Dynamic Transverse Stability for High Speed Craft

Abstract

Even in calm water, high-speed vessels can display unstable behaviors, such as chine-walking, sudden large heel, and porpoising. Large heel results from the loss of transverse stability due to high forward speed. When a planing craft begins to plane, the hydrodynamic lift forces raise the hull out of the water, reducing the underwater submergence. The available righting moment due to the hydrostatic buoyancy is, therefore, reduced. As the righting moment due to hydrostatic buoyancy is reduced, the righting moment due to hydrodynamic effects becomes important. These hydrodynamic righting effects are related to the hydrodynamic lift. This paper explores the relationship between the hydrostatic righting moment, the hydrodynamic righting moment, and the total roll restoring moment of a planing craft operating at planing speeds. A series of tow tests using a prismatic hull with a constant deadrise of $20^{\circ }$ measured the righting moment at various angles of heel and at various model velocities. The model was completely constrained in heave, pitch, sway, roll, yaw, and surge. The underwater volume is determined from the known hull configuration and underwater photography of the keel and chine wetted lengths. The results presented include the total righting moment with the hydrostatic and hydrodynamic contributions for various model speeds at two model displacements.