BOILER. There is one boiler of the horizontal fire-tube type, with the tubes returned by the sides of the furnace. The shell is a horizontal cylinder of 49 inches outside diameter, and 6 feet 6 inches extreme length, with flat ends. The front end is the front tube-plate for the tubes, and the uptake is of sheet-iron, made separately, and bolted to the front of the shell. There is one furnace, and it is contained in a cylinder of 2 feet inner diameter, and 4 feet 11 inches extreme length. In this cylinder are the grate-bars and the bridge-wall. The grate-bars are 4 feet 3 inches long, and the average breadth of the grate-surface is 1.96 feet. The top of the grate-bars, at the front of the furnace, is one foot below the furnacecrown; and, at the back of the furnace, 1 foot 4 inches below this crown; the breadth of each grate-bar is inch, and the width of the air-spaces between them is § inch. The least water-space between the furnace and the shell is at the bottom of the latter, and is 3 inches wide, including thicknesses of metal. The opening for the furnace-door is a semicircle of 20 inches radius. The door is of wrought iron, hinged at the bottom and latched at the top. It has a perforated lining-plate for the distribution of air, and two registers for the admission of air above the incandescent fuel. The aggregate air-opening in the two registers is 13.5 square inches. The bridge-wall is an iron casting faced with brick. Its top is 6 inches above the top of the grate-bars, and its width is 5 inches. The height from the crown of the furnace to the top of the bridge-wall is 10 inches. The back smoke-connection has a flat top, a flat back, and a flat front. The sides and bottom are concentric with the boiler-shell, from which they are separated by a waterspace 3 inches wide, including thicknesses of metal. The flat water-space between the back of the connection and the end of the shell is 3 inches wide, including thicknesses of metal. The extreme height of the connection in the clear is 29 inches. The front of the connection is the back tube-plate of the tubes. The tubes are returned along each side of the furnace, the top of the upper row being 3 inches above the furnace-crown. The tubes are of iron, lap-welded. Six of them are 2 inches in outside diameter, and the remaining fifty-four are 2 inches in outside diameter. Their metal is of an inch in thickness. The tubes of each row, horizontally, are placed opposite the spaces between the tubes of the row, above and below. The least water-space between the tubes is of an inch in the clear. The tube-plates are of -inch thick metal, and the length of the tubes in the clear of the plates is 4 feet 10% inches. The uptake is a construction of sheet-iron separate from the boiler-shell, and bolted to it. The outer periphery is concentric with the boiler-shell, and the inner periphery is concentric with the furnace. The front projects over the fire-room 4 inches at the bottom and 13 inches at the top. On this inclined surface are two uptake-doors opposite the tubes. They are hinged at the top and latched at the bottom, and are of sufficient area to embrace all the tubes. From the top of the uptake, (at the level of the top of the boiler-shell,) which is rectangular in horizontal section, the chimney is drawn in to a circle of 10 inches inner diameter at the height of 20 inches above the top of the shell. At this height the upper cylindrical part, 4 feet 6 inches high, is hinged on. The chimney, for the whole height above the top of the shell, is surrounded by an air-jacket of 144 inches outside diameter, perforated with a row of holes at top and bottom. Immediately over the boiler-shell, and connected to it by a pipe of 8 inches diameter, is a boiler-plate cylinder with flat ends serving for steam-room additional to what the upper part of the shell contains. The inner diameter of this cylinder is 15 inches, and its inner length is 4 feet 11 inches. It is of -inch thick iron, and its upper part contains a dry-pipe, of 3 inches diameter, extending its whole length and perforated along the upper side. The steam-pipe to the engines is an extension of this dry-pipe. The hole in the top of the boiler-shell within the 8 inches diameter pipe is 4 inches diameter, and through it the steam passes to the cylindrical steam-room from the shell. The space between the top of the boiler-shell and the bottom of the cylinder is 34 inches. The cylindrical portion of the shell is of 4-inch thick iron. Its flat ends, and the flat back of the smoke-connection, are of inch thick plate. All seams are double riveted. In the front of the shell, opening into the uptake, is an elliptical man-hole with diameters of 11 and 14 inches. And in the lower portion of this front, beneath the uptake, are two elliptical hand-holes, with diameters of 24 and 5 inches. The entire exterior of the boiler-shell is felted, lagged, and covered with sheet-iron. The following are the principal dimensions and proportions of the boiler : Diameter of the shell.... Length of the shell proper.... 4 feet 1 inch. 6 feet 6 inches. Length of grate-bars. Area of grate-surface Total number of tubes.. Outside diameter of six of the above tubes Outside diameter of fifty-four of the above tubes... Height of the chimney above the level of the grate-bars. Cross area for draught over the bridge-wall.. Cross area of the chimney.. Heating-surface in the furnace.. Heating-surface in the back smoke-connection Heating-surface in the tubes, calculated for their inner circumference. Heating-surface in the uptake... Total water-heating surface Steam-superheating surface in the uptake Ratio of the water-heating to the grate surface....... Ratio of the steam-superheating to the grate surface Ratio of the grate-surface to the cross area over the bridge-wall. Weight of water in the boiler 7 feet 7 inches. 