in wave resistance, thus total resistance or effective horsepower, is practically attainable through the wave interactions upon cautious hull modifications. The side bulbs also increase payload capacity and improve ship stability. A parent hull was selected and modified with increased beam gradually up to 20% using various add-on side bulbs. The model was built at a scale of 13.75:1. Systematical experimental investigations were conducted in towing tank to understand the effects of beam increment, influences of longitudinal location of maximum beam and fairing extensions on hull form factor as well as resistance characteristics. Wave resistances were calculated upon experimental determination of form factors using Hughes-Prohaska's method. Additionally, direct wave resistance values were calculated by applying Sharma's longitudinal wave cut method on a rectangular patch of wave pattern. The wave elevations in the patch were acquired during experiment by a laser-camera wave pattern profiling system. Limitations of the system were addressed. The direct wave resistance values calculated from experiments were used to support the theoretical wave resistance predicted using Michell's integral from thin ship theory. Qualitative and quantitative comparisons between experimental and theoretical results were made. Upon studying using the add-on bulbs, an optimized hull form with built-in parabolization is conceived and built. The resistance-reduction capability of this recommended parabolized hull was substantiated by experiments. Over the targeted speed range, the model scale results have shown the consistent 25% reduction in wave resistance leads to the encouraging 15% reduction in total resistance (R[sub TM]) and 38% improvement in stability (GM[sub M]).

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