In practice, the writer has found
1.4 square feet per indicated horse power to maintain a steady vacuum of
271/2 inches.
Mr. Marshall has just completed six pairs of engines for three twin
screw ships, having steel shafts of 10 inches diameter, and has in each
case run the engines at 120 revolutions per minute, while indicating
1,380 horse power from each pair for ten to fifteen hours without
stopping; and in no case has a single bearing or crank pin warmed or
had water applied, the surfaces on examination being perfect. In these
engines all working bolts, pins, and rods, except the piston and
connecting rods, are of steel, all rods in tension being loaded to 8,000
lb. per square inch. The boilers are of the Navy type, made throughout
of Siemens-Martin steel plates, riveted with steel rivets, all holes
drilled. Furnaces are welded and flanged; the tubes are of brass. In
comparison with an ordinary merchant steamer's iron boilers of the
double ended type, they weigh, including water and all appurtenances, as
follows:
Double ended Type. Navy Type.
Weight, tons............ 135 ........... 146
I. H. P................. 1,400 .......... 2,760
Draught................ Natural ......... Forced.
SCREW PROPELLERS.
The screw propeller is still to a great extent an unsolved problem.
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