INTERACTION OF DUCTED PROPULSOR AND STERN FOR AN AXISYMMETRIC HULL

A numerical exploration of propulsor-stern interaction, with a predominately potentialflow representation of the flow field, is undertaken to evaluate thrust deduction and control-surface loads for an accelerating-duct, decelerating-duct and open propulsor of relatively large diameter on the same axisymmetric hullform. A boundary-element model of the annular foil shroud and the hull is coupled with a lifting-line model of the rotor to determine the flow field. Interaction fields are taken as axi-symmetric. For the examples considered, each propulsor is at nearly the diameter for minimum shaft horsepower. The estimates show that the superior open-water efficiency of the accelerating-duct propulsor is degraded by an increased thrust-deduction force component in the axial force balance for the propulsor-hull combination system while the propulsive efficiency of a decelerating-duct propulsor approaches that of the open propeller because of hull interaction, making the propulsive efficiency nearly the same for all three propulsors. Based on estimates from a lifting-line model of side force on a representative rudder located forward of the propulsor, increased course-keeping ability from the control surfaces is anticipated for an accelerating-duct propulsor relative to both an open propeller and decelerating-duct propulsor.