Scientific American Supplement, No. 561 | Page 5

square of its speed. Thus if a screw sends 100 tons of water astern at a speed of 10 ft. per second per second, the work wasted will be 156 foot tons per second in round numbers. If a hydraulic propeller sent 10 tons astern at 100 ft. per second per second, the work done on it would be 1,562 foot tons per second, or ten times as much. But the reaction effort, or thrust on the ship, would be the same in both cases. The waste of energy would, under such circumstances, be ten times as great with the hydraulic propeller as with the screw. In other words, the slip would be magnified in that proportion. Of course, it will be understood that we are not taking into account resistances, and defects proper to the screw, from which hydraulic propulsion is free, nor are we considering certain drawbacks to the efficiency of the hydraulic propeller, from which the screw is exempt; all that we are dealing with is the waste of power in the shape of work done in moving water astern which we do not want to move, but cannot help moving. If our readers have followed us so far, they will now understand the bearing of Rankine's proposition, that that propeller is best which moves the greatest quantity of water astern at the slowest speed. The weight of water moved is one factor of the thrust, and consequently the greater that weight, other things being equal, the greater the propelling force brought to bear on the ship.
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