1 1.96 foot. 4 feet 3 inches. 8. 33 square feet. 60 24 inches. 2 inches. 4 feet 10 inches 10 inches. 140. 3494 square feet. 3. 4290 square feet. 185, 6657 square feet. 2.5153 square feet. 22.289 to 1.000 0.266 to 1.000 6.734 to 1.000 7.630 to 1.000 13.853 to 1,000 5,050 pounds. SPACE OCCUPIED IN THE VESSEL BY THE MACHINERY, AND ITS WEIGHT. The length in the vessel occupied by the machinery, including the fire-room, feedwater tanks, and coal-bunker, is 19 feet 8 inches. The feed-water tanks are placed along each side of the engines and boiler, so that the entire breadth of the vessel is occupied by the machinery and its appendages. The coal-bunker is forward of the boiler. The weights of the machinery are as follows, namely: Pounds. Net weight of the engines proper, including crank-shaft, but excluding piping, flooring, &c... Weight of the stern-bearing pipe in dead-wood, and the dead-wood stuffing-box 1,400 Weight of the screw-propeller... Weight of the line-shafting and its couplings 141 590 Weight of all the piping.. 250 Weight of the water in the boiler. Weight of the boiler, including grate-bars, bearers, chimney, and all doors and plates.... 150 5, 050 Weight of the felt, lagging, gum, putty, and paint on the engines and boiler 2,290 129 Total weight of feed-water and its tanks, and of coal and its bunker. Total weight of all objects in the engineer department..... or 12 tous. 16,800 26,800 SCREWS. The different screws employed in these experiments are of brass, and will be designated by letters. They are all of the same diameter, and have the same diameter of hub, except the Griffith screw H. Screws A, C, E and F, were formed in the following manner: Two true screws were very carefully swept up in the sand by the same moulder from the same iron guides, and were cast of the same metal at the same time. Each of these screws has two blades, one opposite the other, and is 54 inches long in the direction of its axis. The pitch is uniform, and, by accurate measurement of the screws after they were cast, 5.136 feet. If the blades are viewed in projection on a plane parallel to the axis, their forward and after edges are parallel to each other and at right angles to the axis. The outboard end of the screw-shaft was made to receive both screws at the same time, one being placed immediately in front of the other and touching, so that by bringing the after edge of the blades of the forward screw to coincide with the forward edge of the blades of the after screw, the propelling surfaces of both screws would be continuous, and they would thus form one two-bladed screw A, 11 inches long in the direction of the axis. Or, the blades of the after screw could be placed immediately behind those of the forward screw, in the direction of the axis, and they would thus form the Mangin screw F, 11 inches long in the direction of the axis. Or, the blades of the forward screw could be placed at right angles to those of the after screw, and thus form the four-bladed screw E, 54 inches long in the direction of the axis; for the fact that the blades of the after screw are recessed, as it were, 5 inches back of those of the forward screw, does not affect the results in the slightest degree, and the screw was the same as though the four blades had been on the same hub of 5 inches length. Or, one of the screws could be used alone, when it was the two-bladed screw C, 5 inches long in the direction of the axis. After the completion of the experiments with the screws formed as above described, one of them was cut through at right angles to the axis, so as to leave it 3 inches long in the direction of the axis and make the two-bladed screw D. By using screw D in connection with screw C, bringing their propelling surfaces to be continuous, the two-bladed screw B was formed 8 inches long in the direction of the axis. It will thus be seen that all the screws from A to F, both inclusive, are composed of exactly the same physical surface, governed by the same co-efficient of friction on the water, and have exactly the same helicoidal form; the results from them are, therefore, free from the doubt which attends trials of screws having different physical surfaces, and, consequently, possibly different helicoidal forms, and different co-efficients of friction, though intended to be exactly the same. Screw G is a three-bladed screw, with a pitch expanding gradually from 6 feet 6 inches at the forward edge of the blades, to 7 feet 6 inches at the after edge, making the mean pitch 7 feet, which it had by close measurement. The length of the blades, in the direction of the axis, at the periphery of the screw, is 7 inches; gradually increasing thence to 11 inches length, in the direction of the axis, at the radius of 19 inches; from which point it gradually decreases to 6 inches length, in the direction of the axis, at the hub. When the blades are viewed in projection on a plane parallel to the axis of the screw, their forward edge is nearly perpendicular to the axis. If the most forward part of this edge is made to touch this perpendicular, the contact will be at 19 