Three designs of a floating spar platform for a vertical-axis wind turbine (VAWT) are considered, including two novel designs. The novel designs convert the rotary motion of the platform to a translational motion (namely, heave), to reduce the maximum yaw stiffness requirements of the mooring system. In typical operational conditions of a VAWT, the mooring system needs to be stiff to prevent the yaw rotation of the platform, allowing power to be taken off from the rotary electromagnetic generator. The first design considered is a simple spar platform with mooring lines in a spread-mooring configuration. The second design, a novel design, incorporates a lead-screw to couple the rotational motion to a translational motion. This design can take advantage of the hydrostatic restoring force present in heave to reduce the mooring stiffness. The third design, also a novel design, uses a spar-torus combination platform and a lead-screw to counteract the torque on the stator of the generator. Numerical models in the time domain are developed to simulate the dynamics of these three platforms in regular waves and constant-wind conditions and the results are reported. It was found that the stiffness requirements of the second design can be reduced by 25% without changing the transient dynamics. If the wave climate is energetic enough, the third design can not only reduce the yaw stiffness requirements but also increase the mean power produced. Advantages and disadvantages of each design due to the relative complexity of each system, as well as the mooring, generator, and platform dynamics are discussed.

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