MARINE ENGINES.

SU

The power varies approximately as the cube of the speed of vessel(see p. 513).—A vessel of a certain form and tonnage, and of 200horse-power, has a speed of 12 knots per hour, and we wish anothervessel of the same lines and tonnage to attain a speed of 15 knotsper hour: the cube of 12 is 1728, and of 15 is 3375; thus—

3375 X 2001728

= 390 horse-power.

Thus very nearly double the power is required to give an additionalspeed of 3 knots per hour to the vessel, and this is a serious matterwhen we remember that the amount of coal consumed is in thesame ratio. For example: supposing the vessel had to steam over3000 knots, going at a speed of 12 knots per hour it would take250 hours to run the distance, and going at 15 knots per hour itwould take 200 hours; therefore taking the horse-power as indi-cated, and the coal consumed per horse-power per hour at 2^ lbs.,we have—

200 x 2*5 x 250 = 125,000 lbs. of coal.

390 x 2*5 x 200 — 195,000 lbs. of coal.

or, in round numbers, 31 tons niore coal is required to perform thesame distance at the quicker rate of speed. In the construction ofall steam vessels, respect must of course be had to the facilities forcoaling at the stations on which they are intended to run, and theircarrying capacity for coal adapted accordingly. Theoretically andpractically it has been proved, under all circumstances, that a lowrate of speed is the most advantageous in an economical point ofview. Thus when ships of war are cruising at sea it is advisable towork the engines at half-power, or such a fractional part of thepower as may be thought best under the circumstances; but on noaccount should the steam power be decreased in cases where thefull power may be demanded at a moment’s notice,—economy incoal being but a secondary consideration when the safety of thevessel is in question.

Proportion of power to tonnage. — In the merchant Service theproportion of power to tonnage for vessels of about 2000 tons oltonnage ranges from 1 to 5'6 to 1 to 3'8; in a number of vessels,with a horse-power ranging from 80 to 700, the average proportionwas 1 horse-power to 4'6 tons of tonnage. This is only the nominalor commercial horse-power allowed; but the actual or indicatedhorse-power depends on the lines and draught of water of thevessel, taking into consideration the speed that may be required.