Before the calculation of velocity distributions in ship wake, three kinds of experiments are carried out in order to get theoretical models as to the separation phenomena and the diffusivity coefficient; (a) observation of the limiting streamlines by the oil film method, (b) the velocity measurement near the separation position and (c) the vorticity measurement in ship wake. The separation position is determined experimentally which can be predicted roughly using some suitable parameters of the boundary layer calculation. It has been made clear from the velocity measurements that the velocity near the seperation position makes remarkable changes and the newly-generated-vorticity affects very much the down stream pattern. It has also been made clear that not only the x-axis vorticity but also the y- and z-axis vorticity compose the stern vortex. Using the measured velocity and vorticity distributions the diffusivity coefficient is determined, which amounts to about 300 times of the molecular one. The velocity calculation of a tanker model is carried out after these experimental considerations, which shows a fairly well agreements with the measured one.