Unconventional Propulsion — Silverleaf 



revolutions. These points are important in considering the development of pro- 

 pulsion devices, other than conventional open propellers, because such devices 

 often only have advantages where restrictions exist, and selection of the most 

 profitable research topics therefore involves prediction of the likely trend in 

 removing these restrictions. 



The principal reasons for investigating the possible usefulness of uncon- 

 ventional marine propulsion devices can thus be summarized as: 



(i) An attempt to maintain, at higher thrust and power loading coeffi- 

 cients (C^,Cp) and lower cavitation values (a-^), the high efficien- 

 cies which can be achieved with conventional open marine screws 

 under less onerous operating conditions. 



(ii) At lighter loadings to improve still further the efficiency obtain- 

 able and to reduce liability to cavitation damage. 



(iii) To minimize vibration due to propeller excitation resulting from 

 operation in a nonuniform inflow or through the free -space oscil- 

 lating pressure field. 



Comparative Features of Some Unconventional Devices: 



Table 5 attempts to summarize some of the principal features which are 

 important in any realistic comparison of practicable marine propulsion devices. 

 Some comments on these comparisons may be helpful: 



1. Open (unshrouded) propellers: 



(a) Controllable pitch — These are so well established that it is doubtful 

 whether they should be considered as unconventional devices. How- 

 ever, although they have advantages from the point of view of the engine 

 builder in providing a better match between powerplant characteristics 

 and changing thrust requirements, there is still considerable reluctance 

 to adopt controllable -pitch propellers, even though they are now avail- 

 able for fairly high power outputs. In a recent paper (Ref. (6)) this re- 

 luctance has been primarily ascribed to the much higher capital cost of 

 C propellers, which may be as much as 33% of the main engine cost 

 compared with 8% for a fixed pitch propeller; a secondary reason is 

 doubt about the realiability of any propulsion device which involves a 

 special actuating mechanism. 



(b) Fully cavitating propellers — Intensive efforts have been made, particu- 

 larly in the past decade, to develop fully cavitating propellers primar- 

 ily for high-speed craft. The emphasis has been on high efficiency 

 under extreme cavitation conditions, and in consequence the present 

 use of fully cavitating propellers has been limited to operating condi- 

 tions in which both high speed and high rate of rotation are either nec- 

 essary or desirable, as implied in Fig. 9, which is derived from Ref. 

 (7). However, there is some indication that fully cavitating propellers 

 give a much better propeller -hull interaction than conventional fully 



906 



