Prospects for Unconventional Marine Propulsion Devices 



For these reasons the speed-displacement ratio may be regarded as the 

 simplest single parameter which is of use in defining the desirable overall 

 characteristics of the propulsion system. Some useful guidance affecting the 

 development of marine propulsion devices can be obtained from Table 3 and 

 Figs. 2-8. Thus: 



(i) At low speed-displacement values, corresponding to those for most 

 merchant ships, power-displacement ratios are low, and machinery 

 and fuel weight ratios are not high enough to justify expensive 

 light-weight propulsion systems. Equally, even where propulsive 

 efficiency and low power requirements are important, they are 

 seldom dominant factors in determining the type of propulsion 

 system. 



(ii) At high speed-displacement values, corresponding to those for 

 high-speed marine craft, it is essential to minimize machinery 

 power and weight if reasonable range and payload are to be ob- 

 tained. 



(iii) At intermediate speed-displacement values, corresponding to those 

 for destroyers and similar craft, it is difficult to choose the power 

 plant unequivocally. Improvements in propeller efficiency are de- 

 sirable but unlikely to have a major effect on design criteria. 



TYPES OF MARINE PROPULSION DEVICES . '■] 



General Classification 



In addition to the marine propulsion devices which already exist, there are 

 many other possible types. These can be classified in several ways and, as 

 suggested in Table 4, it is perhaps most convenient to divide them into the two 

 main classes of reaction-screw-type devices and pure jet devices. 



Reaction-screw-type devices may have many variants. Indeed, it is possi- 

 ble in principle to specify well over two hundred apparently different types of 

 device since: 



(a) The blade section shapes of rotating or fixed parts may be either 

 fully wetted, fully cavitating, base vented, or airscrews may be 

 used. 



(b) The axis of rotation of the propeller or pump may be either longi- 

 tudinal, vertical, or transverse. 



(c) Single- or multi-stage pumps or propellers may be used. 



(d) The pitch of the blades may be either fixed or controllable. 



(e) The rotor may be either open (unshrouded), or enclosed in a duct 

 or shroud which may be either long or short. 



903 



