This paper analyses the results of two series of experiments concerned with the responseof a single vertical cylinder in the inertia regime in steep non-breaking waves. We recordedfirst the loading on a cylinder when it was held stationary, and secondly, its response inthe same waves when it was pivoted just above the floor of the wave flume, and supported atthe top by springs in the horizontal plane. Spring stiffnesses were set to achieve naturalfrequencies (measured in still water) in the range between 3 and 11 times the dominant wavefrequency. The experiments were repeated with cylinders of three different diameters.

Peak loading on stationary cylinders was found to exceed the predictions of a Morisonmodel (based on kinematics computed from a numerical model of the measured waves), thoughimprovements were achieved through the inclusion of slender-body terms. Measured ringingresponses are generally in good agreement with those computed on a quasi-static basis fromthe measured loading history, but in some conditions, particularly at low frequency ratios,there is clearly some feedback from the motion to the excitation. Peak accelerations in thesteepest waves are found to be limited approximately to those that would occur if themaximum loading were applied as a step change. Particular attention is given to a rapidcycle of loading that occurs after the crest has passed the cylinder's axis, and to imagesof the flow around the cylinder at the water surface.