This paper presents theoretical investigations on the performances of several kinds of fishtail rudder sections and hydrodynamic mechanism for generating higher lift forces due to the fish tails. The hydrodynamic forces acting on the sections and the flows passing them are calculated by using a CFD commercial code "Fluent". At first, validation of the computed results is done by comparing the results with experimental results. Secondly, the effect of Reynolds number on the hydrodynamic forces are investigated. Thirdly, hydrodynamic characteristics of several kinds of rudder sections with different shape and fish tails are computed to find the effects of the cross section shapes and the size of fish tail on them.