Two practical approaches — based on an analysis of experimental data given in the literature or simple flow computations — for estimating the sinkage and trim of a ship that advances at constant speed in calm water are considered. The ‘experimental approach’, based on measurements for 22 ship models, requires no flow computations and yields particularly simple relations. The ‘numerical approach’ involves flow computations based on the Neumann-Michell theory (a practical linear potential-flow method) for the ship hull in equilibrium position at rest; i.e., sinkage and trim effects on the position of the ship hull are ignored in these flow computations. Both approaches are found to yield reasonable predictions of sinkage and trim for a wide range of ships at Froude numbers F ≤ 0.45.
The pressure distribution around a ship hull, with mean wetted surface ΣH, that advances at a constant speed V in calm water evidently differs from the hydrostatic pressure distribution around the wetted hull surface ΣH0 of the ship at rest, i.e. at zero speed V = 0. As a result, the ship experiences a hydrodynamic lift and pitch moment, and a related vertical displacement and rotation that are commonly called sinkage and trim, as well known.
The viscous and wave drags determined from flow computations around the actual mean wetted ship hull surface surface ΣH or the related hull surface surface ΣH0 can differ significantly. Practical methods, notably methods that do not require iterative flow computations for several hull positions, to predict the sinkage and trim experienced by a ship hull are then of practical importance.
