A ship with a
hydraulic propeller can sail without let or hindrance; a powerful pump
is provided, which will deal with an enormous leak, and so on. If all
the good things which hydraulic propulsion promises could be had
combined with a fair efficiency, then the days of the screw propeller
and the paddle wheel would be numbered; but the efficiency of the
hydraulic propeller is very low, and we hope to make the reason why it
is low intelligible to readers who are ignorant of mathematics. Those
who are not ignorant of them will find no difficulty in applying them
to what we have to say, and arriving at similar conclusions in a
different way.
Professor Greenhill has advanced in our pages a new theory of the
screw propeller. As the series of papers in which he puts forward his
theory is not complete, we shall not in any way criticise it; but we
must point out that the view he takes is not that taken by other
writers and reasoners on the subject, and in any case it will not
apply to hydraulic propulsion. For these reasons we shall adhere in
what we are about to advance to the propositions laid down by
Professor Rankine, as the exponent of the hitherto received theory of
the whole subject. When a screw or paddle wheel is put in motion, a
body of water is driven astern and the ship is driven ahead.
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