Computer model and data interpretation for a rubber cable tethered wave amplitude measuring buoy

Experimentation with rubber cable tethered buoys, originally used to reduce the watch circle of navigation buoys, has led to a unique wave measuring system. A tensiometer and two angle measurement transducers added to the anchor end of the rubber cable allows measurement of the cable tension and the angle of the cable with the vertical. For small fractional change in length, the cable exhibits a linear displacement-tension characteristic. The system thus allows an indirect measurement of the vertical buoy displacement. The two dimensional computer model consists of a coupled pair of non-linear differential equations formed by summing the component vertical and horizontal forces to the virtual mass times component accelerations. The equations are coupled through the tether force and are non-linear in the drag and buoyant force terms. The driving function for the model is a traveling sinusoidal wave which affects the buoyant force and the drag forces through the orbital water particle velocity. The equations are solved numerically and all component forces, displacements, velocities, and accelerations are available as outputs. The mathematical model of the system predicts that the buoy is a wave follower as long as the buoyant force is predominant over the tether and drag forces.