Ötzi's Bow

by Dick Baugh, Vttorio Brizzi & Tim Baker
(January 20, 2006)

Introduction
In 1991, the archaeological find of the century was made in the Ötzaler Alps near the Austrian-Italian border. In addition to Ötzi's body frozen in the snow there were clothing, tools of stone, copper, bone and antler and archery equipment. This was a snapshot of the technology of 5,300 years ago. It is a challenge to derive as much information from the scanty set of data that he left for us. Oh, for a time machine!

What can we conclude from examining Ötzi's bow and arrows? Was it a finished ready-to shoot weapon or a work in progress? Several features can be examined to determine the answer to this question. We know the dimensions of his bow, the material it was made from, the dimensions of arrows, both finished and unfinished, in his quiver; the dimensions of an alleged bowstring and Ötzi's height and physique. Each of these contains clues to the mystery of the Ice Man.

Throughout this article we have based conclusions on the physical dimensions of artifacts found with Ötzi. Small errors in measuring these dimensions can have major consequences in our conclusions. For example, a 1 mm error in measuring the thickness of Ötzi's bow would cause a 10 % error in computing the draw weight and a 10 % error in measuring the thickness of Ötzi's bow would cause a 30 % error in computing the draw weight.

Details on the bow
The bow was made of yew (Taxus baccata) with a total length of 1.82 meters (71.6 inches). Detailed dimensions were generously provided by Vittorio Brizzi of UniversitB degli Studi di Ferrara, Dipartimento delle risorse Naturali e Culturali, Via Ercole I d'Este, Ferrara. It was quite symmetric with an elliptic cross section throughout its length. The dimensions at the center, one half and three quarters of the way to the tip, 28 cm from the tip and at the tip are given in the following table. 28 cm (11 in) from the tip a detailed cross section was obtained via computer aided tomography ( a CAT scan).

From the dimensions, cross sectional shape and assumed modulus of elasticity of yew wood, we can compute the draw weight and internal strain in the bow limbs versus draw length. This is a fairly straightforward calculation that would be very familiar to a structural engineer. We can also compare the dimensions of Ötzi's bow with other finished yew longbows.

For comparison, the dimensions of a 72 inches long-50 pound yew longbow were obtained from L.E. Stemmler's Essentials of Archery, a classic first published in 1942. The limbs were semi-elliptical (D shaped) in cross section.

Stemmler 50 pound yew longbow

This represents good bowyery in the early part of the twentieth century. The main difference between the two is the greater thickness of Ötzi's. The proportions are different also. The Stemmler bow shrinks abruptly in thickness above and below the handle section whereas there is very little taper in Ötzi's. Consequently the Stemmler bow has a rigid, unbending handle section whereas Ötzi's bends in the handle and is much, much more powerful.

The Mary Rose bows
The Mary Rose, an English war ship, sank in the English Channel in 1545. A marine archaeology team recovered the wreck in 1978. Among the artifacts were a large number of yew longbows. W.F. Paterson gave a few of the dimensions of some of them in the "Journal of the Society of Archer Antiquaries".

Bob Kooi, a scientist from the Netherlands who has done extensive modeling of bows ancient and modern, supplied detailed information on a particular Mary Rose bow. The data was in the form of cross-sectional moment of inertia instead of width, thickness and cross-sectional shape. That's completely sufficient because in order to compute the draw weight you must convert the cross-sectional data into moment of inertia. Kooi's data is summarized in the following table assuming that the cross-section is elliptical and the thickness is 80 % of the width:

The Mary Rose bows have dimensions very similar to Ötzi's. Does that mean that he, like the military archers of Henry VIII's England, shot a very powerful bow quite often or was Ötzi's alleged weapon really an unfinished stave?

Surface texture of the limbs
The limbs show very regular tool marks. It looks as if someone had very methodically gone over the surface with a sharp edged tool. The surface treatment reminds me of the adze marks seen on a Pacific Northwest Indian totem pole. Is this finish work or just preliminary shaping? If the computed draw weight and strain in the wood are excessive, then it is just preliminary shaping.

