Daily Kennebec journal. [microfilm reel] (Augusta, Me.) 1870-1975, 1903-03-13
INCREASED OCEAN SPEED.
Gains Effected Merely by Changing the
Form of the Screw.

Although the idea of using a screw
propeller on steamships is at least a
century DID, the invention itself is even
yet undergoing Improvement. One of
the newest British Warships, the Drake
developed the exceptionally fine spe ID
of twenty-three knots on her official
trial, and was all that the contractors
promised. Yet, by the substitution of
new twin screws for those first employ
ed, gain of one whole knot was af-
fected, which was equivalent to an ad-
dition of Is or 20 per cent to her engine
power. All of which goes to show that
perfection has not been reached in this
important device; or, at least, had not
up to a 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. 1t grows out of new condi-
tions. 1t is the result of the attempt
to drive a 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 perceive
the difficulty which has lately been en-
countered, if he will remember what
takes place in the water when the shaft
of a steamship revolves. Perhaps it
will be easier if the phenomena in a ro-
tary pump cylinder is first considered.
In the latter apparatus the piston is
a screw whose constant whirling sucks
in fluid at one end of the cylinder and
forces it out at the other. There the
water is confined on it's sides for an in
Sta IN no tho pirc1ly1
stances are a little different, but the
principle is the same. In both stu
atoms a thrust is exerted against the
water by the curved blades of the rota-
ting screw. The effect in one case is to
force the water to move; i'll the other,
while some water is sent astern, the
machinery itself and the ship contain
ing it are driven forward.
In either case the highest efficiency
is attained only when the real or imag-
mary column of water remains solid.
Should the screw be driven so rapidly
in the pump that the fluid energies fast
er than it can come in. hollows will
form in the cylinder, and power viii be
wasted. So, too, with the steamship
propeller. What engineers call ''cavi-
tation" occurs in the water when too
high speed is developed. Eddies form
between the blades and the ship, Great
ing a suction which retards the latter's
progress. The influence may be slight.
but it is real. 1t prevents the engines
from converting their energy into the
maximum possible progress. Parsons
observed this hindrance when he first
tried his steam turbine on an expert
mental boat several years ago. That
type of engine develops exceptionally
rapid rotation. He did not want to re-
duce 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 propel-
ling vessel scarcely larger than a
steam yacht among nine crews! The
change increased the boats speed from
about thirty to thirty-five knots. AS
yet the old reciprocating engine is re-
tanned for ocean liners and fast cruis-
ers. but their shafts revolve faster than
did those in use a few years ago. The
same trouble has been encountered,
though in less degree. and though
met in different manner.
The precise nature of the notifications
adopted by the British Admiralty for the
Drake and three other warships of the
same class has not been announced Pos
siby the blades were enlarged. 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 are being revised
80, M ,anyx DC