Prospects for Unconventional Marine Propulsion Devices 



show that large low-speed tankers and bulk carriers operate at thrust and power 

 loadings much higher than those for all other merchant ships, except tugs. This 

 means that only those devices which operate well at high loadings need be con- 

 sidered as alternatives to the conventional open propeller. Indeed, only devices 

 which offer significant advantages in propulsive efficiency, without any accom- 

 panying disadvantage of complexity or liability to damage, can be seriously con- 

 sidered, and merchant-ship studies have already shown that a substantial gain in 

 a thorough techno -economic assessment is an essential prerequisite to a depart- 

 ture from the conventional open marine propeller; indeed, there is even a reluct- 

 ance to move away from single-screw systems. The ducted propeller is clearly 

 the most obvious alternative for large, low-speed ships, but the need for a small 

 tip clearance to obtain the best performance may be an inhibiting factor. 



High-Speed Merchant Ships •'• _ ■ ■ • > 



There has been a great deal of discussion about high-speed cargo liners and 

 similar apparently novel merchant ships. However, even though the diameter of 

 propellers for such ships may be severely restricted by draft limitations, the 

 thrust and power loadings at which they operate are not high; thus, conventional 

 open propellers can still serve very efficiently. The main problem may well be 

 that the higher absolute powers for such vessels may lead to more severe pro- 

 peller excited vibration. Thus, while devices such as contrarotating propellers 

 may show appreciable gains in propulsive efficiency, their increased mechanical 

 complexity and much higher capital cost suggests that their adoption, except on 

 an experimental basis, is unlikely unless they also appreciably reduce propeller- 

 excited vibration. 



Very-High-Speed Marine Craft 



Foilcraft, hovercraft, and very-high-speed displacement craft clearly need 

 unconventional propulsion devices, since the conventional open marine screw 

 cannot be developed to perform efficiently under the extreme speed and cavita- 

 tion conditions at which these vessels operate. For such craft it is particularly 

 important not to consider the propulsion device in isolation but as part of the 

 overall propulsion system. When this is done it would seem from information 

 presently available that the overall efficiencies of fully cavitating propellers, 

 and of current waterjet installations, are not sufficiently different to be deci- 

 sive. Further, almost all present and projected very -high -speed merchant 

 ships of this kind are not intended for long-range operation, and thus fuel con- 

 sumption and cost are less important in the overall assessment of priorities. 

 Equally, it must be recognized that such craft represent only a very small part 

 of the whole world merchant and naval fleets, and this is most unlikely to change 

 for many years. Development of propulsion devices for high-speed marine craft 

 can easily absorb a disproportionate part of the total effort available for such 

 activities. 



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