A primary design for a moderate size spar-buoy floater suitable for the 10 MW DTU wind turbine is introduced. Due to the high aerodynamic thrust acting on such large-rotor wind turbine, the tower pitching can become relatively high, which makes it a key point to be considered in the primary design stage. In addition, the floating platform is required to provide a sufficient dynamic stability to maintain stable energy production. However, it should stay cost-effective and limit the need to a complicated and expensive control system. The metacentric height plays a vital role in the dynamic stability, since it is related to the hydrodynamic stiffness of the floating system. Thus, in this study, minimizing the trim of the floating wind turbine was achieved through increasing the metacentric height by using ballast materials with different densities. The effect of increasing the platform stiffness on the spar-buoy pitch response was investigated with regard to the amplitude, natural frequency, damping ratio, nacelle acceleration and energy production. The potentials and limitations of this approach were investigated analytically and numerically by using the aero-hydro-servo-elastic simulation tool OpenFAST. The simulations were performed at the rated wind velocity which is supposed to cause the maximal pitch inclination and with the blades pitch control system deactivated. The results showed that with a moderate substructure size for the large-rotor wind turbine “DTU 10-MW”, a satisfactory dynamic stability can be obtained only by using high density, commercially available, ballast materials.