New-York tribune., 1903-03-01
INCREASING OCEAN SPEED.
GAINS EFFECTED MERELY BY CHANGING
THE FORM OF THE SCREW.


THE FORM OF THE SCREW.
Although the idea OF using screw propeller
on steamships is at least 8 century old, the IN
vention itself is even yet undergoing improve
ment. One of the newest British warships, the
Drake, developed the exceptionally fine speed OF
twenty-three knots her official trial, and was
ali that the contractors promised. Yet by the
substitution Of new twin screws for those hrst
employed, gain of one whole knot was ehecteo'
which was equivalent to an addition OF 10 or
20 per cent to her engine power. All of which
goes to show that perfection has not oeen
reached in this important device; or, at least'
had not up to year or two ago.
The necessity for change does not prove that
the form Of the propeller hitherto in service was
not admirably suited to the work performed
few years ago. It grows out OF new conditions.
It Is the result Of the attempt to drive ship
through the water faster than was customary
a decade or so ago, and of rotating the screw
more rapidly than before. One will readily per-
ceive the difficulty which has lately been en-
countered II he will remember what takes place
in the water when the shaft of steamship
revOlves. Perhaps it will be easier II the phe-
nomena in a rotary pump cylinder is first con-
sidered.
In the latter apparatus the piston is screw
whose constant Whirling sucks in fluid at one
other. There the water is confined on Its SIDES
for an instant. In the open ocean the circum-
stances are little different but the principle Is
the same. In both situations a thrust is ex-
erted against the water by the curved blades OF
the rotating screw. The effect in one case Is TO
force the water to move; in the other, while
some water is sent astern, the machinery itself
and the ship containing it driven forward
In either the highest efficiency is attained
remains solid. Should the screw be driven SO
rapIdly in the pump that the fluid emerges
faster than it can come in, holoWs will form in
the cylinder, and power will be wasted So, too
with the steamship propeller What engineers
call "cavitation" occurs in the water when too
high speed is developed. Eddies form between
the blades and the ship, creating a suction
which retards the latter's progress. The in
fuence may be slight, but it is real 1t pre-"
vents the engines from converting their energy
into the maximum possible progress. Parson''
observed this hindrance when he first tried HIS
steam turbine on an experimental boat several
years ago. That type Of engine develops ex-
ceptionally rapid rotation. He did not want TO
reduce this with gearing. Instead, he adopted
the plan of putting three screws on each of his
three shafts, so that he divided up the work Of
propeIling vessel scarcely larger than steam
yacht among nine screws! The change in
thirty-fve knots. AS yet the old reciprocating
engine is retained for ocean liners and fast
cruisers but their shafts revolve faster than did
those in few years ago. The same trouble
has been encountered, though in less degree,
and though met in a different manner.
The precise nature of the notifications adopt
ed by the British Admiralty for the Drake and
three other warships of the same class has not
been announced. Possibly the blades were en-
larged or their pitch, or twist altered. But it
is apparent not only from this action, but also
from the discussions of marine engineers else
where, that old formulas being revised to
meet new requirements. Only few weeks ago
a well known French expert, J. A. Normand,
read a paper on the subject before the French
Academy. No doubt the matter is being studied
extensively elsewhere. While it is by no means
certain that the conclusions will be
reached by all authorities it is clear that prog
ress involves the creation, well the solu-
tion of problems