Wake Adapted Ducted Propellers
For the application behind ships two different types of ducted propellers are considered; the ducted propeller where the nozzle accelerates the flow at the propeller and the ducted propeller where the nozzle decelerates the flow at the propeller. The first type of ducted propeller is now extensively used in cases where the ship screw is heavily loaded or where the screw is restricted in diameter. The accelerating nozzle offers a means of increasing the efficiency of heavily loaded propellers. The second type of ducted propeller is used to increase the static pressure at the impeller. This nozzle may be used if retardation of propeller cavitation is desired. For the design of a ducted propeller it is attractive to have a theoretical calculation method available supported by experiments. Tests on ducted propellers are scarce and most of the tests are restricted to isolated applications. The work described here deals with systematic experiments on ducted propellers and may contribute to fill up the lack of experimental data. The potentialities of ducted propellers both with nozzles of the accelerating and the decelerating flow type are discussed based on simplified theories. The open-water test results of systematic series of flow accelerating and flow decelerating nozzles are given. These data are of importance for the design of optimum ducted propeller systems as well from the viewpoint of efficiency as from the viewpoint of cavitation. In addition, the results of open-water tests with ringpropellers and ringpropellers in nozzles are given. Finally, the design and the application of non-axisymmetrical nozzles which are adapted to the wake behind the ship, are discussed. In the case of single-screw ships (tankers and bulkcarriers) the nozzle is designed in such a way that the flow at the propeller plane becomes more uniform. Such a nozzle offers, next to a reduction in DHP, a means of minimizing propeller induced vibration and cavitation problems. In the case of twin-screw ships (fast naval ships; frigates, destroyers), the propellers operate in a varying inflow, due to the shaft inclination. The non-axisymmetrical nozzle is designed here, from the viewpoint of retardation of screw cavitation, in such a way that the actual effective incidence changes of the blade sections of the impeller will be as low as possible during a revolution.