Fully Cavitating Propeller for a Hydrofoil Ship 



loading, although minor compromises, largely due to engine characteristics at 

 the extreme thrust and rpm conditions, are necessary for this. Fortunately, 

 fully cavitating propellers behave reasonably well in this respect, when com- 

 pared with other propulsion devices such as present-day water jets. This re- 

 quirement is essential for hydrofoil ships when, in addition to the calm-water 

 resistance variation, extra loads may be imposed because of high sea states and 

 variable depth sonar (VDS) towing requirements. 



Much has been done in recent years towards producing a design method for 

 fully cavitating propellers, and the work at NSRDC (Refs. 4 and 5) may be cited, 

 together with the interesting qualitative observations of Tulin in Ref. 6. Much 

 still remains to be done, however, in the field of fully cavitating propeller theory 

 to put it on an equal footing with noncavitating propeller theory. Steps in this di- 

 rection are being taken, and numerical results from the work by Cox (Ref. 7) 

 are awaited with interest, while at NPL evaluation of the proposals made in Ref. 

 8 are continuing. At NPL, considerable emphasis is given to obtaining a de- 

 tailed quantitative physical insight into the flows created by fully cavitating and 

 ventilated propellers, and partly for this purpose the instrumentation described 

 in Ref. 9 has been built. 



The design situation with highly loaded, fully cavitating propellers, as may 

 occur in the hydrofoil ship takeoff condition, is more difficult than for the mod- 

 erately loaded case, since there is a dearth of empirical information at off- 

 design advance ratios. Consequently, model testing is even more essential for 

 an actual fully cavitating propeller design. With this in mind, therefore, the ap- 

 proach used in designing the Bras d'Or screws has been to consider the flying 

 condition first, following this with tests on propeller models over a wide range 

 of loading and cavitation number to determine if the other critical conditions 

 were satisfied. 



In parallel with the specific design work for the Bras d'Or screws, a num- 

 ber of additional experiments were conducted at NPL on a model screw made to 

 the T95 design described in Ref. 8. This screw was used mainly for convenience 

 and because information was required before the design for the Bras d'Or was 

 finalised. These tests were undertaken to determine the importance of some de- 

 sign features on performance, and included the observation of the cavity heights 

 above the backs of the blade surfaces in the vicinity of the leading edges over a 

 range of operating conditions, the effect of leading edge thickness on perform- 

 ance, and the determination of stress levels in a region near the leading edges. 

 Attention was given to these aspects of design, since it was recognised from the 

 experience of others that leading edge geometry is a crucial factor in the pro- 

 duction of a reliable, full-scale, fully cavitating propeller. 



MEASUREMENT OF WAKE CONDITIONS AND 

 SIMULATION IN THE WATER TUNNEL 



The Bras d'Or screws are of the pusher type and are mounted at the stern 

 of the propulsion pods. Considerable care was taken in designing the propulsion 

 pods and the junctions with the struts, the main foil, and dihedral foils, in order 

 to minimise the irregular wakes created by these components. The method of 

 streamline contouring was used for this purpose. Nevertheless, it was considered 



965 



