A trim tab for mounting the transom of a boat has an upper surface and an undulating bottom surface with down-turned flaps on opposite sides of the tab. The trim tab is formed of a molded composite material. The top surface includes a hinge platform at its forward end and mount platform at a rearward end. The trim tab tapers from a minimum thickness at its leading end to a maximum thickness at its trailing end. The shape of the bottom surface and the down-turned flaps provide a bottom surface area with enhanced lift characteristics.