It may be taken as fairly correct that the N.H.P. of the old
jet condensing engines when well made was about 80 per cent,
at full speed. In the newer engines with surface condensers, and
the circulating of water done by centrifugal pumps, the efficiency at
full speed may have been 85 to 87£ per cent.; in newer engines
still, having three cranks and only the air pump worked by the
main engine, 90 to 93 per cent, is not an extravagant estimate for
the best made vertical ones, judging by some experiments made on
engines running with and without the propellers.
The ordinary friction of a marine engine practically varies with
the revolutions, for although at very slow speeds the friction per
revolution increases, the rate of revolution is then much less than
generally obtains when dealing with propeller problems.
In a general way the friction of an engine is proportioned to the
size of its cylinders, that is, to the nominal horse-power ; for this
piirpose Norn. H.P. may be estimated by multiplying the diameter by
the stroke of the low-pressure cylinder (both in inches) and dividing
by 15 for a compound, 13 for a triple, 10"5 for a quadruple engine.
A fair allowance for internal resistance of a modern engine is
"006 per Nom. H.P. per revolution.
Example. — What is the frictional or internal resistance of a triple
engine having cylinders 20 inches, 32 inches, and 52 inches diameter
and 36 inches stroke, when running at 150 revolutions ?

Norn. H.P. = 52 x 36 -f 13 = 144.
Friction=*006 x 150 x 144 = 129*6 I.H.P.
At this speed the I.H.P. would be about 1800.
The efficiency is therefore 92*7 per cent.
128 MARINE PROPELLERS.
