The influence of using a ducted propeller on motions and speed loss of a ship in waves

Comparisons of experimental and numerical investigations into the effect of duct on motion variables and speed loss for a given ship model are presented. The purpose of these comparisons is to verify the advantage of using ducted propeller on sea keeping performance and added power or speed loss. Relevant damping theories concerning the lift characteristic of ducts are described as foundation for the numerical calculations of using an equivalent flat plate that represents the duct of ducted propeller in software of SHIPX. In SHIPX, a case ship of RR NVC Nor-lines 120m cargo vessel is defined and several plug-in are used to do simulations for bare ship (refers to vessel with open propeller) and ship with foil (represents vessel with ducted propeller). The duct‟s effect on RAOs for motion variables, added resistance and speed loss are evaluated respectively under 5 different wave conditions. A series of experiments on M3006 respectively with an open propeller and a ducted propeller are carried out in both calm water and 8 regular wave conditions in the large towing tank of MARINTEK center. Repetitions of tests under same wave conditions are used to make uncertainty analysis of measured variables. The effect of wave height and wave period affecting the duct‟s effect on motion variables and added resistance are discussed and compared with numerical results obtained from SHIPX. Additionally, by interpolation of propeller rotational speed given tow rope force at self-propulsion point, corresponding thrust and torque are obtained. Then the interpolated variables are used to calculate propulsive coefficients including thrust deduction, wake fraction and relative rotative efficiency in both calm water and regular waves. The effect of wave height and wave period on propulsive coefficients are studied and compared for both open and ducted propeller. Furthermore, corrections are made for interpolation results to give more precise power prediction. The effect of propeller loading on wake fraction and propeller efficiency varying with 5 different wave conditions are presented. Finally the duct‟s effect on added power is studied and compared with numerical results from SHIPX