144 ELECTRIC PROPULSION OF SHIPS. 



It is hardly fair to compare the New Mexico and Tennessee with the Idaho and Missis- 

 sippi, as the two latter are fitted with direct-drive main turbines and geared cruising turbines. 

 A considerable amount of the saving was due to the propellers. 



Mr. Thau neglects to state that, while the battleships and battle cruisers of our Navy are, 

 or are to be, fitted with the electric drive, the light cruisers and destroyers are being equipped 

 with geared turbines. The U. S. Navy refrained from using the latter in the battle cruisers 

 where weight is of great importance, because they did not believe that gears could be built 

 that would transmit the necessary power (45,000 S. H. P. per shaft). However, the British 

 Admiralty has been able to transmit 39,000 S. H. P. per shaft successfully. 



Commander S. M. Robinson, Member (Communicated) : — Mr. Thau's paper has so 

 completely covered the range of subjects involved in a discussion of electric propulsion that 

 there are not many new points that can be added, but I think some of them might be empha- 

 sized in order to bring out their very great importance. 



Turbo-electric propulsion is being used to such an extent at the present time and is so 

 well known to the marine engineering world that it hardly seems necessary to dwell much on 

 this subject, but Diesel-electric propulsion is a comparatively new art and still has its way 

 to make in the marine field. It seems to me to be an ideal system of propulsion for cargo 

 vessels. By its use it should be possible to greatly increase the reliability of Diesel engines, 

 since it will not be necessary to start them up under load nor reverse them. The importance 

 of these two points is very great and cannot be emphasized too much in any comparison of 

 direct-connected Diesel engines and Diesel electric propulsion ; submarine experience has shown 

 that many of the troubles with Diesel engines are directly traceable to these causes and, while 

 air starting is still fitted on these engines, it is only used in case of emergency when the mo- 

 tors are out of commission. ; 



With Diesel electric propulsion it will be possible to use multiple units, thus keeping 

 down piston diameters ; this will add greatly to the reliability and will also operate to reduce 

 the maintenance cost. It also makes it possible to carry out engine repairs on one engine at 

 sea without suffering any very great reduction in the speed of the ship. 



Diesel electric propulsion is frequently criticized as being about 12 per cent less efficient 

 than the direct Diesel drive. No general statement can be made which will cover all types of 

 ships, but for cargo vessels of the usual low horse-power it can be said that there will be little 

 difference in the overall economy of the two systems, and what difference there is will gen- 

 erally be in favor of the Diesel electric. This arises from the fact that it is necessary to use 

 twin screws running at comparatively high propeller speeds for the direct Diesel, while a 

 single screw of lower speed can be used with the Diesel electric ; with the latter arrangement 

 the losses in the struts are done away with and a more efficient screw is provided due to the 

 lower speed at which it runs. 



I believe that the voltage proposed by Mr. Thau is somewhat higher than should be 

 used on board ship, and this will not be necessary if the number of units is reduced; a three- 

 generator and two-motor installation would seem to give all the flexibility desired and at the 

 same time obviate the necessity for the use of such a high direct-current voltage. This will 

 give three combinations of generators and motors, the first being all units in use, the second 

 being two generators and two motors, and the third being one generator and one motor. At 

 the maximum capacity of each of these conditions the motor and generator efficiency will 

 be practically the same as at full power. 



