35" Yew short bow - sinew failure - Lifting and hairline cracking

[2nocks]

Hi guys !
New to PA, please forgive the long-ish post. Hopefully some of the details won't put everyone to sleep.

Probably way out of my league in terms of (my lack of) skill here, but there's one little bow in the British Museum I've been trying to replicate. It's a tiny 34.5" extremely recurved almost U-shape little yew wood bow beautifully decorated in porcupine quills. There's no recurve in the tips and all the (un-braced) recurve is mid-limb.The final braced and full-drawn shape of this bow is a bit of a mystery (to me) although the sinew build-up at the nocks would suggest it might have been quite heavily drawn back and they didn't want the string to slip off. I'm guessing about 1/3 - 1/2 thickness of the bow is the sinew. I'd have loved to post some 'non-failure' results, but as we all know that's not how things go in the land of bow making, so please if any of you have some advice please feel free to chime in.

I did two light courses of sinew and knee/floor tillered it to even out the limbs out. Then I did 3 more layers of sinew to bring it in to a more U-shape reflex. I can't even get a string on it yet or brace it. But I began the tillering by bending it over my knee and re-checking the flex of each limb, I started to thin it down and bring it closer to final dimensions. I've been going very gentle and slowly with this bow and definitely haven't pushed it or pulled on it more than what I feel the wood can handle (and never more than in the 2nd pic). but I noticed a spot flexing/ working way more on one limb. And then also noticed the sinew beginning to lift or fail in that spot. A closer look reveals two strands of sinew that are lifting but also little hairline fissures branching off that indicate a more serious failure. I'm kind of surprised, because I haven't heard any sounds or cracking or anything to make me believe the wood failed underneath in tension and there are no signs of compression fretting or fatigue (with good yew can this even happen?)

Is this game over ? Or can sinew failure like this be recovered ? It's just at the beginning of failing, so I haven't creased or hinged or fatigued the wood, but is it best to tear it down, recycle the sinew, look for wood core failure and start over ? I didn't think sinew ever failed without a wood core failure underneath.

Any help or shared experiences would be greatly appreciated.
Thanks !
-Jonathan

[bradsmith2010]

  I have had sinew do that,, when I did not have the glue thick enough,, usually when the sinew glues really well it wont lift a strand,,, I am just guessing,, it could be core failure,, it would not take that long to take off the sinew and see if there is a problem,, if no problem with the core then your sinew application might be improved,,
also I have had different quality of sinew,, if you can break a small strand in your hand,, it might not be the best,,
maybe it rotted a bit before drying or some unknown,,again I am just guessing,,\\
  you could sinew wrap over that,, using more glue,, or put another layer on the whole bow,, I hate so say this but I think taking it off and starting over would have the highest rate of success,, Im sure others will chime in,, and you can find a solution that will work for you,,

[Hamish]

I'm not very experienced with the use of sinew, but that induced reflex  looks like one hell of an ask even for yew.

[DC]

I'm not very experienced with the use of sinew, but that induced reflex  looks like one hell of an ask even for yew.

That was my thought too. Sure is a pretty bend though

[PatM]

If Perry reflex is real, why can a wood and sinew bow not be reflexed to a ridiculous degree?  If it doesn't  "feel" compression force until it's bent back to its original presumably flatter shape.

[DC]

If Perry reflex is real, why can a wood and sinew bow not be reflexed to a ridiculous degree?  If it doesn't  "feel" compression force until it's bent back to its original presumably flatter shape.

Hmmmmm, give me an hour or so

[2nocks]

Thanks for the replies so far.
After reading what Perry reflex is I think this would qualify. I didn't use blocks at the tips or clamp the grip down, but more like Turkish or Korean bows I pulled the tips together with string while the sinew was still wet and drying.

I'm leaning towards a bad glue up or lack of knowledge and experience in sinewing. I had read that if you use too thick a glue then it can give you cracks and scary sounds as the crystalline structure of a heavily glued-up bow shatters apart. So pretty sure I steered more towards a thinner solution of glue to try and avoid this. Today also have new delaminating at the edges on the tips between layers 2 and 3 of sinew. I can now see this might be putting some lift pressure on the layer of sinew as the wrapped edge is flexing around the bend. Though I seem to remember may other types of sinew bows doing this and not having any difficulty.

Under a good amount of light I can see another similar little hairline crack developing in the sinew -though nothing lifting yet. What concerns me is that these cracks appear to go really deep into the whole thickness of all 5 layers of sinew. If it were just little shattering cracks on the surface I could chalk it up as the glue stretching more than it's 8-10% at the very outer layer of sinew and keep going. But it appears these cracks are setting up very deep into the sinew even into the base layers where the stretch shouldn't be hitting it's limit yet.

