Interceptor and trim tab combination to prevent interceptor's unfit effects

Abstract

Trim tabs, small surfaces connected to the end of the craft to control the trim by adjusting the angle of tab, relative to the larger surface, have been used to optimize the running trim of displacement, semi-planning, and planning vessels for many years. Interceptors are the same as trim tabs, but are vertically installed at the end of the craft to control the trim by changing the height. As demonstrated in this paper, the same size as the interceptor and trim tab (when span and chord of trim tab are respectively equal to span and height of interceptor), the interceptor shows better efficiency (better trim and resistance reduction). While efficiency of trim tab just depends on the trim tab angle, the effective factors on the interceptor effectiveness are little complicated. The interceptor efficiency highly depends on the interceptor height and boundary layer thickness at transom. Although the higher interceptor increases the amount of lift force, but that could create a very strong moment against trim moment and consequently negative trim angle. The results of this investigation prove that the combination of an interceptor with a trim tab shows better performance compared to an interceptor or a trim tab. Also instead of increasing the interceptor height to gain more lift, which could make intense negative trim, it is better to use integrated interceptor with trim tab. To do so, a comprehensive series of dynamic CFD simulations have been performed in the case of a simple planning boat model with three different trim control appendages. Unsteady Reynolds Average Navier-Stokes equations (URANS) are applied to model the flow around the considered model with interceptor, trim tab and combination of an interceptor and a trim tab at an equal span length. The model is analyzed based on finite volume method and SIMPLE algorithm using dynamic meshes in the Fluent computational code. For validation of the CFD results, the Savitsky planning boat calculations (only for model boat) and the grid convergence index (GCI) were used to estimate the uncertainties due to grid-spacing and time-step.