Prospects for Unconventional Marine Propulsion Devices 



power of such devices to about 2,000 hp, and even so for craft requiring less 

 than 1,000 hp the use of vertical axis propellers has recently been severely 

 challenged by other propulsion devices, such as the steerable open propeller 

 and the Pleuger activated rudder, which also have good maneuvering quali- 

 ties. 



4. Paddle wheels: . 



These again scarcely rank as unconventional propulsion devices, but recent 

 intensive studies of their performance suggest that they may not yet be en- 

 tirely dead. Their value in remote areas in less developed countries indi- 

 cates the importance of engineering simplicity and reliability in the overall 

 choice of propulsion system. 



-J. ' '•' 



5. Airscrews: 



While airscrews have often been proposed as propulsion devices for marine 

 craft they have only been used for this purpose since the advent of the am- 

 phibious hovercraft, for which they are obviously well suited. However, ex- 

 perience has shown that the performance of an open unducted airscrew is 

 much more strongly affected by wind, wave, and power variations than a 

 marine propeller. Further, limitations in propeller size may well limit 

 their application to fairly small craft, quite apart from other factors such 

 as noise. t .a := ; .- .:. ^ ■ -i ■ r -_ 



6. Pure jet devices: .,. -. v ; ^ /-^ - 



Most pure jet devices have very low propulsive efficiencies in any operating 

 conditions resembling those for present or projected marine craft. How- 

 ever, theoretical studies of an air-blown ramjet (Ref. 9) have suggested that 

 reasonable propulsive efficiencies, perhaps exceeding 40%, might be ob- 

 tained, though these values have so far not been confirmed by experiment. 

 If further work shows that reasonable propulsive efficiencies can be 

 achieved then the simple air-blown ramjet might be employed as a booster 

 unit, if not as the main propulsion device, for some high speed craft. It 

 should be added that the effective thrust loading coefficient C^. for which 

 reasonable efficiencies might be achieved is low, probably not much exceed- 

 ing 0.15. 



SHIP TYPES AND THEIR PROPULSION REQUIREMENTS 

 General Criteria 



For ship propulsion systems, of which the propulsion device forms a critical 

 part, the order of priority for design criteria is different for merchant ships and 

 for naval ships; Table 6 shows these priorities as summarized in Ref. (5). It will 

 be seen that reliability rates highly in all applications, but that low capital cost 

 disappears from the naval list as a primary aim, while low weight and compact- 

 ness become more important. Further, fuel consumption is judged on a weight 

 basis for naval applications and on a cost basis for the Merchant Navy. 



911 



