|In this thesis I am making a computational fluid dynamics(CFD) simulation of a SWATH (Small Waterplane Area Twin Hull) wind turbine service vessel moving in still water at different speeds by using the CFD tool STAR-CCM+. Since I did not have any prior experience in CFD, a substantial part of the thesis is dedicated to theory in CFD. First of all theory for fluid dynamics and CFD methods are described. Based on this theory, models and solvers for the simulation in STARCCM + are chosen with the required boundary conditions and initial values. A major part of the simulation work is to obtain a good mesh before the solution is achieved. The paper consider the total hull resistance of the vessel at different speeds due to pressure and shear forces. The results are compared with a still water performance test for a scaled model. Wave making resistance is also considered in the comparison. The resistance on the four holes in the hull, where the ballast tanks are placed, are compared with the resistance on the hull. Secondly the paper examines how the water level inside the ballast tanks, which is open to sea at the front and at the back, are affected at different speeds.