The marine boiler as now made is very efficient, but if the quantity of
steam used be considered in relation to the increased pressure, it will
be seen that the boiler of to-day is little if anymore efficient than
that of ten years ago. The present boiler has an evaporative efficiency
of about 75 per cent., and cannot be much improved so long as air
is supplied to the furnace by the natural draught. To increase the
efficiency from 75 to 82.5 per cent. would require about double the
heating surface, the weight of boiler and water being also doubled,
while the gain would be only 10 per cent. Mr. Blechynden's formula, used
in Mr. Marshall's works for weights of cylindrical marine boilers of the
ordinary type, and for pressures varying from 50 lb. to 150 lb., is as
follows: