The Art & Science of Throwing

The Plumbata Revisited


Plumbata

For such a simple weapon, the Roman war dart known as the plumbata has been misunderstood by researchers for decades.  Consisting of a wooden shaft, a ball of lead, a point of iron, and feather fletching – its lack of complexity would seem to offer little resistance to a complete understanding of its attributes and capabilities. 

That assumption would be wrong.

-Bruce Pruett, Independent Researcher & Historian

A year ago, at Athenaeum, I presented my research on the plumbata. Since then I’ve continued my experiments in plumbata manufacture and design and in throwing them. Oh, what a difference a year can make. This exhibit is a continuation of my previous work showcasing what I’ve learned and the path I’ve followed. To gain a greater understanding of my presentation it would be helpful to review last year’s exhibit, linked below.

The Art and Science of Throwing – Athenaeum


My Path of Discovery

Shortly after Athenaeum last year I was contacted by a researcher who specializes in the historical use of Roman weapons and tactics. He is a few years ahead of me in experimenting with plumbata and wanted to share his research with me. He has studied body mechanics as it relates to over – vs – under hand throwing, the physics of weight and drag on thrown weapons, and the practicality of the over – vs – under hand throw in a Roman formation. Some of his thoughts and ideas pointed me down paths I had not considered and have helped me in my own journey. My experimentation this year has focused on weapon design, construction methods, and throwing technique. Based on my latest design, and the effectiveness of the new design, I had to change my assumption on the most effective way to throw plumbata.


Weapon Design

Variations in Length of Hand Grip

My first attempts at creating plumbata were based on what I thought looked good. I had purchased a few weapons online that looked to my untrained eye like they would be effective and deadly. I then began to tweak this basic design making incremental changes and noting how these changes affected overall flight performance. As you can see by the pictures, these changes were initially to the overall length of the weapon, the size of the hand grip between the fletching and the end of the weapon, and the fletching size itself. I kept the same shaft size for my first experiments but eventually tried a narrower shaft. To this point, I have not changed the size of the lead weight so the overall mass of my weapons has varied by only a few grams. I’m trying to limit the number of variables I change so I can see the effect of each one before moving on to the next.


Weapon Length

In changing the overall length of the weapon I’ve modified the distance from the head to the fletching and from the fletching to the end of the shaft, the grip area. When considering overall length I need to remember that Roman soldiers carried these on the back of their shields. This seems to indicate that they could not be too long or they would become unwieldy and difficult to draw and throw. I’ve also noted that a longer grip allows for better control of the plumbata which makes them much easier to throw.

Varying Lengths of Plumbata
Conclusions:
  • Weapons that are longer do fly further
  • The longer grips allow better control and increased flight distance
  • Longer weapons with longer grips makes my overhand throws travel greater distances than my underhand throws, this is a major change from my previous experiments
Questions for further Study:
  • What is the optimum length for maximum throwing distance?
  • Would a heavier/denser wood shaft fly further than a lighter one?

Shaft Diameter

The diameter of the shaft on the first weapons I bought was always ½ inch so this is the measurement I first used. I’ve made several weapons of varying lengths using this shaft size. One of the suggestions that was made to me by my fellow researcher was to decrease the shaft diameter. In his experience this increased flight distance. My latest weapons use a ⅜ inch shaft. Making this change forced me to also change the size of the iron tip insert. I could no longer use the ¼ inch square iron tip material, so I changed to a 3/16 inch round stock. This meant that there was less iron in the tip which would mean less weight. However, the weight difference was offset because I was using the same mold to pour the lead. With the narrower shaft and tip material there was additional room in the mold for more lead. The overall weight difference between the weapons with the ⅜ inch shaft and the ½ inch shaft was minimal.

The image to the left shows 2 plumbata heads found at Burgh Castle located on the east coast of England. The example on the left side demonstrates how I chose to construct my plumbata. This illustration shows the shaft with a diameter of approximately 1cm, or 3/8 of an inch. I modified the design of my most recent plumbata to reflect this example.


Throwing Test #1

Comparing 2 darts of the same length with different shaft sizes throwing both over and under hand.

Underhand Throws
½ inch shaft3/8 inch shaft
117’115’
116’123’
123’131’
114’130’
120’138’
118’ Average127.4’ Average
Overhand Throws
1/2 inch shaft3/8 inch shaft
115′121′
128′133′
126′142′
131′144′
129′144′
125.8′ Average136.8′ Average

This was my first trial with newly redesigned darts. I had increased the overall length of these plumbata, reduced the fletching size, and made the grip area behind the fletching longer as well. Unlike the tests with my earlier designs these showed that throwing overhand produced greater distances that my underhand throws. I believe this was due to having a longer grip which allowed me to throw the weapon more like it was designed to be thrown and utilize the greater force that throwing overhand produces. This test also showed that the smaller diameter shaft resulted in greater flight distances.

Throwing Test #2

Underhand Throws
½ inch shaft3/8 inch shaft
122′123′
132′130′
125′136′
124′135′
124′129′
123′143′
122′132′
123′129′
127′134′
125′149′
124.7 Average134′ Average
Overhand Throws
½ inch shaft3/8 inch shaft
132′140′
128′134′
136′139′
138′140′
127′137′
137′139′
138′140′
141′138′
134′148′
141′142′
133′135′
124.1 Average139.3 Average

This second test produced similar results as the first test with these weapons. I also observed that the greater distances were obtained from throws that had the least in-flight wobble. I believe that learning how to throw these weapons with minimal wobble would come from practicing with them. I also think I could get more distance from decreasing the fletching size even more.


