Theoretical and experimental investigation of yaw motion instability in a submerged axi-symmetric wave energy converter is presented. The device is a truncated vertical cylinder
which is taut-moored via three tethers. Assuming linear hydrodynamics, but retaining non-linear geometry associated with the tethers, governing equations are derived in 6 degrees of freedom.
Due to the axi-symmetry of the system, there is no hydrodynamic excitation moment in yaw. However, the yaw governing equation - correct to second order in buoy motions - reveals a time-varying restoring moment coefficient. Such systems can undergo large oscillations given a small initial perturbation, through the well known Mathieu instability. Targeted regular wave experiments were used to verify the model predictions on the onset of yaw motion instability in the first two instability branches. The yaw motion in a three-tethered system is analogous
to sway motion in a single-tethered device. The yaw instability and the transverse/sway motion instability both arise due to coupling with heave. Due to small damping, the instabilities can be prevalent. The theoretical analysis presented is applicable to other floating WECs.