This paper presents an analysis technique for investigating the effects of gyroscopic couples on the behaviour of a large vertical axis wind turbine mounted on a floating semi-submersible vessel. Equations of motion in 6 degrees of freedom are formulated in the frequency domain. Time domain aerodynamic loads are derived from a double multiple stream tube model based Paraschivoiu [13]. These loads are transposed to the frequency domain and integrated with a formulation of the gyroscopic moments acting for the wind turbine rotating and the semi-submersible rotating in roll and pitch due to ocean waves. The paper uses this model to explore the effect of gyroscopic effects on the motions of the turbine and semisubmersible. It is shown that the gyroscopic effect has negligible effects on some modes of motion but that it introduces significant motions at key frequencies related to rotor speed. (C) 2012 Published by Elsevier Ltd.