THE ATLATL RESEARCH OF BOB PERKINS
Pronounce it "ott-lottle"
Bob Perkins of BPS Engineering has been interested in atlatl and dart mechanics for decades. BPS engineering claims credit for four major and half a dozen minor discoveries in the field. Onagocag Publishing is pleased to be able to present Mr. Perkins papers on Atlatl mechanics as part of The History and Primitive Technology Page.
EFFECTS OF STONE PROJECTILE POINTS AS MASS
Stone projectile points represent the single most durable artifact occuring in the archaeological record. A great deal of research and speculation have gone into their interpretive qualities. Materials in knapping techniques have been studied, point styles have been typed, categorized, dated, and volumes of information have been published relating directly and indirectly to stone projectile points. But, other than explaning that the sharp edges and point are for piercing hide, flesh, tendon, and smashing bone and cartilage in order to inflict traumatic wounds, their function and effects within the mechanical system of the atlatl and dart have been largely ignored.
The primary consideration of projectile technology is to make a smaller particle go faster. The atlatl and dart is a deceptively complex mechanical system, highly sophistocated even by today's standards, and the mass of the stone projectile point plays an integral role within this system. In the mathematical expression for energy, one half the mass of a particle multiplied by the velocity squared, it can be readily seen that velocity plays a more significant role in increasing energy than mass. Over a time improvements in projectile technology have been marked by making smaller, lighter particles go faster. However important the velocity of a particle is, when analyzing a weapons system the mass of the projectile, its effects within the system, and how that projectile is accelerated, are paramount in the interpretation of that weapons system.
Prehistorically the concept of making smaller projectiles travel faster can be generally traced in the study of stone projectile points with heavier points occuring earlier and lighter points occuring more recently in the archaeological record. Studies of stone points from sites known to have utilized either atlatl or bow and arrow systems has shown a trend toward lighter points for arrows (three grams or less) and heavier points for darts (four grams and greater). Generally speaking the atlatl predates the bow by a considerable margin, and in fact, the atlatl has enjoyed such an extended and widespread tradition that comparatively speaking the bow and arrow is a recent development in projectile technology. In North America the atlatl can be traced back in the archaeological record some 8-10 thousand years, where as the bow has been generally accepted as being introduced only 1500 to 2000 years ago. The atlatl and dart was used in North America longer than any other weapons system to date. Therefore a detailed study of the projectile point mass and its effects on this system, establishing parameters for minimum and maximum mass will help, through a process of elimination, and distinguishing between arrow, dart, and lance points.
The atlatl and dart is defined as a spring mass mechanical system. (See Atlatl and Dart Mechanics at the bottom of this page. -wrk- ). The flexible dart is the mechanical foundation of this system and the mass of the point plays a critical role within it by causing the dart to flex and store spring energy.
When the flexible dart is accelerated by the atlatl the point mass at the opposite end of the dart resists that acceleration and causes the dart to flex and compress, storing spring energy to be used for the launch. The potential energy available to the dart is dependent upon three things, the length and flexibility of the dart itself and the mass of the projectile point at the end of the dart. The mass of the point directly controls the amount of energy available to the system. Without point mass the system will not function to its full potential.
The mathematical perameters of length and flexibility as they relate to the atlatl and dart can result in a number of design variations and performance capabilities. This results in a wide range of point masses that can be used. Somewhat like bullets today, each particular atlatl and dart design requires a particular size projectile point. Basically point mass is proportional to dart length. The longer the dart, the greater the spring force required for efficient operation and the greater point mass required to cause its compression. Once a system is designed and fixed with length and flexibility within functioning perameters the particular mass of the projectile point is fixed and any deviation from that mass will cause the efficiency of the system to deteriorate. The perameters of the point mass for any particular system design can vary from two to three grams, but for peak efficiency a consistant point mass is required. A dart designed to use a six gram projectile point will not function properly with a twelve gram point. It can however, function within acceptable parameters with a five or seven gram point but to its full potential.
