TRANSACTIONS OF SECTION G. I'^l 



horse-power expected aud to settle the initial pressure, for on this point there is 

 still much timiditj- among steam users, so that the engine builder has to adapt 

 himself in this, and other particulars, to the needs or prejudices of his customer. 



In marine engines, again, the difficulty is still greater, because the only measure 

 of the effective power of the engines is the speed of the ship under given conditions 

 of immersion. But the resistance of ships is a complicated matter, not perfectly 

 ascertained yet, so that the speed attained in any new combination of engines and 

 hull is by no means a certainty ; hence some recognised measure of the power of a 

 marine engine, depending only on the measurement of the cylinders and boilers, 

 becomes very desirable. 



So strongly has the want of a standard horse-power been felt by shipbuilders 

 and marine engine makers that the council of the North-East Coast Institution of 

 Engineers and Shipbuilders appointed a Committee to investigate the subject, and 

 to devise, if possible, a set of rules which would be generally acceptable. The 

 Committee made its report in the spring of 1888. They took as their basis the Indi- 

 cated Horse-Power, under certain normal conditions, and propose to call this the 

 Normal Indicated Horse-Power (N. I. II. P.). The normal conditions are, briefly, 

 the following: — 



1. That the steam, of whatever boiler-pressure, is expanded to the same 

 terminal pressure. 



2. That the expansion is effected by all engines with the same degree of 

 efficiency. 



3. That the piston speeds of engines of different lengths of stroke are propor- 

 tional to the cube root of their respective strokes, and, further, that the actual 

 loaded trial-trip value of piston-speed may be taken as 144 times the cube root of 



the stroke in inches (l44«yS"J- 



4. That in cases in which the engines and boilers bear to each other such pro- 

 ])Ortions as to prevent condition 1 from being fulfilled without thereby violating 

 condition 3, the coal consumption per I. II. P. will not be affected, but will be 

 constant for the same boiler-pressure. 



5. That the boilers are constructed in accordance with what will be generally 

 recognised as the average practice of the present day in respect of the allowance of 

 steam room in relation to power,"the diameter, area, and pitch of tubes, the rela- 

 tion of grate to heating surface, and the area of uptakes and funnel ; that average 

 natural chimney-draught is used, or, if forced draught be employed, that it does 

 not exceed the natural draught ; that the horse-power is proportional to the heating 

 surface (II.) and to the cube root of the pressure (^P.) ; and, further, that the 

 actual loaded trial-trip horse-power may be taken as equal to one-sixteenth of the 



heating surface multiplied by the cube root of the pressure ^ "^ '' 



6. That the efficiency of the engine mechanism is constant, and tliat the pro- 

 peller is such as to secure that the engines will utilise the boiler power referred to 

 in condition 5 in the manner prescribed by conditions 3 and 4. 



Subject to these conditions the normal indicated horse-power is found by mul- 

 tiplying the square of the diameter of the low-pressure cylinder in inches by the 

 cube root of the stroke in inches, adding to the product three times the heating 

 surface of the boiler in square feet, multiplying the sum by the cube root of the 

 pressure, and dividing the product by 100. 



i^ t „ p _( D->ys-f3ii)yp - 



It is evident from this formula, and from the conditions, that account is takea 

 of all the variables, aud that the boiler is regarded as an integral part of the 

 engine. The report gives several useful formulse deduced from the above. 

 Whether the expressions given are the most convenient possible for general marine 

 practice or not I am not competent to say, but it seems to me that a step has been 

 taken in the right direction in the attempt which has been made to measure marine 

 engines by some rational standard. The members of the Committee were all 



