Powering and seakeeping characteristics of a displacement hullform with waterline parabolization

Abstract

Waterline parabolization is a design procedure used for displacement vessels to decrease the wave resistance of the hullform through the addition of amidships bulbs. The bow and shoulder wave system of a parent hullform are interfered with by the wave system produced by the amidships bulb. Despite an overall increase in vessel beam, amidships bulbs can produce enough wave cancellations to decrease the total resistance. The designer must pay close attention to the amidships bulbs longitudinal positioning and fairing. Two design approaches can be taken: one the amidships bulbs are "retro- fit" to the existing parent hullform increasing the vessels displacement, and second the displacement is held constant producing an entirely new "optimized" design with shallower entrance and exit angles. Optimal shapes for the amidships bulbs were developed numerically using a potential flow code based on Dawson's method coupled with a quasi-Newton nonlinear programming algorithm, Calisal et al.(2009a). Tow-tank tests at Istanbul Technical University (ITU) confirmed that amidships bulbs could reduce the effective power by 15%. Given a significant improvement in powering, this paper compares the seakeeping performance of the parent, optimized, and retro-fit hullforms at different sea state conditions and quantifies fuel consumption and acceleration levels. SHIPMO PC, a ship motion program based on strip theory is used to compare the three different hullforms. Three speeds are considered: the design speed of 12.5 knots, a reduced speed of 11 knots associated with the expected loss of speed from added resistance, and 6 knots to represent significant speed reduction. Roll, pitch and heave motions along with added resistance are estimated. Accelerations at the bridge are used to evaluate effects on the crew. For various sea states the most significant motion is roll in beam seas and is incurred at low speeds. The only significant difference in response between all models was for the retro-fit design; the increased displacement from adding the amidships bulbs and holding the draught constant increased the added resistance. Powering and acceleration levels for all models in head seas will be verified in tow-tank tests at ITU. Copyright © 2010 by ASME.