SINKAGE, TRIM, DRAG OF A COMMON FREELY FLOATING MONOHULL SHIP

A practical method well suited for early ship design and hull form optimization for estimating the sinkage, the trim and the drag of a freely-floating common monohull ship at moderate Froude numbers F <= 0.45 is considered. The sinkage and the trim are realistically estimated via two alternative simple methods: an experimental approach based on an analysis of experimental measurements (involving no flow computations), and a numerical approach based on a practical linear potential-flow theory (the Neumann-Michell theory) that only requires simple flow computations for the hull surface Sigma(H)(0) of the ship at rest. The drag is also estimated in a simple way, based on the classical Froude decomposition into viscous and wave components: well-known semi empirical expressions for the friction drag, the viscous drag and the drag due to hull roughness are used, and the wave drag is evaluated via the Neumann-Michell theory. The drag is more sensitive to the hull position than the sinkage and the trim. Accordingly, it must be computed for a 'dynamic' ship hull surface Sigma(H)(st) that accounts for sinkage and trim effects, although the hull surface Sigma(H)(st) does not need to be very precise. In fact, the total drag computed for the hull surface Sigma(H)(st) chosen as the hull surface qi predicted by the numerical approach, or as the hull surface Sigma(H)(alpha) predicted by the even simpler experimental approach, are nearly identical. Moreover, the drag of the hull surface Sigma(H)(1) and the (nearly identical) drag of the hull surface Sigma(H)(alpha) are significantly higher, and also in much better agreement with experimental measurements, than the drag of the hull surface Sigma(H)(0) of the ship at rest at high Froude numbers.