Twin skeg types for merchant ships are now under construction as they have dimensional limitation due to cruising route, the lack of required power with a single engine, station keeping ability, and so on. The ships of wide-beam and shallow-draught are investigated in recent days. It has been well known that the effect of propeller rotational direction on propulsive efficiency and cavitation behavior is significant in such ships, which is somewhat depending on the skeg hull form design. However, it is not easy to find the relevant papers related to the subject. In this research, using a wide beam tanker and her propellers, we performed calm water tests including wake survey, and analysis of propeller rotational direction effects. Through numerical cavity simulation, cavitation behavior was predicted and cavitation tests were also performed. We compared the cavitation test results with those of simulation and also surveyed the difference of the cavitation behavior between inward and outward propeller rotational direction. Propulsive efficiency and cavitation behavior showed much difference according to the rotational direction of the propeller. The research results will provide valuable information for the design of propeller and the choice of rotational direction.