A mathematical model has been tested in calm water and total resistance, trim and vertical forces measured. Measurements are compared with calculations made using a linearised theory and it is concluded that neglect of non-linearities contributes more to the poor agreement between calculation and measurement than the neglect of viscosity. A relation between vertical force, wave resistance and friction form resistance is proposed and it is suggested that there is a relation between trimming moment, wave resistance and viscous pressure resistance. In order to test these deductions, a large ram bulb was designed to modify the flow over the hull. Total resistance, wave pattern resistance, sinkage, trim and vertical forces were measured after the addition of the bulb. This second series of tests showed that a ram bow can markedly alter the viscous pressure resistance and running trim of a model
