Fully Cavitating Propeller for a Hydrofoil Ship 



In connection with devices employing air/water mixtures, it appears that 

 the efficiency and effectiveness with which energy can be transferred from gas 

 to water, using the natural mixing process, is considerably less than that 

 achieved when energy is transferred to water with the blades of propellers or 

 pumps, where, for example efficiencies in the range of 80 to 90 percent are 

 commonplace. If in the mixing process the efficiency of energy transference is 

 only in the range of 20 to 40 percent, then this is too large a deficiency to make 

 up by factors which have a second-order influence on the overall efficiency. 



Water propulsion is the only alternative method and this must be accom- 

 plished by means of rotodynamic machines such as propeller -type devices or 

 pumps. The principal difference in the application of these two types of device 

 is that the propeller type, i.e., fully cavitating screws or ducted propellers of 

 the so-called pump-jet type (Ref. 25), must be submerged beneath the surface, 

 whereas those employing pumps, e.g., waterjets, may be installed within the 

 hull. When mechanical power transmission is used, then pumps installed within 

 the hull possess the obvious practical engineering advantage of a simpler me- 

 chanical drive system, and this weighs heavily in their favour. Also the ques- 

 tion of the vulnerability of complicated underwater equipment such as screws, 

 ducts, gears, etc., must be considered, and while this is of secondary impor- 

 tance in the case of oceangoing vehicles such as the Bras d'Or, it may be of pri- 

 mary importance in vehicles employed on coastal and inland routes. 



Clearly then, the future choice of a propulsion device will depend largely on 

 the role of the vehicle and on any developments that occur in power transmission 

 systems. In the authors' opinion, any advances in geared or possibly hydraulic 

 transmissions will strengthen the case for the fully cavitating propeller, on ac- 

 count of its relatively high efficiency and ability to accommodate a wide range 

 of loading without additional complications. 



Ducted propellers, similar to the type described in Ref. 25, in which the 

 noncavitating screw or impeller is situated within a decelerating duct that is 

 either base -ventilated or surface -ventilated on the external surface, could be 

 contenders in the future. Mounted on the aft end of propulsion pods similar to 

 those of the Bras d'Or, the device would employ a mechanical shaft drive, and 

 the overall complication would be slightly greater than that with fully cavitating 

 propellers. However, the presence of the duct and a support strut may enable 

 more flexible alternative drives to be considered, e.g., hydraulic or pneumatic 

 drives, in which the strut and duct are utilised for the power transmission, pos- 

 sibly to the tips of the rotor instead of the axis. With a device of this type it is 

 conceivable that the duct could replace the propulsion pod and the strut and foils 

 could then be attached to it. 



Finally, the waterjet is another alternative, although in many respects it 

 can be considered as a particular type of ducted propeller. Intense interest has 

 been shown in these devices over the past few years, and the work mainly spon- 

 sored by the US Navy Department, Bureau of Ships, has given fresh impetus to 

 this subject. The current position and thoughts in relation to waterjet propul- 

 sion are contained in the numerous papers that have been published recently, for 

 example, Refs. 19, 26, 27, 28, 29 and 30. While some advanced proposals have 

 been made, most of the component testing appears to have been conducted on 

 pumps of established and proven design. A number of troublesome design areas 



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