The "Bowstring" and arrows found in his quiver
A piece of two-ply cord made from bast fiber obtained from the bark of a lime tree (Tilia sp.), neatly coiled, was found in the bottom of Ötzi's quiver. According to Spindler's book it tapered uniformly from 3.5 mm (.138 in) at one end to 7 mm (.276 in) at the other end and was approximately 1.9 to 2.1 meters (74.8 to 82.7 in) long. The thicker end was knotted If this truly were Ötzi's bowstring, how powerful a bow could be used with it? The obvious way to find out would be to make a cord of the same material with the same dimensions and test its breaking strength. The simplest rule of thumb for bowstring strength is to make the breaking strength of the string equal to four times the full draw weight of the bow.

I measured the diameter of a bowstring made from excellent quality linen by Tim Baker for a 50 pound longbow at 3.5 mm. A string made for a 50 pound bow out of poorer material would have to be even thicker. Spindler's book states that the nocks on Ötzi's finished arrows were 4 mm wide. That sounds reasonable for a 50 pound bow but not much more.

Someone needs to duplicate his bowstring in lime tree or basswood fiber and determine its breaking strength and consequently the draw weight of the bow for which it was intended.

Arrow length is not always an acceptable metric for draw length. Native archers of the western United States frequently used short wide bows with short draw length and arrows that were several inches longer than their draw length. Tim Baker pointed out that extra long arrows do a better job of shooting where you point them when using a bow with an excessively wide handle section. We can, however, learn a little bit about the strength of Ötzi's bow from his arrows. Powerful bows require thick bowstrings and thick bowstrings require wide nocks on the arrows. The nock of one of the finished arrows was 12 mm deep and 4 mm wide. We can estimate from the width of the nock the maximum bowstring diameter and from the bowstring diameter plus an assumed tensile strength of the material we can infer the maximum allowable draw weight of the bow.

Ötzi's stature and physique
He was about 5 ft-3 in (160.5 cm) tall. Using a corner-of-the-mouth or under-the-chin anchor, he would have a draw length of possibly 26 inches. Is there any evidence that he was excessively muscular? If not, he would not have been able to shoot a bow exceeding 60 pounds draw weight at his 26 inch draw length. The English longbowmen of the twelfth-fifteenth centuries shot with bows exceeding 100 pounds, based on dimensions of the bows recovered from the Mary Rose, but they were compelled by law to practice at least once a week. Because of his small stature it is doubtful that Ötzi could have drawn a bow of more than 50 to 60 pounds (25 kgf).

Was he a fugitive and could he have used the bow in its existing state?
One possible scenario is that Ötzi was "on the lam", had hastily grabbed a roughed out bow stave, some partially completed arrows, a bowstring, some other supplies and fled. As a temporary expedient, he could tie the bowstring loosely to the ends of the bow, shoot it as needed and do the finish tillering later at his leisure. This is probably a moot point because detailed X-ray examination of Ötzi showed that he had a stone arrow point lodged in his back that would have prohibited any attempt at shooting his bow.

Green or seasoned wood?
There are plenty of examples of Native American bowyers beginning their work with green wood instead of the well seasoned wood beloved by the traditional bowyers of the last few centuries.. The reason is obvious to anyone who has tried to do woodworking with stone tools. Green wood is a lot softer than dry seasoned wood. If Ötzi's roughed out bow had been green wood, the draw weight would be somewhat less.

Lack of nocks on the bow
There was no evidence of nocks cut into the tips of the bow, reinforcing the conjecture that this was an unfinished weapon. Is the absence of nocks on the bow tips further evidence that the bow was not yet finished? There are plenty of examples of "primitive" bows that lack nocks. Instead, the bowstring was simply tied to the bow tips when used. The Mary Rose bows were initially thought to be unfinished because the nocks were very shallow. Later it was pointed out that the shallow nocks were compelling evidence that the bows had originally had cow horn nocks. The process of adding cow horn nocks to a longbow frequently includes increasing the depth of the grooves after the cow horn nocks have been fastened. Then the grooves penetrate into the wood. After the undersea microorganisms eat away the horn, then the bow is left with shallow grooves where the horn nocks used to be.

