The Free Decay of Coupled Heave and Pitch Motions of a Model Frigate

Abstract

A ship motions program has been found to over predict damping especially at resonant frequencies, resulting in there being no significant peak in the transfer function of pitch at resonance. Presented here is a series of experiments that measured the decay time histories of a model frigate free to move in the vertical plane, in an attempt to identify where the problems might be occurring. This paper provides a comparison of the heave and pitch motion decay time histories together with the analytical decay time histories synthesised using the hydrodynamic coefficients predicted by the PAT suite, a suite of ship motion computer programs. The authors make judgements concerning the components in the heave pitch motion using the terms in the full two dimensional strip theory heave and pitch motion equations to explain the trends shown. The investigation has shown that the sectional damping coefficient appears to be over predicted. This results in a relatively poor prediction of heave decay curves at low speed and over estimates the damping in the pitch decay curves. The heaving motion of a ship is dominated at low frequencies by the stiffness terms. The amount of damping at these frequencies are negligible and any errors in the damping terms has little effect on the transfer functions. The actual values of heave damping at resonance appears to be correct, but because of the nature of the coupled system of equations the heave sectional damping has a magnified effect in the pitch equation. however, the small errors identified in the time history comparisons do not appear large enough to account for the differences seen in t he transfer functions. Therefore, it appears that the discrepancies seen in the heave pitch transfer functions could be die to the wave excitation being predicted incorrectly. The paper also illustrates the need to consider only small amplitude motions when carrying out experiments of a linear nature.