Prospects for Unconventional Marine Propulsion Devices 



while unconventional devices are the only practical means of propelling some 

 marine vehicles, for the overwhelming majority of conventional ships and for 

 most marine craft— even some relatively unorthodox vessels— the conventional 

 open marine screw propeller is both practical and highly efficient; it is not easy 

 to beat. 



GENERAL CONSIDERATIONS ^, . ^ ; V . v./":^ 



Basis of Comparison . :,( ,r ., - : 



Efficiency and other criteria which can be expressed in direct numerical 

 terms are seldom decisive in determining the choice of propulsive device for a 

 particular ship or other marine craft. This choice should be made as part of an 

 overall system design, in which the real target is minimum total operating cost 

 to carry a specified payload over a stated range at optimum speed. The payload 

 may be either a deadweight cargo such as oil, a light-weight load of passengers, 

 or a weapons system or other mixed weight and volume load. The range is gen- 

 erally an independent operational variable, but optimum speed, though often 

 treated as another independent factor, should more properly be regarded as a 

 derived variable, depending on payload and range. Since total operating costs 

 include both direct costs for fuel, crew, and maintenance, and also indirect 

 costs which reflect initial capital expenditure, any attempt to minimize total 

 costs will ensure that the most efficient ship has the most suitable propulsion 

 system. The power plant and the propulsion device themselves affect the prin- 

 cipal characteristics of the ship; dimensions, shape and displacement to carry a 

 fixed payload will vary with the required power output and with the power-weight 

 ratio and specific fuel consumption of the primary mover. For these reasons 

 realistic comparisons of different propulsion devices should, in principle, form 

 part of complete design studies for particular vessels, but clearly this is not 

 practicable here. 



A more limited, but reasonably realistic, basis of comparing different types 

 of propulsion device is to consider their application to ships with total displace- 

 ment, speed, and range all fixed. The emphasis is then placed on the propulsive 

 efficiency of the device and the corresponding engine power required; the over- 

 all weight of the propulsion system and of the necessary fuel will then depend 

 primarily on the primary mover selected, and this will in turn affect the avail- 

 able payload, which can be expressed, if desired, as a transport efficiency cri- 

 terion. Although far from entirely satisfactory, such an approach is better than 

 comparisons which consider different propulsion devices in isolation, without 

 taking any serious account of their interaction with the ship which is to be pro- 

 pelled. On this basis of comparison, some efficiency criteria can be used to 

 give general guidance about the likely prospects for unconventional marine pro- 

 pulsion devices. 



Propulsive Efficiency 



In the past, many accounts of novel propulsion devices have claimed advan- 

 tages based on inadequate or even incorrect efficiency criteria. Fortunately, 

 recent papers comparing different devices have adopted more realistic and cor- 

 rect criteria, but it is still important to stress that the definition and usage of 



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