The effect of spray rails, chine strips and V-shaped spray interceptors on the performance of low planing high-speed craft in calm water

Abstract

The design of spray rails has been under-reported in recent decades; besides general guidelines, few studies focus on the design of spray rails on a planing craft. The influence of the breadth and the bottom angle of the spray rails on the performance of the craft is not clear yet. Although known to naval architects, few studies of chine strips have been conducted. V-shaped spray interceptors (VSI) are a novel concept of reducing the resistance of a planing craft by deflecting the spray forward of the stagnation line. Although the VSIs have been shown to reduce the resistance of a planing craft at Fr∇ > 3.65, it is not known whether they yield the same benefits at lower planing speeds. This study presents a series of model tests of a low planing craft (Fr∇ = 1.742–3.261) equipped with different spray rails, chine strips, and a V-shaped spray interceptor (VSI). At planing speeds Fr∇ > 3, the spray rails reduced the resistance by up to 2.4%, while at semi-displacement speeds Fr∇ < 3, spray rails did not improve the performance. At Fr∇ > 3, none of the spray rails changed the trim (± 0.05°) significantly. Narrow spray rails with sharp bottom angles provided equal or better performance compared with conventional wide horizontal spray rails. Unnecessarily long spray rails increase the resistance, negating the benefits of spray deflection. Chine strips had a significant influence on the total resistance, trim and sinkage of both the bare hull and the hull equipped with spray rails. At the planing speed, chine strips reduced the running trim by up to 0.3° and the total resistance by up to 3%. However, at semi-displacement speeds, the effect of the chine strips was proportionally opposite. When the VSI was located forward of the stagnation line, it deflected the oncoming spray, thereby inducing a reaction force on its deflection surface in the direction of the craft's motion, hence reducing the total resistance by up to 4.2%. When the VSI was behind the stagnation line, it increased the total resistance by up to 8.4%.