UNLOADING THE (PROPELLER-BLADE) TIPS

The Author, of the Ship Research Institute of Norway, discusses some current problems in propeller cavitation and propeller-induced hull vibration, with particular reference to reducing the excitation forces by unloading the blade tips. For reducing the waterborne forces and pressures induced by the cavitating propeller, the main parameters are pitch distribution and blade-area ratio. The best way to reduce the tendency for the blade to cavitate in its top position is to decrease the pitch towards the tip or to increase the blade area. If these measures are overdone, the risk of pressure-side cavitation will be high; the unloading of the tip will also lower the ship's propulsive efficiency. Where extreme unloading could result in cavitation erosion, it has always been possible (at the Ship Research Institute of Norway) to use experience gained with the cavitation tunnel to modify the design so as to avoid serious mid-chord bubble-cavitation, pressure-side cavitation, and unstable sheet-cavitation. Results of full-scale measurements, and model tests in large cavitation-tunnels, have shown that the quest for high propulsive efficiency will often lead to unacceptable pressure-fluctuations on the hull. Some of these results, and the alleviation of the trouble by unloading the blade tips, are briefly discussed, and it is mentioned that it is "tempting" to conclude (i) that marked reductions in propeller-excited pressure pulses are obtained by allowing reductions in propulsive efficiency through significant unloading of the blade tips, and (ii) that the cavitation tunnel is not only a means of investigating the risk of cavitation erosion but is also useful for the study of pressure forces induced on the hull by the cavitating propeller.