Unconventional Propulsion — Silver leaf 



Table 6 



Ship Propulsion Systems: 



Order of Priority for Design Criteria 



Several important conclusions are implicit in this statement of priorities. 

 For example, for most merchant ships it strongly suggests that, unless an un- 

 conventional propulsion device gives such an increase in propulsive efficiency 

 that the consequent reduction in required power can be reflected in the capital 

 cost of the machinery installation (including the propulsion device itself), then it 

 is very unlikely to be regarded favorably, since the simple open marine propel- 

 ler is clearly superior so far as all the other criteria are concerned. On this 

 basis it is not unreasonable to suggest that the minimum reduction in required 

 power due to adopting an unconventional marine propulsion device must be not 

 much less than 10% to justify its widespread adoption; further, the attainable 

 power reduction should increase with the complexity of the device, so that the 

 improvement in performance needed to justify a mechanically complex contra- 

 rotating system must be greater than that to justify the relatively simple single 

 propeller in a short duct or nozzle, although even this may well have significant 

 disadvantages in maintenance and total weight. For naval ships the emphasis on 

 rapid maneuverability and ease of operation creates a more favourable climate 

 for the adoption of propulsion devices which may not show any significant gain in 

 propulsive efficiency or in overall power required; thus, water jet systems may 

 have advantages from this point of view, particularly if they can be designed to 

 operate more quietly than unshrouded open marine screws. Because of the wide 

 differences in the requirements for differnet classes of ship it is necessary to 

 consider each main type separately. 



Low-Speed Merchant Ships 



Tankers and bulk carriers now dominate the world merchant fleet and so 

 clearly their propulsion needs should be paramount in any civil research and 

 development programs. The typical values of propulsion parameters in Table 3 



912. 



