Spar is one of the promising floating structures which support wind turbine at sea. In general, some lower part of spar is used as a ballast tank. It is possible to replace this part with other shapes without water-tightness because this part does not contribute to buoyancy nor stability to support the weight or heeling moment of wind turbine. In the field of oil and gas production, truss spar has been developed with this concept and the lower part consists of heave plates. However, for wind turbine, pitching motion is more important than heaving. The authors changed this part to four vertical damping plates to reduce this motion. The effect of the central gap and holes in the damping plates were investigated because these parameters have effects on flow separation and hydrodynamic forces. The motions in waves of the new spars were compared with a classic spar of the same dimensions and stability. The proposed vertical damping plates, with central gap or holes, work to reduce the pitching motion in wide range of wave frequency. Considering the expected cost reduction and some motion reduction, the promising nature of the proposed spar revealed.

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