Experimental evaluation of a new stern shape

Article type: Research Article

Authors: Tommasi, Giovanni B.

Abstract: The paper describes a new hull form, intended to improve the efficiency of ships propellers by so shaping the afterbody that it forms a stator. Model tests were carried out to compare a hull having the new afterbody form with a conventional twin-screw hull which had the same forebody, an identical midship section, and an afterbody whose curve of immersed transverse sectional areas was very close to that of the new form. These tests showed that in the design speed range a maximum saving on the propulsive power of 24% could be made by adopting on both models twin three-bladed screws of the same form, and of 26% with the same two-bladed propellers, leaving the way open for a further improvement towards a 30% saving. Several factors contribute to the overall saving, each having a comparable degree of influence. All the factors affecting propulsive power, namely the towing resistance, the thrust efficiency, the wake coefficient, the counter-propeller effect and the relative rotative efficiency of the propeller, play their part. The benefit from the improved rotative efficiency is connected with a marked reduction in the mechanical vibrations imposed on the hull by the working conditions of the propeller blades, which makes it possible to adopt two-bladed screws, and so to benefit from their better free water efficiency.

DOI: 10.3233/ISP-1976-2325801

Journal: International Shipbuilding Progress, vol. 23, no. 258, pp. 31-51, 1976