Author Topic: Looking for information on warbow dimensions and draw weight  (Read 822 times)

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Offline JonathanD

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Adam Karpowicz's paper "Ottoman bows an assessment of draw weight, performance and tactical use" includes a formula for approximating the draw weight of a bow based onthe limb dimensions, length and draw weight, and I've recently put together a spreadsheet estimating the draw weights of most of the Mary Rose bows using the data published in Weapons of Warre, which fit reasonably well with the graph based on the calculations using Kooi's computer program. However, this is an imperfect process*, since I only had two reliable examples that had all the data and the original calculations were based on the properties of Pacific, rather than European, Yew.

So, I was wondering, are there any bowyers who would be willing to provide information on bows they've made to produce a more accurate estimate of the range of draw weights for the Mary Rose bows? I need the width and thickness of the center of the bows, plus the length of the bow and the draw weight @ 30" (as per Weapons of Warre). Ideally it would be great to get around 5 examples each for <40 RPI, 40-60 RPI and >60 RPI so I can get a more accurate estimate for each category, but even 5 examples total would be extremely helpful.

If you have any information on bows with reasonably high draw weights (>70lbs) at 28", that would also be handy, as I'm also doing the same process for the Iron Age Germanic bows I have data on, but only have information on replicas made with Pacific Yew. The 28" is important, since the most powerful bows are, without exception, less than 67" NTN and more generally under 65" NTN.

*The whole process is imperfect, since only a full scale simulation such as Kooi's will produce even anything even remotely close to "accurate" and there will always be major variations due to the nature of the wood and the way the bowyer works, but the goal here is to be as least imperfect as possible and come up with ballpark figures.

Offline Del the cat

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    • Derek Hutchison Native Wood Self Bows
Re: Looking for information on warbow dimensions and draw weight
« Reply #1 on: June 28, 2019, 12:01:20 am »
This is all just my opinion...
I don't think you will get many takers.
It is too hard to characterise a bow from a few dimensions, it only need a bit of poor tiller or a thin spot to completely throw out any estimates (I won't call 'em calculations). (I've seen some horribly whip tillered warbows that I wouldn't draw past 28". Data from those would be worthless)
I don't think you'll make any useful improvement on what has already been done and it will be of dubious value.
You can't really expect to make a self bow to a specified poundage from dimensions.. or if you could it would be sucking all the joy and artistry from the process. The best you can achieve is a rough out guide, and anyone in their right mind will always rough out oversize anyway.
Where dimension are useful is when copying an existing bow and making slight changes for a specific purpose, say a flight bow.
When making warbows I treat each one as individual.. I may take a couple of dimensions at the grip to rough it out, but from then on it's all about the tiller.
Del
« Last Edit: June 28, 2019, 12:04:54 am by Del the cat »
Health warning, these posts may contain traces of nut.

Offline peacefullymadewarbows

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Re: Looking for information on warbow dimensions and draw weight
« Reply #2 on: June 28, 2019, 06:52:07 am »
What Del said is pretty much what you will probably hear from most bowyers. RPI has little bearing on the stiffness/density of a given piece of yew. If you look at the replica I just posted below it is a dimensional replica of MR81A3966; one of the bigger bows on the ship. My replica ended up at 118# @ 31". This was 50 RPI pacific yew. This was surprisingly low from what I would have expected. Similar dimensions have produced 130-150lb bows. One of the densest and hardest to carve pieces of yew I've handled was only 18RPI and very pale. But, it was some of the stiffest yew I had worked with. Point being is you can take two pieces of 60 rpi yew with similar color of heartwood, keep the same heart wood sapwood ratios, maybe even from the same tree, with exact same dimensions all around and end up with two bows 40 lbs apart in draw weight. Will Sherman on here has done some in depth work to prove this. I apologize if this is a disappointing answer for you but it is the reason we cannot truly nail down exact draw weights of the historic bows since we have no idea the true properties of the wood. Many of us have tried but like Del said there's something to be said of the art of each bow/piece of wood being totally unique. Each one has a personality. That's what makes it fun for us. When we want to achieve a specific draw weight, we go well over dimensions and carve it down gently until we reach it, but replicas will end up all over the place. Sorry for the rant.