inches radius, from which point the forward edge of the blade curves gradually back until it is, at the hub and at the periphery, 13 inch from the perpendicular. The thickness of the blade just above the fillet joining it to the hub, is 1 inch at the center. The weight of the screw is 250 pounds. Screw H is a three-bladed Griffith screw, formed by trimming the blades of screw G into the Griffith shape, and bolting between them a hub made of wood, to the figure of the frustum of a sphere 15 inches in diameter and 11 inches in height. This hub was well smoothed, painted, and varnished; its diameter is 0.28846 of the diameter of the screw, and both ends are flat and circular. The length of the blades, in the direction of the axis, at the periphery of the screw, is 34 inches, whence they curve gradually outward to the length of 11 inches, in the direction of the axis, at the radius of 19 inches, from which point they curve gradually inward to the hub, at which the length is 7 inches in the direction of the axis. When the blades are viewed in projection on a plane parallel to the axis of the screw, they are pear-shaped, and the forward and after edges are arranged symmetrically on both sides of a perpendicular to the axis passing through the center of the blades. The pitch expands gradually from 6 feet 8 inches at the forward edge of the blade, to 7 feet 4 inches at the after edge, making the mean pitch 7 feet. The fraction used of the pitch in function of the surface and of the propelling efficiency of the surface is 0.24. In the following table will be found the principal dimensions of the screws: For screws G and H, the mean pitch only is given, and the slip is always calculated for it. For these screws, too, the length given is the greatest length of the blades in the direction of the axis. Table containing the principal dimensions of the screws employed in the following experiments. MANNER OF MAKING THE EXPERIMENTS. Before commencing the experiments, a very excellent dynamometer was constructed and applied to the screw-shaft for the purpose of measuring the thrust of the screw. It consisted of a single vertical-lever, stiff enough not to spring under a considerably greater pressure than the screw was capable of giving, bearing by knife-edges of steel against a brass ring free to move on guides in the direction of the screw-shaft, and having a turned recess in which was a loose brass ring carrying lignum-vitæ plugs or cylinders projecting beyond both sides of the loose ring; both ends of the plugs are bearing-surfaces, and are flat and at right angles to the grain of the wood. These surfaces were kept flooded with oil during the trials. The knife-edges bore against pieces of steel let into the movable brass ring. The thrust of the screw was delivered against the lignum-ritæ plugs by a brass collar secured upon the screw-shaft abaft the regular thrust-collars. There were no collars on the screw-shaft abaft the dynamometer. The guides of the movable brass ring carrying the loose ring in which the lignumvitæ plugs were inserted, were two steel pins, one on each side of the shaft, fitting into holes of a little larger diameter bored through lugs cast upon the ring. An accurately graduated steel spiral spring was attached to the upper end of the lever, which end also carried a pencil that traced the line of pressures continuously on a sheet of paper secured around a horizontal large diameter revolving-drum which received its motion from the screw-shaft through worm-wheels and worms. The lower end of the dynamometer lever, the other end of the spiral spring, and the guides of the movable brass ring, were, of course, attached firmly to the vessel. The ratio of the length of the vessel-arm of the lever to the length of the spring-arm, was 1 to 11. The dynamometer-diagram thus obtained, gave the thrust-pressures for every instant during each run of the vessel. Two indicators were used: one of them was kept permanently in position on one cylinder, and the other on the other cylinder, during the experiments. Each indicator communicated with both ends of its cylinder, and before use was put in perfect adjustment, and had its spring tested. A counter was attached to the screw-shaft, and registered the number of its revolutions. The base for the experiments, or the course passed over by the vessel during each run, was a straight line 8,955 feet long, as given by the very accurate survey of Mare Island. It extended from the northern side of the dry-dock dolphins, or guard piers, to the northern side of the magazine wharf. This base was close under the lee of the high ground of the island, the wind over which was always in the same direction, exactly at right angles to the base; and the water smooth. During all the trials, the variation in the vessel's draught of water, and in the trim, was very slight. The velocity of the tide varied from nil to three geographical miles per hour. With each screw eight experiments were made at the speeds, respectively, of 5, 51, 6, 64, 7, 74, 8, and 8 geographical miles per hour, as nearly as could be obtained. Each experiment consisted of six runs over the base, three in each direction, and the time of making them was selected when the tide had but little influence. The vessel's speed through the water during each double run was not only ascertained from the ranging marks at the ends of the base, but by means of a mercurial speed-gauge consisting of Berthon's modification of