rn. 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