The disclosure reduces vertical movement of a floating offshore platform by including a centralized open keel plate coupled to the hull that allows water below and above the keel plate. As the floating platform moves vertically, the keel plate separates the water and causes drag on the platform. The water moving vertically with the plate also increases the dynamic mass. The drag results in less vertical movement of the offshore platform without the need to extend legs of the platform to gain an equivalent reduction in vertical movement. The added dynamic mass increases the natural period of the vertical motion away from the wave excitation period to minimize the wave driven motion. The keel plate generally is above or at the same level of the keel, and therefore would not reduce the clearance between the seabed and the keel of the hull at the quayside.