31" - to draw, or not to draw

[skerm]

Hi!

I am working on a yew bow. This is the first one from a couple of staves I got. It didn't split quite the way I had planned so the stave was only 35mm thick.  I figured that would be a good starting thickness to work my way towards 100#, but it also meant I had to live with a high sapwood to heartwood ratio towards the tip. Sapwood is between 10mm and 12mm thick. As it turned out the sapwood was weaker than I guessed due to having no clue and I had to remove quite some wood to get a good tiller. Consequently I ended with 71#@31" instead of the 100# I was hoping for.
I read that when shortening by 1% the draw weight increases by 5%. So I piked the bow from the initial 79" to 71", which should then make 100#@31".

Here she's braced



and drawn to almost 28".



I only scraped a bit on the right limb, otherwise it is unchanged since the piking, so it is still thick at the tips. I measured the draw weight at various draw lenghts and extrapolated the draw weight at 31". This looks reasonable.



Now the question is what are the chance of the bow surviving the draw to 31"? The decision was more or less made when I sawed of the 8", but I was still a bit reluctant to draw further than in the picture. What do you think?

Daniel

[zenmonkeyman]

I don't know the answer, but if you try it, have a video camera running!

[Badger]

  The bow looks good, how much set has it taken? Check the weight at 28, then go to 29, come back to 28 and check it again and see if it is dropping as you increase the draw length, if you notice it falling I would stop right there. Do you have any idea what the physical weight of the bow is at this point? I would be looking for about 24 oz if I was going to take it to 31" and close to 100#, Steve

[acker]

71" is pretty short for that draw weight and that kind of yew , I wouldn't say it'll be much fun to shoot that bow.
Will probably feel like pulling 130# or more.
If you're able to pull it , do it but don't forget to wear some safety glasses.You know, yew don't just break it explodes with a loud bang.
But the idea with the video cam is very good 
Gruß acker

[skerm]

Thank you for your advice!

I promise I will use a camera should I decide to go for it. I drew it to 28" a couple of times and I didn't notice anything particularly inconvenient about it, but then again I was focused on the draw length and listening if the bow would make a noise.

Steve, I worked the tips to 14mm width, 16mm thickness and tapered the outer 5-6 inches accordingly, now the mass is 607 grams, or nearly 21.5 ounces. I am a little worried now because of the difference to 24 ounces.
I pulled it to 28" for a couple of times and then measured the draw weight. It was 86#, which fits the extrapolated weight from the graph, so what I removed was just dead weight left after piking. Then I went for 29" two times in a row and then pulled it to 28" a few more times. The draw weight went down to 83# and there was minor set in the left limb and an 1" set in the right limb.

This is the unbraced shape before and after the 29" pulls.



I left the bow in the workshop since yesterday and I noticed that the relative humidity increased to 70% since I last checked. Must be because it is finally getting warmer. However, the temperature is only 10°C / 50°F, so I guess this is about the same as 50% at room temperature. I am not sure though. I will have to move the staves and the bows to my room again just to be safe.

Daniel

[Badger]

   I would stop right there and call it a 28" draw bow, if you bring it out to 32" I think the weight will drop considerably and very likely break, Regardless still a very nice bow. Steve

[skerm]

Yes, you are right, it seems I have reached the yield strength of this yew. Thanks again for your advice, I'll keep this procedure in mind! I started debarking a thicker yew branch and will try again. The sapwood is about 1/4" on this one, so that's better, but the outermost ring is so thin that I can remove it with a single pass of the scraper.

Daniel

[Hrothgar]

Nice looking bow, excellent tiller.

[Rod]

A sapwood bow would be weak, be nice to actually see some heartwood on the next one.

Rod.

[bow-toxo]

Hi!


I only scraped a bit on the right limb, otherwise it is unchanged since the piking, so it is still thick at the tips. I measured the draw weight at various draw lenghts and extrapolated the draw weight at 31". This looks reasonable.



Now the question is what are the chance of the bow surviving the draw to 31"? The decision was more or less made when I sawed of the 8", but I was still a bit reluctant to draw further than in the picture. What do you think?

Daniel

 I go for the medieval standards whenever possible. By that measure a 31" arrow would take a 71" bow     [between nocks] for normal shooiing, 3 1/2" less for a flight shooting bow that would have a limited life. Those would be bows tillered to an arc of a circle, without thick tips. For a 100 pound bow, there should be no problem. There ar two possibilities for sapwood. One is 'chasing a ring' which gives you bow tips that are mostly sapwood, the other [my personal preference] is to keep sapwood at 1/4 to 1/3 the thickness through the entire bow. If you want the thinner tips that were thought to increase cast, I suggest shooting for a while first.
                                                                                       Erik
 

[alanesq]

It looks to me like it is on the point of developing a hinge on both limbs and so it would be best to try to stop this happening by removing some wood first - but then you are going to reduce the draw weight....

So I think I would be tempted to leave well alone and call it finished

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btw - Wouldn't a bow which is difficult to draw for a given final draw weight be preferable ?
i.e. the harder it is to draw the more powerful it is going to be   (essentially a higher initial draw weight giving higher draw weight at all draw lengths up to full draw)

as long as it isn't so short it starts stacking at the end of course