It may be urged, and with propriety, that the results obtained in practice with the jet propeller are more favorable than our reasoning would indicate as possible; but it will be seen that we have taken no notice of conditions which seriously affect the performance of a screw. There is no doubt that it puts water in motion not astern. It twists it up in a rope, so to speak. Its skin frictional resistance is very great. In a word, in comparing the hydraulic system with the normal system, we are comparing two very imperfect things together; but the fact remains, and applies up to a certain point, that the hydraulic propeller must be very inefficient, because it, of all propellers, drives the smallest quantity of water astern at the highest velocity.
There is, moreover, another and a very serious defect in the hydraulic propeller as usually made, which is that every ton of water passed through it has the velocity of the ship herself suddenly imparted to it. That is to say, the ship has to drag water with her. To illustrate our meaning, let us suppose that a canal boat passes below a stage or platform a mile long, on which are arranged a series of sacks of corn. Let it further be supposed that as the canal boat passes along the platform, at a speed of say five miles an hour, one sack shall be dropped into the boat and another dropped overboard continuously. It is evident that each sack, while it remains in the boat, will have a speed the same as that of the boat, though it had none before. Work consequently is done on each sack, in overcoming its inertia by imparting a velocity of five miles an hour to it, and all this work must be done by the horse towing on the bank. In like manner the hydraulic propeller boat is continually taking in tons of water, imparting her own velocity to them, and then throwing them overboard. The loss of efficiency from this source may become enormous. So great, indeed, is the resistance due to this cause that it precludes the notion of anything like high speeds being attained. We do not mean to assert that a moderate degree of efficiency may not be got from hydraulic propulsion, but it can only be had by making the quantity of water sent astern as great as possible and its velocity as small as possible. That is to say, very large nozzles must be employed. Again, provision will have to be made for sending the water through the propeller in such a way that it shall have as little as possible of the motion of the ship imparted to it. But as soon as we begin to reduce these principles to practice, it will be seen that we get something very like a paddle wheel hung in the middle of the boat and working through an aperture in her hull, or else a screw propeller put into a tube traversing her from stem to stern.
We may sum up by saying that the hydraulic propeller is less efficient than the screw, because it does more work on the water and less on the boat; and that the boat in turn does more work on the water than does one propelled by a screw, because she has to take in thousands of tons per hour and impart to them a velocity equal to her own. Part of this work is got back again in a way sufficiently obvious, but not all. If it were all wasted, the efficiency of the hydraulic propeller would be so low that nothing would be heard about it, and we certainly should not have written this article.—The Engineer.
THE NEW ARMY GUN.
The cut we give is from a photograph taken shortly after the recent firings. The carriage upon which it is mounted is the one designed by the Department and manufactured by the West Point Foundry, about six months since. It was designed as a proof carriage for this gun and also for the 10 inch steel gun in course of construction. It is adapted to the larger gun by introducing two steel bushing rings fitted into the cheeks of carriage to secure the trunnion of the gun.
The gun represented is an 8 inch, all steel, breech-loading rifle, manufactured by the West Point Foundry, upon designs from the Army Ordnance Bureau. The tube and jacket were obtained from Whitworth, and the hoops and the breech mechanism forgings from the Midvale Steel Company. The total weight of the gun is 13 tons; total length, including breech mechanism, 271 inches; length of bore in front of gas check, 30 calibers; powder space in chamber, 3,109 cubic inches; charge, 100 pounds. The tube extends back to breech recess from muzzle, in one solid piece. The breech block is carried in the jacket, the thread cut in the rear portion of the jacket. The jacket extends forward and is shrunk over the tube about 87½ inches. The re-enforce is strengthened by two rows of steel hoops; the trunnion hoops form one of the outer layers. In front of the jacket a single row of hoops is shrunk on the tube and extends toward the muzzle, leaving 91 inches of the muzzle end of the tube unhooped. The second row of hoops is shrunk on forward of the trunnion hoops for a length of 38 inches to strengthen the gun, and the hoop portion forms three conical frustums. The elastic resistance of the gun to tangential rupture over the powder chamber is computed by Claverino and kindred formulas to be 54,000 lb. per square inch.
THE ARMY 8 INCH STEEL GUN WITH CARRIAGE.
The breech mechanism is modeled after the De Bange system. The block has three smooth and three threaded sectors, and is locked in place by one-sixth of a turn of a block, and secured by the eccentric end of a heavy lever, which revolves into a cut made in the rear breech of the gun. The gas check consists of a pad made of two steel plates or cups, between which is a pad of asbestos and mutton suet formed under heavy pressure. The rifling consists of narrow grooves and bands, 45 of each. The depth of the groove is six one-hundredths of an inch.