ENGINEERING PROGRESS IN THE U. S. NAVY. 63 



6 per cent is estimated to be obtained. At 12 knots, the cruising speed, the 

 saving by the use of superheat hardly exceeds 3 per cent. 



The first experiences with the vessels fitted with superheat were far 

 from satisfactory, due to the rapid deterioration of the valves in the steam 

 lines. These valves had cast-steel bodies and cast-steel valve discs with 

 monel metal seats. The erosion and corrosion of the valve discs was very 

 extensive, and in a short period of service it became necessary to replace the 

 cast-steel valve discs with discs of monel metal. This substitution has been 

 satisfactory and no further trouble has been experienced. 



The superheat has been used only on battleships fitted with reciprocat- 

 ing engines or impulse turbines for propelling purposes and has not as yet 

 been used on any of the destroyers. 



Reduction of clearances, decrease of frictional resistances of steam 

 through the steam ports and reduced back pressures in the low-pressure 

 cylinders have all resulted from one very important change in the design of 

 engine cylinders and valve chests. The original design of these cylinders is 

 shown on Plate 40, and the modern improved design on Plate 39. 



The results obtained by this change from the long, tortuous port to the 

 short, direct port can best be shown by the following table of comparison of 

 cylinder clearances and steam velocities : — 



Old Type. New No. i. Neiv No. 2. New No. j. 

 Diameters of cylinders : — 



H.P 32i" 38I" 32" 35" 



I-P 53" 57" 52" 59" 



L. P 2-61 " 2-76" 2-72 " 2-78" 



Stroke of pistons 48 " 48 " 48 " 48 " 



Per cent clearances : — 



H.P 28 16.17 13-83 13 



I-P 20.8 13.17 12.65 13I 



L. P 18.5 12.485 12.02 12 



Steam velocity, feet per minute : — 



H. P 5,670 6,565 6,723 6,402 



I-P 8,262 6,678 7,883 7,517 



L- P 10,833 10,446 9,947 10,281 



Exhaust velocity : — 



H. P 5,480 5,340 5,289 5,078 



LP 6,670 6,180 5,909 6,087 



L- P 7,450 7,514 7,199 7,298 



To condenser 7, 390 6,637 6,612 6,500 



