The parallel force alone
has the propelling effect; the screw, therefore, should always be so
constructed that its surfaces shall be chiefly employed in driving the
water in a direction parallel to the keel from stem to stern.
[Illustration: Fig. 2--KAPP'S DIAGRAM.]
It is evident that a finely pitched screw, running at a high velocity,
will supply these conditions best. With that beautiful screw lying on
this table, and made by Messrs. Yarrow, 95 per cent. of efficiency
has been obtained when running at a speed of over 800 revolutions per
minute--that is to say, only 5 per cent was lost in slip.
Reviewing the various points of advantage, it appears that electricity
will, in time to come, be largely used for propelling launches, and,
perhaps, something more than launches.
In conclusion, quoting Dr. Lardner's remarks on the subject of steam
navigation of nearly fifty years ago, he said:
"Some, who, being conversant with the actual conditions of steam
engineering as applied to navigation, and aware of various commercial
conditions which must affect the problem, were enabled to estimate
calmly and dispassionately the difficulties and drawbacks, as well as
the disadvantages, of the undertaking, entertained doubts which clouded
the brightness of their hopes, and warned the commercial world against
the indulgence of too sanguine anticipation of the immediate and
unqualified realization of the project.
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