In order to safely design submarines and other underwater vehicles, it is critically important to understand and be able to predict the hydrodynamics forces and moments they will experience in operation. Most methods of predictnig the hydrodynamic forces during maneuvering assume the flow to be quasi-steady. This assumption neglects the motion history effect of the fluid. Using computational fluid dynamics simulations, it is demosntrated that this effect can be significant to a bare submarine hull undergoing sway manevuers at realistic speeds. A quasi-steady model is shown to have relative error of up to 36% for the lateral force and yawing moment. Further, it is demosntrated that the motion history effect on both the lateral force and the yawing moment can be scaled by the hull geometry, and are independent of the hull profile's shape. An unsteady force estimation model is formulated, based on indicial function theory, and is shown to give improved predictions for the unsteady force and moment.

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