Unconventional Propulsion — Silver leaf 



merchant and naval fleets of the world; major changes in the dimensions, 

 speeds, and powers of conventional merchant ships have been accompanied by 

 equally significant changes in naval vessels, and by other spectacular, and per- 

 haps technically more challenging, innovations in high-speed marine craft, of 

 which the most dramatic has been the birth of the hovercraft or waterborne air- 

 cushion vehicle. Largely because of these striking and somewhat unexpected 

 developments, there has been growing activity in exploring the value of propul- 

 sion devices which could supplement the long-established conventional marine 

 screw propeller. These devices cover a very wide range of types and possible 

 applications; some, like ducted, controllable -pitch, and contrarotating propel- 

 lers, have been in regular, if limited, use for many years; others, like fuUy- 

 cavitating propellers and waterjet systems, have undergone considerable engi- 

 neering development in prototype installations; a third group, which includes 

 air-blown ramjets and magnetohydrodynamic devices, are still in the early 

 stages of laboratory investigation and are, in some cases, little more than 

 "ideas in principle." 



Faced with this diversity of possible propulsion devices, and by a barrage 

 of technical and other literature extolling the virtues of each one, the designer 

 of even a relatively conventional ship is faced with a difficult choice; for the de- 

 signer of an unorthodox, advanced marine craft, the choice is often bewildering, 

 and is not made easier by the apparently different standards and criteria used 

 by the advocates of many of these propulsion devices. The principal aim of this 

 paper is to suggest some general criteria, not all of which can readily be quan- 

 tified, which can help in making the best choice of propulsion devices for ships 

 and marine craft of many different types. 



There are several recent papers which ably summarize and compare tech- 

 nical features of different marine propulsion devices (Refs. (1, 2, and 3) are 

 examples), and many papers, including those at the present Symposium to follow 

 this review, which discuss individual devices in considerable detail. For this 

 reason, among others, this review will not contain new information about de- 

 vices which are used for marine propulsion, or are proposed for such purposes. 

 However, many papers about marine propulsion devices tend to emphasize se- 

 lected aspects of their performance, generally concentrating on hydrodynamic 

 efficiency, sometimes including cavitation susceptibility and associated noise 

 generation, but frequently say little or nothing about engineering and operational 

 features, which are often more decisive in the choice both of power plant and 

 propulsion device. While such hydrodynamic studies are necessary, they are 

 far from sufficient; indeed, high efficiency is but one factor among many, and 

 reliability, liability to cause vibration, compactness, simplicity, low first cost 

 and low direct and indirect maintenance costs, are generally of more impor- 

 tance in arriving at the techno -economic balance which determines the final 

 choice. 



Thus, in assessing the prospects for the widespread use of any unconven- 

 tional marine propulsion device, it is essential to recognize that the choice and 

 design of a propulsion system for a ship must not be considered as a series of 

 separate and isolated units, each selected to have maximum component effi- 

 ciency, but as an integrated whole in which the characteristics of main machin- 

 ery, propulsion device, shafting or other connections, and needs for auxiliary 

 power must be closely related. Finally, it is also desirable to recognize that 



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