Davis and English 



in the overall system have been highlighted. These are associated with the 

 avoidance of cavitation erosion of the pump impeller, particularly in the takeoff 

 condition, avoidance of inlet cavitation with fixed inlets and cavitation in the 

 bends, ducting and elevation losses, and the weight of the system, including the ' 

 water in the ducting. Suggestions for alleviating these difficulties include in- 

 stalling variable geometry intakes and nozzles and using inducers to enable the 

 pumps to operate at lower suction specific speeds. However, the engineering 

 development required with variable geometry intakes and their installation are 

 added complications; also, the efficiency of present-day inducers is too low for 

 the large hydrofoil ship application. As regards the future, therefore, the intro- 

 duction of these items can only be viewed with uncertainty. In the meantime, the 

 progress of the US Navy's Tucumcari, Ref. 18, fitted with a gas -turbine -driven 

 waterjet system is observed with great interest, although it will be appreciated 

 that the Bras d'Or power requirement is about six times that of the Tucumcari. 



Perhaps the only firm prophecy that can be made regarding future hydrofoil 

 ship propulsion systems is that they will employ gas turbine power units. Their 

 high power-to-weight ratio, the flexibility of the free-power turbine, and the 

 large amounts of power that can be developed make them invincible for this ap- 

 plication. The aeronautical requirement will also ensure a high level of devel- 

 opment in the future. Developments in the marinised versions will possibly take 

 place, and the interesting suggestion mentioned by Waldo in Ref. 31 of separat- 

 ing the free -power turbine from the gas generator and considering the ducted 

 gases as a replacement for the mechanical drive arouse interesting speculations. 

 For example, possibly alternative engine arrangements could be devised to en- 

 able simpler geared drives to be used for propellers, even including the possi- 

 bility of inclined shaft drives. The relative simplicity of the inclined drive 

 makes it an attractive proposition for use with fully cavitating and ventilated 

 propellers, both of which offer good prospects for reliable, efficient propulsion. 



While fully cavitating propellers have received considerable attention in the 

 past, ventilated propellers have been virtually ignored up to now, although con- 

 siderable benefits are envisaged with this type of propeller since the problems 

 of cavity collapse, cavitation erosion, and particularly underwater noise propa- 

 gation should be greatly reduced. Further this type of propulsion, in which the 

 venting air or gas is delivered through support struts which may be of the base- 

 vented type, will also have application to high-speed displacement ships such as 

 destroyers or frigates and hovercraft of both the sidewall and peripheral-skirted 

 types, in addition to hydrofoils employing either noncavitating or fully cavitating 

 foil systems. 



The structural problems associated with these propellers, such as provid- 

 ing adequate strength and resistance to fatigue and cavitation erosion, may be 

 relieved with the new materials that are emerging from the laboratory; for ex- 

 ample, the boron or carbon-filament reinforced composite materials employing 

 metallic or polymeric matrices may provide a better solution than either Inconel 

 718 or titanium. Also, these materials may be more easily fabricated than In- 

 conel and titanium, thereby eliminating the very costly forging and machining 

 procedures currently employed. 



1008 



