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Bowstrings
Part 4 of 6

In these calculations all references to "strength of thread" mean the strength at rupture. This is warranted in the case of linen because stress-stretch curves show that the strength at the elastic limit is at least 95 percent of the strength at rupture. With silk this is not the case as shown below.

Strength at
Elastic Limit
Strength at
Rupture
Percent
Stretch at
Elastic Limit
Percent
Stretch at
Rupture
Linen 10.5 lb 11.1 lb 2.8 2.9
Silk 6.0 lb 14.8 lb 2.6 11.

This property of linen to develop its high rupturing strength without acquiring a permanent elongation makes it a pre-eminent material for bowstrings.

In an article Dr. R. P. Elmer described an "Oriental knot used for attaching a string to the bow. I have made measurements and can find no detectable reduction of strength due to this knot providing that sufficiently large tennis gut is used for attaching the linen string to the bow. A number 12 gut (diameter 0.07 inches) seems to be sufficient. Use of this "Oriental" knot simplifies string making greatly—all that is necessary is to wind the right amount of thread around two pegs in the wall, fasten the ends under the serving, twist slightly and tie to the bow.

Moisture increases the strength of linen very greatly. By saturating the linen with water the strength is increased about 50 percent over its strength in air dry condition. Its strength seems to be proportional to the moisture content as is shown in the following table:

EFFECT OF MOISTURE
Pull Strength
Coefficient
Treatment using 40/3 button thread (P) W/f (S.C.)
Wet and then dried to air dry condition 10.1 .0400 49800
Wet and then dried to 36.5% water 14.4 .0630 45000
Wet and then dried to 32.2% water 14.4 .0590 48100
Wet and then dried to 27.8% water 12.4 .0554 44100
Wet and then dried to 15.4% water 11.9 .0473 49600
Wet and then dried to 9.5% water 11.9 .0442 53000
Wet and then dried to 5.4% water 10.9 .0423 50800