MARINE ENGINES. 5 13

lengthened, with a view to gain these important advantages, whilstretaining tlieir original engines and boilers.

The following extracts from a recent paper on this subject willshow the conditions involved in this question . 1

“ It is an immediate consequence of the fundamental definitionsof mechanics that if the gross indicated horse-power required topropel a vessel at the rate of V nautical miles per hour be denotedby E, then the gross resistances to the motion of that vessel is

E

expressed by a quantity of the form C y , in which C is a coeffi-

cient dependent upon the construction elements and developmentof power in the engines, or, in an equivalent, but directly oppositepoint of view, the construction elements of the hüll and circum-stances affecting the expenditure of the developed power.

Now, a first approximate Statement of the problem of steamshippropulsion, according to mechanical principles as stated by Smeatonand other mechanicians, is furnished by the well-known formula,M V s

C = -g—; sometimes termed the ‘Admiralty Formula,’ in which M

is the immerged midship area, E the gross indicated horse-powercorresponding to the trial speed, V nautical miles, and C, a numeri-cal coefficient, which, in the cases where M, E, and V are known, iseasily calculated, and which, subject to the following reservations,has been generally adopted as an approximate measure of theefficiency. It is found, however, in trying experiments on the samevessel at different speeds, where there would seem to be no goodreason for supposing great variations in efficiency, the coefficientsthus found vary in an extraordinary degree; also, in passing fromone vessel to another, the variations have been found irregulär, con-tradictory, and perplexing to a degree which, without the aid ofother considerations, principally derived from experience of thevalues which have been obtained in vessels of similar dimensionsand speed, would render these coefficients of doubtful value as ameasure of efficiency; and, although a coefficient of efficiency isundoubtedly involved in this quantity, all experience points to theconclusion that it involves variable elements, and the hypothesis thatit is a constant of one term, is not in accordance with the physicalconditions of the problem, and that this formula requires correctionfor ignored and misrepresented elements.

1 See paper “on Steamship Efficiency,” by Mr, Robert Mansel, Trans, Inst. Engineersand Shipbuilders in Scotland , vol. xxii.

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