Point Mass Deviation
In experiments using darts designed to function most effectively using a point mass of nine grams, a measurable drop in performance was noted when the mass of the point was altered by 1.5 grams. When point mass was increased to 11.5 grams or decreased to 7.5 grams overall distance decreased by 3 to 7 yards from a maximum of approximately 120 yards. Accuracy, being highly subjective, was not tested during this experiment. It was noted however, by an outside observer that launch characteristics did not seem to be as smooth and efficient when point mass was altered from design specifications. This experiment demonstrated the sensitivity of point mass to the overall performance of the system. In the archaeological record the more recent dart points are generally lighter, which if the premise of projectile technology is true, represents a higher degree of sophistocation in the designing of the system. Make a lighter particle go faster. As dart points became lighter in the more recent and advanced systems, the more critical the point mass becomes and the less deviation from design standards can be tolerated. As an example, if a system is designed to use a 12 gram point, mass of 10 to 14 grams can be used efficiently, but if a system is designed for a four gram point the tolerable deviation is less than one gram. It is believed that when ancient people designed their atlatl and dart systems, consistency in this area was taken into consideration and manifests itself in the archaeological record as tool traditions.
Projectile points of a certain tool tradition are primarily identified by style. Size, material, and knapping techniques are noted to a lesser degree, but mass generally not at all. The stated purpose for the various styles is hafting technique and indeed this would seem to be the case due to the variety of notched, stemmed, and fluted points found. Hafting technique certainly plays a role in certain styles of points and the similarity of points of particular tool traditions, but as the primary function, hafting technique falls short when considering the point as a part of a mechanical system.
What can be seen in the similarities of points within a tool tradition when considering the point as part of a mechanical system is production line consistency. For a weapons system of any degree of sophistocation consistency in the manufacture of its various components is paramount for the success of its deployment. It is primarily the mass of the projectile in question which weapons designers have been concerned with.
While studying atlatls and related artifacts at the Smithsonian Institute in Washington D.C., I came across a collection from a site in South Dakota. The collection contained two or three atlatl weights which were my primary interest, but also from the same site were several projectile points and preforms. Three of the points appeared to be new and unused with no indication of resharpening. Two of these points were made from what appeared to be the outer cortex of chert, but the third was definitely knife river flint, an extremely dense, hard material. Width, thickness and notching were all approximately the same but the length of the knife river flint point was a full centimeter shorter than the other two.
In a test of my theory that point mass within the the atlatl and dart system is critical all three were weighed on a digital scale. The results were astounding. All three points weighed essentially the same at 7.6, 7.7, and 7.8 grams respectively. This certainly suggests that mass was an important consideration i the manufacture o fprojectile points. Being that width, thickness, and notching were also approximately the same suggests a secondary importance to these dimensions, with length being adjusted from material type to material type in order to keep mass consistant, standardizing the projectile points of this tool tradition.
Therefore, the similarities between points within a tool tradition can be seen not primarily as a hafting technique or some sort of ceremonial process, but as an attempt to standardize a weapons system's mechanical efficiency.
The fact is stone is not predominantly used in the manufacture of projectile points because of its durability, convenience, or its ability to pierce hide and flesh. Other materials such as bone, antler, and just an old fashioned pointy stick meet these needs and are much easier to work with. But stone possess one quality all others do not -- density. Projectile points of these other materials can be made to have the same mass as stone points, but they would be larger and less efficient and therefore less desirable than stone.
It is the mass of stone points which must be more carefully analyzed in order to gain a more complete understanding of the entire weapon from which it was once an integral part. The lighter the point the more sophistocated the system, but there are limits to the minimum mass which will function in the system. Part of this minimum limit has to do with the materials the dart is manufactured from. The less dense the dart material, keeping in mind the perameters of length and flexibility, the lighter the point mass that can be successfully used in a dart of that material. I have experimented with several types of dart material and have found that "locally" (Gallatin Valley) red oisier dogwood is best. With this material a dart with a minimum point mass of about five grams can be designed for peak performance. But being that the density of red oisier is .6 kg/L the lighter the projectile point the less influence it has on this material. Dart materials of lesser density, such as cedar, can be designed to function efficiently with lighter point masses.
I have designed darts made from cedar which function efficiently with point masses of three to four grams. Darts of this design were about 54 inches in length and functioned well with 18 to 20 inch atlatls at an effective killing range of about 25 yards. Beyond this it is my opinion, that attempting to design a dart that will function properly with a less than three gram point mass will render the overall mechanics of the system ineffective. As the dart shortens the effective range of the system also shortens in order to maintain the proper timing match between atlatl and dart. Also, being that the atlatl's acceleration is angular rather than linear, less velocity is achieved by the shorter atlatls which are required to function with shorter darts.
