Here W = weight in tons.
P = working pressure as on gauge.
S = heating surface, in square feet.
D = diameter, in feet.
L = length, in feet.
C = a constant divisor, depending on the class of
riveting, etc. For boilers to Lloyds' rules,
and with iron shells having 75 per cent.
strength of solid plate, C = 13,200.

This formula, if correct--and it is almost strictly so--would give the
relative weight of boilers per sq. ft. of heating surface, for 105 lb.
and 150 lb. total pressure, assuming we wish to increase the efficiency
10 per cent, as follows:

Weight at 105 lb. = 105 x 1 / C

Weight at 150 lb. = 150 x 1.75 / C = 263 / C

Hence the ratio of weight = 263 / 105 = 2.5

In other words, the boiler with the higher efficiency would weigh two
and a half times that with the lower efficiency. In the case of a vessel
of 3,000 tons, with engines and boilers of 1,500 indicated horse power,
the introduction of locomotive boilers with forced draught would place
at the disposal of the owner 150 tons of cargo space, representing
L1,500 per annum in addition to the present earnings of such a vessel.