View non-flash version
T here are a number of device types that can be added in the sub-surface propeller region of a ship to improve propulsive eciency. An older, well-known type is the accelerating duct, or Kort nozzle. New and less well-known types include the propeller bossing cap ns (PBCF). In all cases, however, if properly selected and designed, the device captures uid kinetic energy that would otherwise be lost to the stream and converts it into productive load development. ese devices all function in fundamentally the same way. e velocity elds induced by the lift-pro- ducing devices interact with the induced velocity eld of the propeller blades. is results in secondary loads that contribute to propulsion, either by increasing thrust or reducing delivered power. is is according to the for- mula for Froude propulsive eciency: where T is the primary propeller thrust, Va is the speed of advance, and Pd is the delivered power. e interacting induced velocities increase in magnitude with increase in the propeller thrust loading. e thrust loading coecient is Here, ! is the water density, V is the ship speed, and Ap is the propeller disk area. Small CT implies light loading, which, by this formula, usually corresponds to high-speed craft (planing boats) or to ships with large- diameter propellers and moderate thrust. e feed? for the subject devices is propeller induction. erefore, for lightly-loaded propellers the induced velocity eld can be so small that any devices added for eciency enhancement produce mainly drag, and act to reduce system eciency rather than increase it. In that case it would be concluded that hydrodynamic devices would not add to the quality of the ship performance. Figure 1. Compound propulsor, propeller, and PBCF device = TVaPdCT = T1/2 !V2 Ap October 2011 www.sname.org/sname/mt (mt notes )e Sub-Surface Propeller Region How hydrodynamic devices can improve ship propulsive eciency BY WILLIAM S.VORUS, FUWEI ZHANG, AND YUSONG CAO