Tunnel flow of a planing trimaran and effects on resistance

Abstract

It is significant to clarify the tunnel flow mechanism and its effect in a wide speed range for the design of a planing trimaran with large loading capacity and long cruising radius. A numerical investigation of bare hull hydrodynamics of a planing trimaran, designed for waterjet propulsion, is conducted at 0.68 < Fr▽<6.16. The discussion is divided into two conditions of low & hump speed and planing speed, as tunnel effect varies with speed. At Fr▽<1.03, air cushion partly occurs in tunnel, and it is considerable to improve the deceleration and distortion of tunnel flow for resistance reduction. The added wetted area of concave tunnel and the strong wave interaction on tunnel accounts for the earlier hump speed and large hump resistance. Apart from tunnel lift generated by itself, its promotion effect on lift of main hull plays a major role in the rapid hull lifting and moderate resistance growth at 2.74 < Fr▽<5.34. Although the tunnel aerodynamic lift remains small, no more than 3% of total displacement, it helps to decrease resistance by providing bow-lifting moment to keep hull trim unchanged and thus reduce main hull wetted length at 5.34 < Fr▽<6.16.