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Elongated floaters drifting in propagating water waves slowly rotate towardsa preferential state of orientation. Short and heavy floaters tend to alignlongitudinally, along the direction of wave propagation, whereas long and lightfloaters align transversely, parallel to the wave crests and troughs. Weinvestigate this phenomenon for homogeneous parallelepiped floaters bycombining laboratory experiments with numerical simulations and asymptotictheory. For floaters small with respect to wavelength and for low amplitudewaves, we show that the floater orientation is controlled by thenon-dimensional number $F = k L_x^2 / \beta L_z$, with $k$ is the wavenumber,$\beta$ the floater-to-water density ratio, and $L_x$ and $L_z$ the floaterlength and thickness. Theory places the longitudinal-transverse transition atthe critical value $F_c = 60$, in fair agreement with the experiments. Using asimplified physical model, we elucidate the physical origin of the preferentialorientation. Through its motion, the floater probes the velocity gradientsalong its surface. Next to a small mean displacement (Stokes drift), thisresults in a net torque which, for short floaters, always favors thelongitudinal orientation. This net torque arises from a phase correlationbetween the instantaneous buoyancy torque and the instantaneous yaw angle ofthe floater, a mechanism analogous to the Kapitza pendulum. The transverseequilibrium of longer floaters has a different origin and arises from thevariation of the submersion depth along their long axis. This varyingsubmersion significantly increases the torque in the trough positions, when thetips are more submersed, and always pushes towards the transverse orientation.

Preferential orientation of floaters drifting in water waves