Hydrodynamic Resistance Assessment of Non-Monohedral Planing Hull Forms based on Savitsky’s Method

Planing hard chine hull forms are widely used for high speed small craft. While monohedral* deep V forms have been commonly used for more than 25 years the recent trends are toward non-monohedral forms in which the most of hydrodynamic lift is produced by low deadrise area in the after part of the hull. Then the deadrise gradually increases in the center and in the forebody allowing higher angles of incidence that are beneficial to reduce wave impact forces and ship motions. Also wetted surface is reduced in comparison to the standard monohedral form resulting in smaller frictional resistance. The origin of this trend is in the search of better seakeeping performances and has been possible through a general reduction in ship weight due to main engines higher power/weight ratio and lower structural weight. Resistance predictions based on Savitsky’s method are commonly used although this procedure had been developed on the hypothesis of monohedral geometry for the wetted part of the hull. The results are consequently affected by errors due to wrong assessment of both hydrodynamic lift and center of pressure. In this paper two procedures for the application of Savitsky’s method to non-monohedral hulls are proposed. The results relative to a recently developed non-monohedral hullform are compared to those ones obtained by standard Savitsky’s method and by towing tank tests. Furthermore a study for the analysis of error propagation in Savitsky’s long form procedure is proposed.