Prospects for Unconventional Marine Propulsion Devices 



(f ) For ducted or shrouded propellers the duct may have an accelerat- 

 ing or a diffusing nozzle, and may either be clear of vanes or have 

 stationary or rotating inlet or exit guide vanes. 



(g) Finally, the duct may be fixed in position, or be steerable so that it 

 can be used as a rudder or control surface as part of the propul- 

 sion device. 



Most commonly used screw-type propulsion devices have a fore-and-aft 

 axis of rotation; conventional open marine propellers, ducted propellers, con- 

 trollable pitch, contrarotating, tandem, and most waterjet systems are of this 

 type. However, vertical-axis propellers are not uncommon; the well-known 

 Voith-Schneider propeller, and the Flettner rotor, are examples of this type. 

 Paddle wheels are the most common form of device with a transverse axis, but 

 in principle the centrifugal pump in a waterjet system should be included in this 

 group. 



Pure jet types of propulsion device may be subdivided thus: [ '■ 



(a) Air jets: these may be like those used for aircraft (as in the 

 "Lucy Ashton" experiments), or water-augmented to increase the 

 density of the fluid at jet exit and thus increase the thrust. 



(b) Underwater jets: in principle these may be of three types: 



(i) Water as a working medium with water -reactive fuels 

 (ii) Air-blown ramjet or other hydropneumatic device 

 (iii) Magnetohydrodynamic devices. 



Pure jet types have not yet been used for marine craft except in a very lim- 

 ited experimental way. 



Need For Unconventional Devices 



The conventional open unshrouded marine screw with fully wetted sections 

 is a simple, efficient, reliable, cheap, and well-proven propulsion device, and 

 considerable research effort has been expended in its development, particularly 

 during the past twenty years. Why then should it be necessary to develop uncon- 

 ventional propulsion devices for ships? 



Research on propulsion devices has shown that this cannot be isolated from 

 research on hull forms and, in fact, the stern form, propulsion device, trans- 

 mission, and steering system must be regarded as a whole. If there are no re- 

 strictions on the size and weight of the propulsion device, and on its operating 

 rate of rotation, then in general the best performance will be achieved by a 

 slow-running screw propeller of large diameter. Restrictions on diameter are 

 always likely to be imposed by draught limitations, but restrictions due to diffi- 

 culty of manufacture are likely to be overcome. Restrictions on revolutions are 

 imposed by an insistence on using diesel engines as a primary mover with a di- 

 rect drive to the propeller, but the wider adoption of geared drives, either with 

 diesel engines or steam turbines, allows greater freedom of choice in propeller 



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