Computer modeling versus copying
Once a computer based bow modeling program has been created it becomes very easy to check the hypothetical performance of a given bow design, much more so than actually building and shooting a bow with the same dimensions and material.

Given the elastic modulus (stiffness) of the wood and the dimensions of the bow limbs it is a straightforward procedure to compute the force-draw characteristics of a bow. The same parameters will also give the strain at full draw. Strain, the amount of compression on the inside (face) or amount of extension on the outside (back) of a bow limb is a very valuable indicator of how close to breaking the bow limbs are. One of the virtues of yew as a bow wood is that it can be compressed or stretched more than 1.1 % before rupturing. As a comparison, red oak can only be safely strained about 0.8 %. The computer calculation of the draw weight will be as accurate as the assumed value of the elastic modulus used in the calculation.

The elastic modulus for yew wood has been stated in several publications. Archery-the technical side (page 74) and Hardy's Longbow uses the number 1.46 million pounds per square inch (psi) and maximum strain of 1.16 %. The USDA Forest Service's Center for Wood Anatomy Research states the elastic modulus of yew as 1.35 million psi (dry) and .99 million psi (green) with maximum strain of 1.13 % (dry) and 1.02 % (green). An elastic modulus of 1.46E6 fits Stemmler's data quite well. Assuming the elastic modulus of yew = 1.46 million psi, we have, from the published dimensions of the three bows.

With the stated dimensions of Ötzi's bow it would undoubtedly break if some Herculean bowman attempted to pull a 28 inch arrow because the strain in the wood would be excessive. This is the most compelling reason to claim that Ötzi's bow was a work in progress.

Conclusions
Evidence for and against the hypothesis that Ötzi's bow was an unfinished weapon has been presented. If it were a finished weapon it would mean that Ötzi had a very powerful physique and probably shot his bow on an almost daily basis. Otherwise he would not have the strength to shoot it accurately. It would also mean that X-ray examination of Ötzi's arm and shoulder bones would show that his skeleton had the characteristics of someone who had a very asymmetric upper body muscular development, characteristic of someone who shoots a very powerful bow.

In its original state, the bow was extremely powerful and would have broken on the first shot because the compressive strain in the limbs (1.5%) was so great. The bast fiber bowstring was probably much too thin to support a bow of this weight. The simplest way to transform this stave into a weapon pulling 50 pounds at 26 inches would be to reduce the thickness along the limbs to 69 % of its present value (the cube root of 50/150.9), leaving the width unmodified. This would reduce the maximum strain to 0.97 %, a very safe conservative value for a yew bow.

Further research needs to be done on the capabilities (tensile strength and durability) of bowstrings made from Tilia sp. bark fiber. Someone also needs to make and test a yew bow with similar dimensions to Ötzi's.

References
Spindler, Konrad; 1994, The Man In The Ice, ISBN 0-517-79969-3

Hardy, Robert; Longbow, ISBN 0-9511747-3

Hickman, C.N.; Archery-the technical side, available from PrimitiveWays.com

Stemmler, L.E.; 1942, Archery Essentials, available as a download from

http://www.stavacademy.co.uk/mimir/archeryessentials.htm

Kooi, private communication

(The artwork in the following "Figures" can be seen in the "Bulletin of Primitive Technology" periodical.)
Figure 1. One limb of the bow. Some idiot broke off the other limb.
Figure 2. A close-up of the texture of the bow limbs.
Figure 3. A cross-section 28 cm from the tip

This article was first published in The Bulletin of Primitive Technology (Spring 2006, #31)
E-mail your comments to "Richard A. Baugh" at richardbaugh@att.net