Wave interaction with a horizontal cylinder is examined in a custom-designed wave tank facility, which allows quantitative imaging of wave-cylinder interaction for an
arbitrary direction of view. Simultaneous high-image-density particle image velocimetry and force measurements relate the instantaneous streamline topology and patterns of vorticity to the instantaneous forces. Instantaneous moments of vorticity are interpreted in terms of time-integrals of the horizontal and vertical force
coefficients, Cx and Cy. Large contributions to these moments can occur for vorticity concentrations having relatively small amplitude orbital trajectories; in addition,
vorticity concentrations in the initial phase of a larger orbital trajectory can make substantial contributions. Regarding the effects of nominal submergence of the
cylinder in the wave, the patterns of vorticity generated during a typical wave cycle are shown to have generic forms irrespective of degree of submergence. The particulars of these patterns exhibit significant differences due to phase lags of the vortex shedding relative to the wave motion.