A semi-submersible wind platform with four floats of equal diameter, damping plates and relatively small drafts has been designed to support a 5 MW wind turbine at full scale. Motion and mooring forces from wave basin measurements have been compared with time domain linear diffraction modelling accounting for drag forces, mooring forces and mean forces due to zero-difference frequency components, as is standard, and due to damping associated with radiation and drag forces, not previously considered. The platform response in the form of rms acceleration is quite well predicted although peak values can be underestimated. The mean mooring forces are underestimated and peak values are considerably underestimated. In large waves moorings experience high snatch loads. The measured mean forces were applied in the model for further comparison. The mooring was primarily designed to prevent drift of the floater and improved designs could eliminate or reduce snatch loads. However, it is shown that hub acceleration is quite moderate, with peak values less than 4 m/s2 in even the largest waves. The rms platform acceleration is largely decoupled from the mooring forces, as shown by corresponding spectra. In extreme conditions hydrodynamic mooring forces require nonlinear effects due to steep, sometimes breaking, waves to be accounted for. The wind thrust is included in the model using a coefficient from blade element momentum theory based on relative wind velocity. The peak hydrodynamic force would be significantly larger than the maximum wind thrust although the mean hydrodynamic force is significantly smaller. A practical conclusion is that a semi-sub floater with damping plates giving sufficiently low accelerations for operation in large waves may be of relatively shallow draft, less than the depths of many ports which is convenient for deployment.