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.
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