A numerical approach to resistance reduction of displacement vessels using parabolic waterlines

Abstract

The addition of parabolic side bulbs at the ship's midbody can significantly reduce the wave-making resistance of a vessel. The parabolic bulbs are strategically placed to create a wave pattern that interacts with the shoulder wave system of the base hull at the desired speed range. This concept was first successfully tested on a coaster tanker and then extended to the UBC series hull, a series typical of Canadian West Coast fishing vessels. Systematic tow tank experiments revealed that while parabolization decreases the total resistance (due to a drop in the wave making resistance) the form factor suffered an increase. An integral boundary method solver and a two-dimensional RANS solver both showed that the increase in viscous resistance was mostly due to an increase in viscous pressure drag. The parabolization concept was subsequently extended to a high-speed NPL trimaran, to determine whether resistance reduction using parabolic side bulbs could be achieved for a slender multihull vessel. A Rankine source panel method was used to predict the wave-making characteristics of the trimaran, and an Integral Boundary Layer solver and a RANS solver were used to calculate the viscous drag. A parametric study, varying the size and location of bulbs, was first performed on the centre hull to identify beneficial bulb arrangements. The study was then extended to the trimaran to evaluate the additional wave interactions caused by the outriggers. Experimental work validated the numerically predicted wave interactions, as well as the change in viscous drag. Based on the numerical work, a modified NPL trimaran hull form was designed that reduced the total resistance of the vessel by up to 6% in the design speed range.