108 ELECTRIC PROPULSION OF SHIPS. 



herent electrical losses in. the latter. Generally speaking, the net economy of a properly de- 

 signed and constructed geared-turbine drive should be better than a turbine-electric drive, 

 even though full advantage is taken of every practical source of gain in the latter. The 

 difference, however, is perhaps hardly of sufficient magnitude to be the dominating factor 

 in arriving at a final selection. The economy of a reciprocating engine drive is obviously 

 poorer than that of a properly designed turbine drive of either type, for the reason that the 

 reciprocating engine cannot utilize the same expansion of the steam. 



It has been advanced that the turbine for the turbine-electric system, being a one-direc- 

 tion rotation machine, can utilize higher superheat than the geared turbine, because of the 

 reversing element of the latter, thus resulting in better overall economy. With the present 

 installations this condition is true to some extent. However, with proper attention given to 

 this factor, geared turbines can be designed to operate without detriment with steam super- 

 heated 150° F. to 200° F., which is fairly close to the practical limit of superheat on board 

 ship, as there are other items aside from the turbines which are affected by high superheats 

 to the extent of fixing limitations. 



Obviously from a fuel-consumption standpoint, the direct Diesel and Diesel-electric pro- 

 pulsive equipments offer by far the best economy. Of these two types, it would appear 

 offhand that the direct Diesel drive is decidedly superior to the Diesel electric; however, 

 it will be shown later that there are certain limitations in the direct Diesel drive which offset 

 its advantage in fuel per brake horse-power hour over the Diesel electric. Here again it is 

 obvious that all items related to the propulsive equipment must be taken into consideration 

 to obtain the ultimate answer. 



Weight. — The item of weight would probably show more variance than other items 

 owing to differences of arrangement, design, foundations, etc., in the practice of the several 

 shipbuilders. The importance of weight depends upon the type of ship, cargo and trade 

 route. Usually, however, weight is an important item as it has a direct effect upon the total 

 fuel consumption and the amount of cargo that can be carried with a given displacement. 

 Here again it is necessary to consider all items related to the propulsive equipment. A 

 correct comparison of weight necessitates that the equipments imder consideration be on 

 the same basis relative to overload, factor of safety and arrangement. This is particularly 

 important in comparisons between electric-drive equipments. Where one equipment affords 

 an advantage in flexibility of reserve, however, it is usually at some sacrifice in weight, and 

 an allowance must be made. 



Space. — Space is important in that it has a direct bearing upon the bulk of cargo that 

 can be carried. Its relative importance depends somewhat upon the location of the ma- 

 chinery, i. e., whether the machinery is located amidship or aft, or both. Space is affected 

 by the distribution of machinery, and the relative saving depends somewhat upon the prac- 

 tice of the various shipbuilders in that respect. General analysis of the space factors which 

 have been made thus far gives the advantage to the direct-connected Diesel and Diesel-elec- 

 tric types of propulsive equipment, the saving being effected by the elimination of the boilers 

 and reduction of fuel and water tanks. Of the turbine-electric and geared-turbine types, the 

 space factor is in favor of the geared turbine, except in special cases. The general arrange- 

 ment of the engine-room, number of propellers and the beam of the ship have a direct 

 effect upon the space occupied as a result of machinery distribution. By locating the con- 

 densers underneath the turbines in electric drives, the total floor space can be greatly 

 reduced. 



Cost. — ^A definite comparison of costs is still a difificult task, owing to the continued 



