This paper is concerned with the development and application of hydrodynamic theory required for the prediction of wave drift damping phenomena as introduced by Wichers in 1979 and 1982, ref. [1] and [2]. The concept of wave drift damping necessitates the development of forward speed dependent expressions for the mean wave drift forces and a new 3-D diffraction potential algorithm for forward speed (Wichers and Huijsmans [3] and Hearn and Koon [4]). In this paper computed mean wave drift forces are validated with results from model tests as reported by Wichers [5]. Results will be discussed of model tests and computations for this tanker in irregular waves with and without current.