It is possible to design a dart that will just function without any point mass at all, but we want atlatls that are efficient. A dart can be made to toss down range but we want something efficient enough to hunt or war with - not a toy.
What I am ultimately trying to point out is that when designing this weapons system many factors must be taken into consideration. And when a particular aspect of this system is considered by itself, such as the point mass, and a minimum requirement it established such as minimum mass, many other inter-relating factors must be considered, such as performance requirements and available materials. If I were required to design an atlatl and dart system to function efficiently using a two gram point mass from locally available materials, I could come up with a design using red oisier dogwood which could have an effective range of ten or fifteen meters. The dart would be approximately one meter in length and the atlatl one third of that length using an influencing atlatl weight mass of 20 grams. But would this be a viable weapon? Probably not.
Maximum Point Mass
In considering what is the maximum point mass that can be tolerated in the atlatl and dart system, all aspects of whatwere considered for minimum point mass are reversed. With greater point mass goes greater dart and atlatl length. The aborigines of Australia are known to use darts up to 12 feet in length with correspondingly long atlatls of more than three feet in length. Although I have never had the opportunity to weigh any Australian dart points, I have seen many of them on these extremely long darts in the Smithsonian collections. They are quite large and certainly in excess of 20 grams, but it is my opinion that systems of thismassive of a design are far less efficient than those of more reasonable dimensions. Also in cases of dart lengths in excess of eight feet, point mass becomes less critical for efficient operation due to the leading mass of the dart material itself helping in compression along the entire length of the dart. Indeed, some Australian darts had no stone points at all and did not appear to be designed for any. Knwing the mechanics of the system the way I do I am sure these darts functioned quite adequuately. So long as the dart is sufficiently flexible, extreme length gives stability. However, it is my opinion that the Australian atlatl systems represent extremely crude technology. One based on brut force rather than efficiency, and merely one step above hand thrown spears in their developement.
The mass of the projectile point plays a critical role in the mechanics of the atlatl and dart system. With a sufficiently light dart material a point mass of three grams can be used in an efficient design. With a sufficiently long dart no point mass at all need be applied, but the cumbersome nature of this design and it's inherent lack of efficiency excludes it completely.
The lighter the point mass the more technologically advanced the system. And the more advanced the atlatl system the greater the consistancy of the point mass required for uniform standard performance. A good test of this theory would be to weigh points from a single site of a known culture.
There is also what I call "effective mass." This is the mass of the point after it has been hafted onto a shaft. In experiments conducted in hafting projectile points I have found that the glue, sinew bindings, and the surrounding wood of the notch itself add an average of one gram to the overall mass of the point. So in effect a 2.5 gram point has the effective mass of 3.5 grams. At this weight is it a dart point or an arrow point?
On the high end of distinguishing between dart and lance points the question becomes a bit grey. A 25 gram point could certainly be part of a less technologically advanced atlatl system, but it might also be a lance point or even a knife blade.
In my research I have found that the overall average mass of dart points is approximately 9 grams, and in fact, this is the mass I prefer and use most often in my atlatl and darat systems. But I've also designed systems that use a mass as light as 3 to 4 grams and as heavy as 15 to 20 grams. What I have definitely found is that once a system is designed to use a particular point mass, stick with it. Deviating form that mass or for that matter anything else such as dart length, atlatl length, or flexibility changes the entire feel of the system entirely. This system is so sensitive that changing the length of the dart by so much as one inch changes the entire feel of the system and causes the dart to launch earlier or later in the swing, thus changing its point of impact on the target. This is why foreshafts were used. When a dart is launched down range the chances of the stone point breaking upon impact are greater than 50%. Sometimes the breakage can be repaired with some loss of the mass, but still remain within functioning parameters. But if the point snaps and takes part of the haft with it, a one piece dart would be shortened considerably when a new point is rehafted onto it. A shorter dart is a stiffer dart, but with the advent of foreshafts this problem is completely avoided.
Point mass and it's consistancy certainly play an integral role in the mechanics of an atlatl and dart system. But no more so than dart length, dart flexibility, and atlatal weight mass, not tomention the materials that go into the construction of the system. A complete understanding of this impressively complex weapon must be achieved before any single component can be properly analized. So long as it is thought of and referred to a "spear thrower" that will never happen.
Click the links below for more of Bob Perkins' atlatl papers.
This page was last updated on 07 March 2012.
Copyright © 1998 & 2012 by Wyatt R. Knapp