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Constructing The Bow With Rectangular Limb Section
Part 4 of 4

Outer ends of sections M and F may be rounded for horn tips.

I have recently completed a series of measurements of bow efficiency, in which the following four bows were used:

1. Osage, conventional limbs; 63½ inches long, drawing weight, 34½ lb. at 27 inches.

2. Osage, rectangular limb section, made from same stave as No. 1. Length, 63½ inches; 36 lb. at 27 inches.

3. Yew, kiln-dried, conventional limbs; 71 inches long; 39 lb. at 27 inches.

4. Yew, kiln-dried, rectangular limbs; 68 inches long; 36 lb. at 27 inches.

Efficiencies were obtained with two different weights of arrows, 324 and 427 grain, respectively. These arrows were shot repeatedly by hand and the time required to travel 3.44 ft. was measured with an Aberdeen Chronograph. Close agreement in the values of time for the individual shots was obtained in all cases. From these data velocities and kinetic energy values were computed. Each bow was then carefully weighed at five different lengths of draw, and the energy expended in drawing the arrow found. The efficiency is the ratio of the two energy values for each case.

Bow No. 1, wt. lbs. at 27 inches, 34.5; velocity, ft. per sec. 324 gr. 151.3, 427 gr. 136; efficiency, 324 gr. 64.3%, 427 gr. 69.9%.

Bow No. 2, wt. lbs. at 27 inches, 36; velocity ft. per sec. 324 gr. 161.5, 427 gr. 148.4; efficiency, 324 gr. 67.8%, 427 gr. 77.6%.

Bow No. 3, wt., lbs. at 27 inches, 39; velocity, ft. per sec. 427 gr. 143; efficiency, 427 gr. 63%.

Bow No. 4, wt., lbs. at 27 inches, 36.5; velocity, ft. per sec. 324 gr. 171.2, 427 gr. 154.3; efficiency, 324 gr. 72.2%, 427 gr. 82.7%.

These results are, of course, not conclusive, but merely indicative. They are given weight by the fact that in every case thus far investigated, I have found the bow of rectangular section to have higher efficiency than a corresponding bow of traditional design.

It seems probable that the higher efficiency in the bow with rectangular limb section is accounted for by the higher "energy density" per unit volume of limb that is possible without endangering the wood, and the uniform distribution of the energy in the limbs of the drawn bow. This was touched upon m the December article. There is, in the writer's opinion, little question but that Dr. Hickman has solved the problem of the best cross-section of a bow. Experimental evidence which is accumulating shows that a decided forward step has been achieved in the new design.