Pitot's tube. This gauge was composed of a glass tube bent into the U-form; the ends of the tube were open, and the curved portion and a portion of the legs were filled with mercury. The top of each leg communicated by a gum pipe with the bottom of a separate airchamber, and the top of each chamber communicated by another gum pipe with the upper portion of a brass tube closed at both ends. One of these brass tubes was placed within the other, the inner tube passing a few inches through the ends of the outer one by stuffing-boxes. The upper ends of the brass tubes were inside the vessel, and their lower ends protruded about 6 inches below the bottom of the vessel, 12 inches from the nearest side of the keel, and at about the middle of the vessel's length. The inner tube was the pressure-tube, and its interior received the pressure of the water through a hole of of an inch diameter in its side, a little above its bottom, and in the directly ahead direction of the vessel. The larger tube was the neutral tube, and in its side, a little above its bottom, was a hole of of an inch diameter with its axis at the angle of 414 degrees from the directly ahead direction. The diameter of the outer brass tube was 1 inch, and of the inner brass tube of an inch. A properly graduated scale being attached to the legs of the glass tube, measured by the difference of the level of the mercury in those legs, the vessel's speed in geographical miles per hour. When the vessel was motionless in still water, the mercury in the two legs stood at the same level. The vessel's speed by this gauge in a calm and at dead high or low water, being frequently compared with its speed at the same time according to the shore-marks, was always found to exactly correspond. In making the experiments, the vessel, at the intended speed, was brought opposite one end of the base and then run uniformly to the other, being kept in a straight line by an expert steersman. After passing the last end of the base à sufficient distance, the vessel was turned and the run repeated back in the same manner. The throttlevalve was always carried wide open, during the turnings as well as during the runs, and the steam-pressure varied but slightly throughout an experiment, the supply of steam required being always within the capacity of the boiler to furnish. From the commencement of each run to its end, indicator-diagrams were taken as rapidly as possible from each end of each cylinder. The assistant engineers charged with this duty being very expert, and having the pencils and paper all previously prepared, the diagrams were taken with so little interval of time, that they may be considered continuous. The dynamometer-diagram, taken by another engineer, was continuous from the beginning to the end of the run. An observer stationed always at the same part of the vessel, gave the signal.the instant he was opposite the ranges at the ends of the base; and, at the same moment, two other observers took, one the time to a second, and the other the number on the counter. Thus, the time of making each run, and the number of revolutions made by the screw in that time, were exactly ascertained. During each run, an observer noted at the end of each half minute the vessel's speed through the water, by the speed-gange; and at the end of every minute the steam-pressure in the boiler, as given by a spring-gauge. There were also noted during each run, the temperatures of the external atmosphere, of the engine-room, of the feed-water entering the boiler, and of the sea-water: also, the atmospheric pressure as given by an aneroid barometer. Every care was observed in the conduct of the experiments to insure extreme accuracy. Although many of the quantities noted were not necessary to the main purpose of the experiment, yet the results from them are interesting in other points of view. Explanation of tables 1 to 6, both inclusive, containing the data and results of the experiments made with screws A, B, C, D, E, F, G, and H, to determine their relative economic efficiencies. In the following tables, numbered 1 to 6, both inclusive, will be found the data and results of all the experiments made with screws A, B, C, D, E, F, G, and H, to determine their relative economic efficiencies when applied to the propulsion of steam-launch No. 4. For facility of reference, the lines containing the quantities are numbered and arranged in groups; and the columns containing the data and results for the different speeds of vessel at which the experiments were made are lettered. screw. These quantities were obtained, for each screw, in the following manner, namely: On a straight line, taken for a base, all the experimental speeds of the vessel were laid off by scale as abscisse, and on ordinates erected from these abscissæ, at right angles to the base, were laid off, by scale, the corresponding experimental slips of the A fair curve was then passed through the ends of these ordinates, dividing them as equally as possible. Finally, there were laid off, by scale on the base, abscissæ representing the speeds of vessel given in line 1 of the table; and from these abscissæ right-angled ordinates were erected until they cut the curve, and on them were measured by scale the distances between the curve and the base, which distances gave the true slips of the screw, as shown in line 2 of the tables, and corresponding to the speeds of vessel shown in line 1. The speeds in line 1 are given in geographical |