282 ELECTRIC PROPULSION OF MERCHANT SHIPS. 



and speed ; then the same turbine was connected to the same generator without gear- 

 ing, and tests were run with the same conditions and the same degree of steam 

 flow. All this was done on a testing stand where conditions are uniform and accu- 

 rate, the gears ran with perfect smoothness, and all conditions were favorable. Since 

 the comparison gives the loss of two gears, the differences are considerable and 

 the determination should be very close to the correct value. This test showed that 

 the performance of a single gear is as follows : — 



In addition to these gear losses, we must also consider the loss in friction of 

 the reversing turbine, which is estimated from reliable data to be 28 horse-power, 

 and we must also consider the bearing losses on about 100 feet of shaft, which in 

 perfect alignment will be 8.5 horse-power. These additional losses reduce the trans- 

 mission efficiency to 93.5 per cent, leaving only 1.9 per cent advantage to the gear- 

 ing. With shaft more or less out of line and gears operating under sea condi- 

 tions, it is probable that the losses given would be greatly increased. Noise is an 

 indication of loss, and most marine gears are at times noisy, while the gears 

 in this test were almost silent. The gears tested in this case were of the General 

 Electric Alquist type, and it might be claimed that other kinds of gears would be 

 more efficient, but it is obvious that, under' fixed load and with similar gear speeds 

 and diameters, there could be no advantage in any other type even if it ran with 

 equal smoothness. 



Cost. — In the present condition of prices it is very difficult to compare costs, 

 but the cost estimates of the General Electric Company on electric equipments for 

 cargo boats and geared equipments of recent design indicate that the electric is 

 slightly cheaper. If we consider savings in shafting support, shaft alley, oiling 

 system, etc., the saving with electric drive should be as much as 20 per cent of 

 the cost of the driving machinery. 



Propeller Speeds. — In ships requiring less than 3,000 horse-power, there is 

 some practical disadvantage in using propeller speeds below 100 revolutions per 

 minute because of the large number of motor poles required if a high-speed turbine 

 is adopted. Studies recently made by the Navy Department and elsewhere have 

 indicated that there is practically no disadvantage in using a propeller speed of 100 

 revolutions per minute on an 11 -knot, 2,400-horse-power ship, but in all cases of 

 electric drive the matter of propeller speed should be carefully studied. In ships 

 of higher power it is not desirable to use extremely high turbine speeds, and there- 

 fore there can be no difficulty about propeller speeds. Even in low-power ships, 

 lower turbine speeds could be used if expedient, but this is disadvantageous to a 

 small turbine, and the relative advantages and disadvantages should be duly 

 considered. 