Offline JonathanD

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Re: Looking for information on warbow dimensions and draw weight
« Reply #3 on: June 28, 2019, 03:15:30 pm »
Thank you both for your replies.

As you've guessed, while I was aware that variations would naturally occur, I wasn't really aware of how large those could be. With that said, I'm not looking at this as a tool for estimating draw weight when making bows, and I should have been clearer. My goal is to use these estimates to look at the bows in their historical context and try to work out what any trends that reveal themselves mean. For instance, my current data set for the Nydam shows that 17 out of 26 bows had draw weights between 55lbs and 75lbs, and only 3 were over 75lbs. While some of the weakest bows could, say, have actually been made with very stiff yew and had a 95lb draw weight, the general grouping indicates that the most common bows were not purpose built warbows. What are the implications of that? I'm not sure, I'm still running that over in my head.

However, this is all based on two replicas made from a different kind of wood (Pacific rather than European Yew) and I'd like to get a large sample size to get a better estimate. A large sample size would also, in light of your comments, help demonstrate the variations in wood and provide a very rough guide to the range of draw weights you could expect for a given bow. The bigger the sample size the more "Average" the average is and the better variations show up.

I do understand your reluctance with regards to my project, and thank you for helping me to better understand the issues in estimating historical bows. It's something for me to think on further.

Offline peacefullymadewarbows

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Re: Looking for information on warbow dimensions and draw weight
« Reply #4 on: July 01, 2019, 06:24:43 am »
I guess one of the issues with your project is simply procuring a mountain of different yew staves with different characteristics and making a bunch of replicas to get a good bell curve sample spread of draw weights. My guess is that there simply haven't been enough replicas made to give you enough data points. Again I am only saying disappointing things. I apologize.

Offline Del the cat

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Re: Looking for information on warbow dimensions and draw weight
« Reply #5 on: July 01, 2019, 06:52:48 am »
I guess one of the issues with your project is simply procuring a mountain of different yew staves with different characteristics and making a bunch of replicas to get a good bell curve sample spread of draw weights. My guess is that there simply haven't been enough replicas made to give you enough data points. Again I am only saying disappointing things. I apologize.
There may have been enough made, but they won't have all been measured at the required intervals!  ;D
Del
Health warning, these posts may contain traces of nut.

Offline JonathanD

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Re: Looking for information on warbow dimensions and draw weight
« Reply #6 on: July 01, 2019, 12:01:20 pm »
All I really need to get started is the width and thickness at the center, length, and draw weight @ 30" for bows made from European yew. I'd be surprised if there weren't five of those based on a Mary Rose cross section out there. If that looks promising I can always go crazy and estimate draw weight based on the average of dimensions from every 10cm of the bow.

I was always dubious about getting the same information for shorter bows at 28", though.

Offline meanewood

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Re: Looking for information on warbow dimensions and draw weight
« Reply #7 on: July 01, 2019, 12:57:53 pm »
The dimensions of the 'Mary Rose' bows vary as does the ring count.

Lots of those bows were recovered in storage boxes which may mean they were part of the ships store rather than personal weapons.

Common sense tells you that the draw weight of these bows would be closer than what the dimensions would suggest.

That backs up what most bowyers would say due to their experience with all woods.


 

Offline WillS

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Re: Looking for information on warbow dimensions and draw weight
« Reply #8 on: July 18, 2019, 05:56:42 am »
Just thought I'd stick this in, as it somewhat applies here.

This is from Roy King himself, as he documented his original Mary Rose approximation bows.

"Sapwood worked down slightly to remove small bumps.  A good stave; no need to follow bumps...

Sides worked down dead square to back, to what should be finished width, leaving just a little extra width at tips... slightly convex taper...

Back generously radiused over full width, dropping a little extra at edges.  Then there appears as if by magic the Mary Rose sharp edge...

Coming to the conclusion that these bows were made almost by numbers.  That given the quality of the wood... a formula for manufacture could be closely followed... a system that isn't used in Victorian bows..."