Sunday, January 31, 2010

Bow Strings and Cables

If you bend a bow too far and the angle that the string meets the tip of the bow exceeds 90 degrees, then the string is likely to slide off and the bow will fly forward until it hits something, like a solid oak chair or your dining room wall (photo). For that reason, the Tuktut Nogait bow has a maximum draw of 24 inches. If the bow is drawn farther than that,
then the string slides off the end.
Its possible to make nocks that will hold a string on so that the string angle can exceed 90 degrees, and there are bows made in the Arctic with flaring or T-shaped nocks that were probably designed for just that purpose, but the Tuktut Nogait bow didn't appear to have that style of string nock.
For the bow string, I made a 2 ply cord of twisted sinew. The sinew fibres are separated into very fine silky threads that are twisted together while wet into a thread that is about 1/8th of an inch thick. This thread is made a little over twice as long as the finished string needs to be. I let that thread dry over night and then looped it in the middle and twisted it into a simple two ply cord. While that was drying I twisted the two ends together to create a second loop at the opposite end of the cord and let it dry under tension.
The end result is a continuous loop of sinew thread which has been twisted into a cord. As it dries it shrinks and everything sticks to itself. I didn't cut any loose threads off, instead I smoothed down any loose ends with warm water and hide glue. The final length of the bow string can be adjusted by twisting or untwisting it, although it can never be untwisted completely.

To add tension to the cord backing of the bow a sinew twister is used. The antler sinew twister is slid through the middle of the cord bundle and used as a wrench to twist the cord into a cable. To hold tension in the cable, I tied a leather lashing through the middle of the cable around the handle of the bow before removing the sinew twister. While the bow is strung, I can put two full twists in the cable, however, if I unstring the bow I can twist the cable three times.
The sinew twister is also handy to use when twisting the bow string and you can slide the string right off the twister onto the bow just like slipping on a shoe with a shoe horn. The tension in the bow string and the tension in the cable backing make a big difference in the performance of the bow.

I shot the bow with some store bought arrows yesterday. The first attempts were with the sinew cord untwisted and the bow string just tight enough to brace the bow (bracing the bow means stringing it so that the wood is flexed and it has the classic D-shaped profile). The arrows were travelling about 50 paces. I unstrung the bow and added a half dozen twists to the bowstring to make it shorter and added a couple full rotations of tension to the cable. With those two little changes the bow started shooting arrows 85 paces.

Tuktut Nogait Bow
from top; the original Inuvialuit artifact, with the Parks Canada reproduction sitting behind it.
The unstrung bow with the cable backing on (Western Yew, 50 inches, 127 cm long).
The braced bow.
The bow at full draw - 24 inches.

The effects of the cable backing are shown in the graph below. This time I was testing the draw weight of the bow by drawing the bow using a fish scale, and recording how many pounds were needed to pull it to 12 inches, 14 inches, 16 inches, etc. The red line shows the draw weight of the bow with no twist in the cable and the green line shows the draw weight of the bow with two twists in the cable. In both tests the tension in the bow string was the same. Without twists in the cable the draw weight of the bow at 24 inches is 28 pounds, with two twists in the cable the draw weight of the bow at 24 inches is 32 pounds - a 14% increase. The 3 twist test was done this morning, so the tension in the bow string might be different, but the results are consistent with the first two tests, this time it draws 34 pounds at 24 inches - a 21% increase. At the moment, 3 twists is pretty much the maximum tension that I can get in the cable.

Its a fun bow to play with because it has so many little ways to adjust it. I'll need to make some reproduction arrows to experiment with. Partly for authenticity and partly because the sinew bow string is thicker than commercial bow string and the little plastic nocks on the store bought arrows don't fit on it properly, so they break a lot.

Photo Credits: Tim Rast

Photo Captions:
First: The bow string at 90 degree angle to the limb - any farther and the string slips off.
Second: Holes in the wall and a dent in the chair from the bow flying across the room when the string slipped off.
Third: 2 ply sinew bowstring loop
Fourth: drying the twisted sinew bow string with tension
Fifth: Sinew twister in the braided sinew cable
Sixth: Tying down the cable using leather lashing
Seventh: Composite photo of the Elfshot reproductions and the original Tuktut Nogait bow
Eighth: Graph showing the effect of cable tension on bow draw weight
Ninth: Wrapping the sinew backing. 70 feet of braided sinew was used.


  1. This reminds me of an article on an Inuit bow in the SPT book. It looks fantastic; where'd you get so much sinew though?

  2. Thanks Sam! About 45 feet of the sinew came from 3 moose leg tendons that I collected several winters ago. I couldn't believe how much cord came out of just those three. The other 25 feet came from a half dozen or more strips of deer back strap sinew that I bought online from 3 Rivers Archery. In total, it took about 3 days of braiding to get the 70 feet. I watched a lot of movies while doing it.

  3. Beautiful looking bow! Will you make some reproduction arrows for it?

  4. Thanks! Yeah as soon as I get a minute I'll start making arrows for it. The first couple will be bird blunts based on the blunt from Ivavvik. Its a different site than where the bow was found, but its a good enough starting point and I hope that they'll be durable enough to survive shooting with only the snow to catch them. Pointed arrows won't last as long unless I can find haybales or something to shoot.

    I didn't really get into bow making because I have a lot of experience with archery, I got into it because I like chipping arrowheads, which leads to arrows, which leads to bows. The ironic thing is that the first bow I made is from a culture that didn't even use knapped arrowheads, so flintknapping isn't going to be a big help with making these arrows.


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