THE Franklin Institute is not responsible for the statements and opinions advanced by contributors to the JOURNAL. AN ACCOUNT OF EXPERIMENTS MADE BY A BOARD OF By Chief-Engineer ISHERWOOD, U. S. Navy. In the following Table No. 1 will be found the data of the experiThe quantities therein given are so fully described as to ments. need no further explanation. The friction of the engine, per se, that is to say, the pressure required to work the unloaded engine, is taken at 2 pounds per square inch of the pistons of the two cylinders. The remainders of the indicated pressures, after deduction of these two pounds, are the net pressures upon the pistons. In order to obtain a single expression for the indicated and for the net pressures, they have been reduced to what they would have been had the engine consisted of the large cylinder alone. WHOLE NO. VOL. CXII.-(THIRD SERIES, Vol. lxxxii.) 6 EXPLANATION OF TABLE No. 2. In Table No. 2 will be found the distribution of the power during each of the experiments whose data are recorded in Table No. 1, the capital letters at the heads of the columns, in both tables, indicating experiments with the screws designated by the same letters. This distribution is necessary in order to understand what portions of the indicated horses-power developed by the engine are expended in the different operations connected with the propulsion of a vessel by a screw. Of the indicated horses-power a portion is consumed in working the engine, per se, that is to say, the unloaded engine; and this portion must first be deducted, because, until the friction of the engine is overcome, no power can be applied to the screw, or externally of the engine; for it is obvious, that until the friction of the engine itself is counterbalanced, the piston cannot move. After the deduction of this power, the remainder of the horsespower, called the net horses-power, is applied to the crankpin and does external work. A friction attends the net power, additional to that of the unloaded engine and proportional to the net power, let the latter be what it may; and the power required to overcome this friction is the power absorbed by the friction of the load, the articulations of the engine moving under the net pressure producing greater friction on them than is due to the mere weight of the moving parts and to the pressure of the packings. Then, there are the horses-power expended in overcoming the skin or surface resistance of the screw blades experienced from the water in which they move. This power can be calculated independently when the data are known. There still remain to be determined the portions of the net horsespower expended in the slip of the screw and in the propulsion of the vessel. These are ascertained as follows: The sum of the powers expended in overcoming the friction of the load and in overcoming the resistance of the water to the surface of the screw blades being deducted from the net horses-power, the remainder is divided between the power expended in the slip of the screw and in the propulsion of the vessel in the ratio of the speeds of the two; the pressure exercised by the screw forward in propelling the vessel, and backward upon the receding mass of water, constituting the slip of the screw, being the same. Hence, if the aforesaid remainder of power be mul tiplied by the speed of the slip expressed in fractions of the axial speed of the screw, the product will be the power expended int he slip, which being subtracted from the above remainder leaves the residue as the power expended in the propulsion of the vessel. On these general principles, the corresponding quantities in Table No. 2 have been calculated. For facility of reference, the quantities have been grouped and the lines containing them numbered. Line 1 contains the dates of the experiments. Line 2 gives the speeds of the vessel per hour, in geographical miles of 6086 feet. Line 3 gives the slips of the screws in per centum of their speed, calculated from their pitches into the number of their revolutions made per hour, and from the speeds of the vessel on line 2. On line 4 are the thrusts of the screws in pounds during each experiment; these quantities are what would have been given by a dynamometer had one been applied directly to the screw shaft and fulcrumed on the vessel. They are calculated from the quantities on line 19, by multiplying the latter by 33,000 and dividing the product by the speed of the vessel in feet per minute. Line 5 contains the number of double strokes made per minute by the pistons of the engine, and of revolutions made per minute by the screw. The quantities on lines 1, 2, 3 and 5 are taken from Table No. 1 and added for the sake of completeness to Table No. 2. Distribution of the Indicated Horses-Power. Lines 13 to 19, both inclusive, show the distribution of the indicated horses-power among the operations contingent on the propulsion of the vessel by the experimental screws. Line 13 contains the indicated horses-power, the quantities being taken from Table No. 1; and line 15 contains the net horses-power applied to the crankpin, also taken from Table No. 1. Line 14 contains the horses-power expended in working the engine, per se, or unloaded, the quantities being the difference between those on lines 13 and 15; they can, however, be obtained independently by calculation, the data being the speed of the piston per minute in feet obtained from line 5, the area in square inches of the piston of the large cylinder alone, and the pressure on line 7. Line 16 contains the horses-power absorbed by the friction of the load; these quantities are obtained by multiplying those on line 15 by 0·075, that fraction being the coefficient of the friction of the load. Line 17 gives the horses-power expended in overcoming the |