THE SUBMARINE OF TO-DAY AND TO-MORROW. 213 



In this discussion it has been assumed throughout that the type of ma- 

 chinery used, both for surface and submerged propulsion, viz., Diesel engines and 

 electric motors, will be adhered to in the near future. The various substitutes 

 which have been proposed, particularly with respect to the submerged power plant, 

 are all as yet in the early experimental stage. Even should any of them achieve 

 such success as to warrant the abandonment of prevailing types of machinery, the 

 results will not show in the defense "submarine of to-morrow," but rather in "those 

 of the day after to-morrow," which are beyond the scope of this paper. 



It remains still to consider the offensive or fleet submarine. If this type is to 

 accompany the battle fleet, a high surface speed is an absolute necessity and with 

 this must be combined a long effective radius. The question as to whether the fleet 

 submarine really belongs among "the submarines of to-morrow" or not depends upon 

 the surface speed which is considered adequate. If a speed of from 20 to 21 knots 

 will suffice, the answer is certainly "yes," but if 25 knots or more are required, 

 there is, to say the least, serious doubt with respect to the matter. 



As the crux of this question lies in the matter of the surface power plant, it is 

 necessary now to examine this subject in some detail. 



At the present time, almost all modern submarines depend for surface propul- 

 sion on Diesel engines, the only exceptions being a certain number of French boats 

 equipped with steam engines. The Diesel engine is in fact the ideal motor for this 

 purpose on account of the following qualities: — 



(a) Compactness and moderate weight. 



(b) Great economy in fuel consumption. 



(c) Ability to start instantly. 



(d) Ease in disposition of products of combustion. 



(e) Freedom from excessive radiation or storage of heat. 



In all respects, except quality (a), the Diesel engine is vastly superior to the steam 

 engine and hence is to be preferred. 



The conditions under which any engine must work in a submarine are very un- 

 favorable. The space and weight available are strictly limited and the demand for 

 speed often so insistent as to force the use of comparatively light-weight, high-speed 

 engines. Complete reliability of operation under such circumstances can only be at- 

 tained after an adequate period of development. The submarine Diesel engine, while 

 a newcomer in the field, has even now been developed to a point where, for moderate 

 powers, entirely satisfactory reliability is obtained, when extremes in the way of 

 light weight and high speed are avoided. The best engineering talent in all countries 

 is being devoted to the development of these engines, which is in fact making rapid 

 strides. Up to the present time, the maximum power per unit which is actually in 

 successful service is about 1,200 horse-power, but considerably larger units are under 

 construction. As is natural, owing to the short time in which this type of engine has 

 been under development, uniformity of practice has not been reached. In Germany, 

 and in other continental European countries, except France and Russia, very high- 

 speed, light-weight engines are most in favor with the two-cycle type generally pre- 



