Prediction of the Seakeeping Characterstics of Hydrofoil Ships

Abstract

Waterjet nozzle designs are, for the most part, simple lowloss, fixed-area, converging nozzles. Their performance is depreciated seriously only when the flow from the pump stator contains appreciable rotational vectors or swirl, which is intensified by the nozzle contraction and can choke the pump flow. Variable-area nozzles for water jets are needed for vessels with a widely varying speed range, and when the inlet ram pressure is a significant portion of the system pressure. Under these conditions, changes in vessel speed cause significant changes in pump flow and tend to drive the unit into off-design conditions. By varying the nozzle area, the pump flow can be adjusted so that the pump operates at best efficiency. Figure 19 shows the effect of variable-area nozzles on thrust for a ship designed for 70 knots when it is operating at higher speeds. At 100 knots, the variable nozzle design gives 15% more thrust than the fixed nozzle. Figure 20 is a typical pump map that shows the extent of overflow and efficiency change at a 50% vessel overspeed. The variable-area nozzle can correct this overflow and adjust the jet velocity ratio to its optimum value. Mechanically, such nozzles are relatively straightforward designs. Figure 21 shows a variable geometry nozzle. Many other schemes are possible.