192 ALQUIST GEARING FOR SHIP PROPULSION. 
Interchangeability—Such interchangeability as this power of disconnection gives con- 
stitutes one of the most important advantages of electric drive in such a ship. With one 
motor out of eight in trouble, only one-eighth of the maximum capacity is lost and the 
ship’s maximum speed is impaired by only about 1 knot. If a generator or turbine is in 
trouble, the maximum speed is reduced only about 2 knots. With two generating units and 
four motors out, the ship can make 26 knots; and with three generators and four motors out, 
she can make 19 knots. If parts give trouble, they are simply cut out and repaired at leis- 
ure or as opportunity affords. The value of this interchangeability in a warship can hardly be 
overestimated ; it largely overcomes the military danger incident to accidents to the driving 
power, a danger which has always been of primary importance. 
Backing Power.—In turbine-driven ships, not electric, it is necessary to provide backing 
turbines which must run idle in the reverse direction when the ship is going ahead. These 
backing turbines involve complications which are very objectionable, and if these are reduced 
to a desirable minimum the backing power will be greatly inferior to that easily provided 
with electric drive. Experiment has shown that a turbine forced in an opposite direction in- 
volves about seven times as much friction loss as when driven in its normal direction. This 
loss is very appreciable in the backing turbines of ships and causes heating which must be 
dissipated by circulating steam, which causes further loss and waste. There are also seri- 
ous difficulties and dangers in high-speed apparatus incident to the abrupt and wide changes 
of temperature where steam is suddenly admitted to a cold reversing turbine. Steam which 
expands without doing work has a high temperature and, if work is done upon it by a re- 
versed turbine, the heat is considerably increased. With superheat these temperatures may 
be extreme and they affect the main turbine as well as the other, since it also must be used 
for reversal of direction and since the two often occupy the same casing. The mechanical 
effect of such abrupt change of steam temperature upon fragile structures presenting very 
extended surfaces is a matter which has been little considered but- which has caused much 
serious trouble. With electric drive the turbine never need be stopped when the ship is 
under way, and its operation is easier than in a power station because the load is steadier. 
With modern high-speed turbines, superheat affords large gains in fuel economy, and the 
ability to safely use high degrees of superheat may constitute one of the important reasons 
for adopting electric drive. 
Efficiency of Transmission.—Various statements and estimates have been published con- 
cerning losses in high-speed gearing for which the authority is not known by the writer. 
Tests at Schenectady have shown losses about double those generally estimated, with rapid 
diminution of efficiency as load falls. These tests were of a most accurate character with 
large smooth-running gears and by the motor generator method where losses alone were 
measured, the same power being transmitted with and without the gearing. 
With the electric drive in these battle cruisers, the losses from turbine to propeller 
shaft at full speed will amount to 7 per cent, and by adjustment of voltage the transmission 
efficiency can be kept equally good at all speeds. Considering gearing losses, reversing tur- 
bine losses, losses incident to necessary subdivisions of the turbine and additional packings, 
it is believed that the actual transmission from turbine blades to shaft at full speed cannot 
be made more than 2 per cent better in the geared equipment than it is in the electric. At 
reduced loads the geared equipment will be much less efficient. The normal cruising speed 
of these ships will require less than one-tenth of full load, and the drag of gearing, bearings, 
reversing turbines and idle parts of main turbines will be very serious. Some of these mat- 
