Added mass and damping on a vertical, surface-piercing, circular cylinder or column with a submerged sharp-cornered circular footing, oscillating in water of finite and infinite depths are measured using a planar motion mechanism. The model is made of aluminum, and the outer diameters of the column and footing are 26.67 cm and 60.96 cm, and their lengths are 90.17 cm and 22.86 cm, respectively. The bodies were forced to oscillate sinusoidally with small amplitudes, for several submergences below a free surface. The added-mass and wave-damping coefficients are shown to be influenced strongly by the free-surface effect and are presented as a function of water depth, frequency and direction of oscillation and of depth of submergence from the free surface. For the vertical oscillation close to the free surface, negative added mass values are measured, and the predictions of the added mass by a 3-D diffraction theory are 10–20% lower than the experimental values at the model submergences tested. The experimental added mass coefficient values for the horizontal oscillation and the wave damping coefficients for the vertical oscillation in finite depth differ more than 100% from the corresponding 3-D theory prediction at a certain at-sea operational frequency range. This set of data provides further experimental information for the improvement of theoretical predictions.
Recently, 3-D (three-dimensional) potential theories and computational methods have made much progress in contributing to the theoretical prediction of hydrodynamic forces and motions of floating structures. However, the accuracy of theoretical predictions needs to be confirmed by experimental results. Previously, Miao, Liu and Chung (1985) showed for a vertical circular cylinder or column with a circular footing that the prediction by a 3-D diffraction theory was more than 100% different at a certain frequency range of horizontal oscillation from the corresponding experimental values.