6o ENGINEERING PROGRESS IN THE U. S. NAVY. 



but only one of the systems has as yet been authorized, and that one is for 

 destroyers. It had not yet been tried out in service, but preUminary shop 

 tests show a good gain in economy of the main propelhng engines at cruising 

 speeds. This system, as applied to the destroyers, depends entirely for its 

 gain upon the greater efficiency of the reciprocating engine at the higher 

 steam pressures over the efficiency of high-pressure turbines of the reaction 

 and the high-pressure nozzles of the impulse type of turbines, no advantage 

 being gained from increased efficiency of propellers, as the reciprocating 

 engines are on the same shafts as the turbines. From some points of view 

 this combination is undesirable, and the gain in service must be considerable 

 to justify its retention.* 



With the other combination systems proposed, calculations indicate 

 that if the propulsive efficiency counted upon can be obtained, these systems 

 will all be very much more efficient than either a straight turbine or straight 

 reciprocating engine drive at maximum power, will hold a big advantage over 

 the straight turbine drive through all ranges of powers, and will hold its 

 advantage over the straight reciprocating engine drive until a minimum 

 speed of about 1 1 knots is reached, when the efficiencies become equal. 



The "if" exists, however, and is caused by the danger of the currents 

 thrown to the rear by the big reciprocating-engine screws seriously affecting 

 the rate of feed and direction of flow of water to the turbine propellers. In 

 addition, there may possibly be another source of loss due to heavy leakage 

 of steam through the large change valves which must be fitted to control the 

 paths of flow of the exhaust steam from the reciprocating engines. 



In all of these systems, to adapt them to naval requirements, it is neces- 

 sary to exhaust from the low-pressure cylinders of the reciprocating engines 

 at a pressure of not less than 25 pounds absolute, when this engine is operat- 

 ing at full power, and to by -pass as few of the stages of the turbine as possible 

 in order to obtain an increased economy of propulsion through a large range 

 of powers. 



TURBINE CHANGKS TO PRODUCE INCREASED ECONOMY. 



The Parsons turbine as it exists in our vessels to-day is, with very few 

 exceptions, the same as the turbines of this type which were fitted in the ini- 

 tial turbine vessel, the Chester. The only improvements which have been 

 made consist of changes in blade angles, particularly in the low-pressure 

 stages, an increase in the number of rows of blades in these same stages, and 

 the fitting of nozzles for the admission of auxiliary exhaust steam at several 

 different locations along the steam path. 



*Since the above was written, this system has been tried and expected economies have been more 

 than realized. 



