The cut-off
in the high-pressure cylinder usually takes place at about 0.6, and
the ratio of expansion has decided the ratio of cylinders. The use of
separate starting valves in both cylinders obviates that necessity.
The difficulties in the way of taking advantage of the higher economic
properties of greater pressures than hitherto used on board ship, are,
it is submitted, not insuperable, and it would be to the interest of all
that they should be firmly and determinedly met. It may be accepted as
an average result that the Woolf engine, as usually arranged, will use
10 per cent. more steam than the receiver engine for the same power.
Of the three-cylinder receiver type the data are insufficient to form a
definite opinion upon; but so far the general working of the Arizona is
stated to be as good, economically, as any of the two-cylinder receiver
class. The surface condenser remains as it was ten years ago, with
scarcely a detail altered. In most engines it remains a portion of the
framing, and as such adds greatly to the weight of the engine.
It is a question seriously worth consideration whether or no the surface
of tubes can be reduced. The practice at present is to make the surface
one-half the boiler surface as a minimum, that is, equal to about 2
square feet per indicated horse power.
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