In this paper, we present a study on the energy-saving devices which is probably called Rudder-Bulb-Fin systems, designed for KVLCC2 tanker. The addition of horizontal fins which is installed on rudder for both sides can significantly reduce drag force in some applications. Several simulating cases of fin such as adjusting the angle of attack of fins, twisted fin, location of fins on rudder or even cutting starboard side fin have been designed and simulated using CFDSHIP-IOWA V4.5 code. The simulation results for those cases are validated by experiment; which are conducted in towing tank of Osaka University with fully loaded conditions at Froude number of 0.142. The flow field is measured in calm water using stereo particle image velocimetry at the downstream. The CFD and experimental fluid dynamics results showed a good agreement with the same test conditions. The total resistance by this design could be reduced a significant percentage amount and improved the propulsive efficiency by comparing with previous designs.
Since the fuel oil price is increasing and the greenhouse effect has escalating rapidly, the world is looking at greener ships with lower carbon emission and better fuel efficiency. It became the springboard and true inspiration for researchers and developers in shipbuilding industry field. Thus, there are many kinds of energy-saving devices (ESDs) around propeller are developed and introduced by companies as well as researchers at universities for a few recent decades.
JMU (Japan United Corporation) and MHI (Mistubishi Heavy Industries, Ltd) designed a contra-rotating propeller which consists of two propellers aligned in front and behind rotating in opposite direction. The grim wheel was proposed using the central part (turbine shaped) to drive the out propeller portion. The propulsive efficiency was improved by recovering the energy loss due to rotational flow behind propeller. Matsumoto and Sakamoto (2009) also succeeded in about 2% fuel consumption reduction with a patent dressing a finned rudder. The starboard side fin was located lower than the rudder bulb (i.e propeller-shaft center) while it was higher for port side fin, the fin length is not so long as the propeller radius. One type of port side fin was designed with an upward inclination. Compared with the conventional rudder, the experiment showed that the ship required less horsepower in order to maintain the same speed. Nakashima Propeller Co. Ltd. developed the “Ultimate Rudder” which was a rudder-bulb extending upstream and close to propeller boss. 4.9-5.4% improvement of self-propulsion efficiency was confirmed by Computational Fluid Dynamics (CFD) and Experimental Fluid Dynamics (EFD) (Kajihama, et al. 2016). Also, the hub vortex could be partially eliminated by this design.