Developing the high efficiency ships is one of the goals that we are heading. The present work is to study the energy saving device, which is a rudder-bulb-fin system, designed for the KVLCC2 tanker. The horizontal fins on the both sides of the rudder bulb share the same foil section, but the suction side faces up on the portside and faces down on the starboard side. The concept is to recover or reduce the energy loss, or gain extra thrust in the propeller rotational flow. The viscous flow simulation was performed by CFDSHIP-Iowa V4.5 with a propeller model. The test condition is at design Froude number 0.142 in calm water for a 3.2m long ship model. The force and flow field were measured in Osaka University towing tank. The propeller rotational rate was determined in EFD (Experimental Fluid Dynamic) self-propulsion test. EFD and CFD (Computational Fluid Dynamic) showed good agreement for rudder flow field and forces for zero degree AOA (angle of attack) for both fins. Furthermore, the AOA combinations from 1 to -2 degree with 1-degree increment for both fins were investigated by CFD only. The best energy saving effect AOA was found; 0 degree for portside fin and -1 degree for starboard side fin. The total resistance was reduced by 2.3% compared with the normal rudder case. Among the cases, its thrust deduction, effective wake and hull efficiency factor were the optimal values.