A note on structural damping of ship hulls

Abstract

Existing information on the structural damping of ships is far from satisfactory. It cannot be calculated and it can only be measured in the presence of hydrodynamic damping, whose nature and magnitude are also somewhat obscure. Yet it is very important. Symmetric responses to wave excitation can be estimated on the basis of existing hydrodynamic theories, with use of rough estimates of hull damping; our limited knowledge of structural damping is then only likely to be a handicap with heavy slender ships and/or fast ships. Much less is known about antisymmetric responses to waves, either as regards the means of estimating them or the appropriate levels of hull damping. Vibration at higher frequencies, due to excitation by machinery (notably propellers), is limited by structural damping to a much greater extent that it is by the fluid actions of the sea. Damping measurements at these frequencies therefore give more accurate estimates of hull damping. Even so, the estimation of ship vibration responses to excitation by machinery remains a matter of considerable difficulty.