Wave pattern Behind the ship

Hi guys,

I am Vijay a Naval architect from India....here i am trying to analyse kelvin wave pattern around the ship for a particular speed. i have chosen one general cargo vessel hull form to perform the analysis..... since i have a past experience in working with fluent, i tried the same for free surface flow as well. The problem i am facing was with the boundary condition at inlet. i am specifying the velocity at inlet but i found after a few iteration the iso surface with 0.5 volume fraction ( water surface ) was getting distorted in shape it was spliting into two or three layers which in turn effects the whole wave pattern around the ship can anybody tell me how to resolve this problem, earlier i did performance estimation for underwater hull form that time i got acurate results.......

Thanks in advance.