A Study on Ship Motion and AddedResisstance in Waves

Abstract

It has been said that the strip method gives good prediction of ship motions at any rate, although hydrodynamic forces consisting of the ship-motion equation are not correctly predicted. To understand a reason of this wonder and a way of improvement in existing ship-motion theories, numerical results by NSM(New Strip Method) and EUT(Enhanced Unified Theory)are compared with experiments newly conducted using a modified Wigley model of L/B=6.667. Although the added mass, damping coefficient, and wave-exciting force in heave predicted by EUT are in good agreement with experiments, the heave amplitude by EUT is overpredicted particularly near resonance. It is found that a reason of this is due to improper prediction in the cross-coupling forces between heave and pitch, despite the value itself being very small. EUT also tends to overpredict the pitch damping coefficient and underpredict slightly the pitch exciting moment in relatively longer wavelength region;which results in underprediction of the pitch amplitude. Using experimental values for the cross-coupling added-mass terms(A_<53> and A_<35>)and the pitch damping coefficient (B_<55>), the results of EUT agree well with experiments of not only ship motions but also the added resistance in waves.