Influence of ducted propeller on seakeeping in waves

It has been claimed that the use of a ducted propeller instead of an open propeller reduces the pitching motions of a ship, and the added resistance in waves. In addition, there is the potential benefit of smaller loss in propulsive efficiency when the propeller loading increases due to added resistance. Although little documentation for the validity of these claims is found, they look reasonable, since the duct has an effect similar to a passive foil, which is proven to reduce the vertical plane motions of high speed vehicles. This paper investigates the effect of a duct on the seakeeping of conventional ships. It is shown how the effect of the duct on the motions can be modelled by an equivalent flat foil. The effect of duct on motions and added resistance is investigated using linear strip theory. Results of model tests using a conventional 120 m single screw cargo vessel with a ducted and an open propeller are presented. The model test results confirm the findings from the numerical calculations, which is that the duct needs to be much larger than what is typical for such a ship in order to have a significant influence on the seakeeping performance.