[Illustration: TWIN SCREW STEAM LAUNCH GEMINI.]
The launch shown in our illustration was built in New Westminster,
British Columbia, Canada. She is 42 ft. keel and 7 ft. beam, and has 4
ft. depth of hold. She has an improved Clarke compound engine, also
shown in an accompanying illustration, with a high pressure piston
four inches in diameter, and a low pressure piston eight inches in
diameter, the stroke being six inches, and the engine driving two
twenty-six inch screws. With 130 pounds of steam, and making 275
revolutions per minute, the launch attains a speed of nine miles per
hour, thus fully demonstrating the adaptability of this engine to the
successful working of twin screws.
[Illustration: THE CLARKE COMPOUND TWIN-SCREW OPERATING ENGINE.]
In the Clarke engine, the exhaust pipe from the high pressure cylinder
leads to the steam chest of the low pressure cylinder, while the
piston in the upper cylinder is secured on a piston rod extending
downward and connected with a piston operating in the lower cylinder,
the exhaust pipe from the latter leading to the outside. On the piston
rod common to both cylinders is secured a crosshead pivotally
connected by two pitmen with opposite crank arms on crank shafts
mounted to turn in suitable bearings on the base, which also supports
a frame carrying the low pressure cylinder, on top of which is a frame
supporting the high pressure cylinder. The valves in the two steam
chests are connected with each other by a valve rod connected at its
lower end in the usual manner with the reversing link, operated from
eccentrics secured on one of the crank shafts.
The crank arms stand at angles to each other, so that the crank shafts
are turned in opposite directions, and the position of the link is
such that it can be readily changed by the reversing lever to
simultaneously reverse the motion of the crank shafts. On the crank
shafts are also formed two other crank arms pivotally connected by
opposite pitmen with a slide mounted in vertical guideways, supported
on a frame erected on the base, the motion of the crank shafts causing
the vertical sliding motion of the slide traveling loosely in the
guideways, and thus serving as a governor, as, in case one of the
propellers becomes disabled, the power of the shaft carrying the
disabled propeller is directly transferred to the other shaft through
the crank arms, pitmen, and slide, and the other propeller is caused
to do all the work. All the parts of the engine are within easy reach
of the engineer, and there are so few working parts in motion that the
friction is reduced to a minimum.
It is said that the plan of construction and the operation of this
engine have been carefully observed by practical engineers, and that,
considering the dimensions of the boat, her speed, the smallness of
the power, the ease with which she passes the centers, the absence of
vibration while running, and the very few working parts in motion, the
engine is a notable success. She can be run at a very high velocity
without injury or risk, and is designed to be very economical in cost
and in weight and space. This engine has been recently patented in the
United States and foreign countries by Mr. James A. Clarke, of New
Westminster.
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