Conclusions:
  • The narrower shafted weapons flew further
  • The weapons with the smaller diameter were weaker and more prone to break
Questions for further study:
  • How would a 5/16 or a 7/16 inch shaft affect flight distance?
  • Is the optimum length for the narrower shaft the same as for the wider shaft?

Fletching Size

Varying Fletching Sizes

I have to admit that I am not an expert on the physics of fletchings on shafted weapons. This being said I didn’t know what I was doing when I fletched my first plumbata. I just guessed the size based on what I thought looked about right. One of my main concerns was wobble in flight which would be reduced by the correct fletchings. I made the assumption that a larger fletching would decrease this. Unfortunately, it also decreases flight distance by increasing drag. I’ve been experimenting with making them smaller while increasing the shaft length. So far, my longest throws have been made with this combination. The picture on the left shows the dark fletching to be smaller in two dimensions. It is both shorter in length and it doesn’t project out from the shaft as far. In some future trials I will experiment with both of these dimensions to try to discover what will give me the best results in terms of flight distance.

Conclusions:
  • Using smaller fletchings seems to have increased flight distance
  • There has been no noticeable increase of in-flight wobble using the smaller fletching
Questions for further study:
  • Does a longer, narrower shaft require smaller fletchings?
  • What is the ideal size to achieve maximum flight distance?

Observations & Conclusions

My experiments in the last year have caused me to change my original assumption about the most effective way to throw these weapons. My previous tests showed that underhand throws produced greater distances than overhand throws. These results were accurate given the design of the weapons I was throwing. As my designs have changed, so to have my results. My most recent tests have shown that, on average, my overhand throws flew further than my underhand throws.

A fellow plumbata researcher, Bruce Pruett, wrote a paper in which he addressed this same issue and provided some explanations as to why this is true. He pointed out that throwing overhand produces a greater force than throwing underhand. He also suggested that poor weapon design and poor throwing technique could also lead to a false conclusion that underhand throwing produces better results. This is exactly what I have found in my own experiments.

My early design flaws did not allow for enough space behind the fletching to grip the weapon properly for an overhand throw. I was trying to throw it like a large pub-style dart by grabbing it in the center. When I increased the length of the grip I was able to properly hold the plumbata behind the fletching. The effect of this was to change the center of mass of the weapon from the palm of my hand to several inches beyond it. This allowed more of a whip-like action just before release, increasing the effective force imparted to the dart. I’ve also found that narrowing the shaft diameter and reducing the fletching size have also led to greater throwing distances. I believe this is due to the decreased drag on the weapon as it flies through the air. 

In the future I will continue to adjust the length and fletching size of my plumbata as I strive to reach a maximum throwing distance. One variable I have not yet experimented with is the weight of the weapon. This will become a primary focus for my next round of experiments with the plumbata.

In this last year I was lucky to have had the opportunity to collaborate with a fellow plumbata researcher, Bruce Pruett, who contacted me after Athenaeum 2020. He has done extensive research into this weapon and how they might have been used by the Romans in battle. His insights have helped me focus my research in how to throw plumbata to achieve greater distance. Our discussions have been interesting and insightful even though our general focus is somewhat different. Bruce has used modern materials and construction techniques in his research while my focus is in traditional and historical techniques. Through our research and experimentation both of us have come to the conclusion that throwing properly constructed, and properly thrown, plumbata overhand will produce the greatest flight distance. This conclusion is in direct contradiction to virtually all other research in this area. It will be interesting to see where this path leads me in the coming years.


References

Pruett, B.R. (2020). Re-testing plumbatae. Setting the record straight via experience, experimentation, and the examination of historical sources. Academia. https://www.academia.edu/42176281/Re_Testing_Plumbatae_Setting_the_Record_Straight_via_Experience_Experimentation_and_the_Examination_of_Historical_Sources?source=swp_share

Sherlock, D. (1978). A Roman mars-barb from Burgh Castle. Suffolk Institute, volume number 34 (part 2), 141-143. http://suffolkinstitute.pdfsrv.co.uk/customers/Suffolk%20Institute/2014/01/10/Volume%20XXXIV%20Part%202%20(1978)_A%20Roman%20Mars-barb%20from%20Burgh%20Castle%20D%20Sherlock_141%20to%20143.pdf

7 Replies to “The Art & Science of Throwing”

  1. I appreciate your systemic approach to analyzing the variables involved in the construction and use of the plumbata. Looking forward to seeing where it leads you.

  2. Great research and observations! I was thinking about your comment that the thinner diameter plumbata flew better but were more fragile. Could this have actually been a desirable trait? I know that a broken plumbata is a disappointment to you, but to a Roman soldier it may have been an advantage that their weapon could not be thrown back at them.

    1. That is exactly my thought on the matter. These things are really pretty easy to make and repair so making them single-use would seem to make sense to me. After all, you would not want your enemy to use your own weapon against you.

  3. Well done,
    I’ve also started experimenting with plumbata, but ou are much farther along in flight testing, I have been focusing on costruction detail. I have found that socketed heads result in a more durable but more labour intensive weapon.
    All has been on hold due to COVID but I will dig out my research .

    1. I would be interested to hear about your construction methods. How do you create your socket? Do you start with a square/rectangle stock and forge it flat or do you use sheet stock to form the iron shaft?

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