Additionally, the size and shape of the centerportion 3 is a function of its ability to provide structural support for the wave forming sections 5, 7.In a slightly different version of the preferred embodiment 101, shown in FIGS. 4-6, the center portion 103 is provided with one or more keel fins or rudder-like members 18, extending downwardly from the center portion, to help stabilize thedevice ho