The diameter of the two propellers should be about
11-1/2 to 11-3/4 inches, with a pitch angle at the extremities of about 25
to 30 degrees as a limit. The "centrale" type (Fig. 135) is to be
preferred. Such propellers can be procured at Messrs. A. W. Gamage, Ltd.,
Holborn, E.C.; Messrs. T. W. K. Clarke and Co., Kingston-on-Thames; and
elsewhere.
For the particular machine which we are considering, the total weight of
the two propellers, including axle and hook for holding the rubber, should
not exceed 3/4 oz. This means considerable labour in cutting and
sandpapering away part of the boss, which is always made much too large in
propellers of this size. It is wonderful what can be done by care and
patience. The writer has in more than one case reduced the weight of a
propeller by more than one-half by such means, and has yet left sufficient
strength.
The combined axle and hook should be made as follows:--Take a piece of thin
steel wire, sharpen one end, and bend it as shown at C (Fig. 136). Pass the
end B through a tight-fitting hole in the centre of the small boss of the
propeller, and drive C into the wood.
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