For those that might be curious I'll post a pic of the original in the museum - unfortunately it's an old grainy pic.

[Pat B]

Could it be the glue that cracked? If too much glue is used the glue can crack and if that is the case I don't think it is a real problem. I don't know why the wood would crack under the sinew/glue backing.

[2nocks]

  I have had sinew do that,, when I did not have the glue thick enough,, usually when the sinew glues really well it wont lift a strand,,, I am just guessing,, it could be core failure,, it would not take that long to take off the sinew and see if there is a problem,, if no problem with the core then your sinew application might be improved,,
also I have had different quality of sinew,, if you can break a small strand in your hand,, it might not be the best,,
maybe it rotted a bit before drying or some unknown,

-I can pretty much rule out bad or rotten sinew. It was good quality fresh Elk leg tendons, and I cannot break a small fibre with my hands.
-I really don't think this is wood core tension failure yet. I haven't even pulled on the bow past neutral where the wood was at in it's natural resting state as a lightly recurved bow stave branch.
-maybe your suggestion that the glue wasn't thick enough. it is showing some signs of delaminating between the sinew layers and coming off in parchment paper light sheets. I may have erred too much on the side of thin glue after reading stories online about thick glue cracking too much under stress. though my pot of glue was like light syrup and when cooled set up as sticky to the touch when fingers dipped in glue and gelatine when cooled in the pot. So wasn't water thin either.

[bradsmith2010]

   I took me a while I did some too thin, and some too thick,, but it seems erring on the side or too thick really did no damage,,im sure there is a point where it would,, but I have not experienced that, I think its a matter of getting the right amount of glue on the sinew,, not squezing out to much,,,,and keeping the glue warm but not too hot,, aot of variables that are hard to explain,, also I think sizing in between layers is good,,,, but have no proof of that,,
   I think where too much glue might happen is some have tried to add glue after the fact to fill gaps and such,, and that might cause it to crack,, I just try not to squeeze it out to much,,,I just   lay it on the bow and dont touch it after its laid down or try to move it,,, if the glue is right thickness it will just stay there,, and if the sinew is soft from being in the glue,, or soaking in water,, I soak mine in water,,, but it will work either way,, I just squeez out the water put in glue and lay it on,,
    I think it sure would not hurt to put some on a scrap piece of wood, one glue thin, and one glue too thick and see which one works the best,,
    I will add that in an effort to reduce weight or mass of the sinew,, I did thin my glue quite a bit, and thats when I had the failures,,,I hit the point of diminishing return ,,,

[bownarra]

It really is quite simple
25  - 30% glue for the first two courses.
20 -  25% for the last layers.
Weigh the dried glue, weigh the water mix
No need to experiment

All that is happening here is the glue cracking.
Carry on
However your outer limbs are far too stiff and the bow is hinging at the spot you marked out.
Yes any yew can chrysal......no  doubt about it and yours isn't going to be far from it unless you fix the hinge first and get the mid outer limbs bending.
Don't expect much on the drawlength side of things either, maybe 18 19 inch at max. It won't shoot very well because it is sooooo sahort.

[2nocks]

However your outer limbs are far too stiff and the bow is hinging at the spot you marked out.
Yes any yew can chrysal......no  doubt about it and yours isn't going to be far from it unless you fix the hinge first and get the mid outer limbs bending.
Don't expect much on the drawlength side of things either, maybe 18 19 inch at max. It won't shoot very well because it is sooooo sahort.

Thanks for this advice.
My first thought was that the bow was starting to hinge at the spot the sinew cracked, because the sinew cracked. and if I tiller in consequence of this then I'd be building a bow and trying to match the tiller of the other limb in consequence of this defective crack (rather than say repair or redo faulty sinew).
Glad to hear I should carry on and these are probably just normal glue cracks.

Could I trouble you to venture a guess as to whether you think the recurve in the outer mid limb should be tillered to the point where the recurve section bends to almost flat ? and if the tips should bend back as well ? Should this bow look more like a D bow when strung ?

I thought the bow in the museum looked a little rigid or stiff in the tips. In the photo it almost looks like the mid-limb is tillered thinner than the tips. but the dimensions are 13mm at grip, 10mm mid limb, and 8.5 at tip. The ratio of sinew is a bit unknown and I have no clue if the tips were super bendy or super rigid. The only clue is that the top specimen is damaged and the wood is completely rotted away near the grip and holding together purely by the sinew. So it looks heavily sinewed almost like a horn bow.

Agree that the recurve needs to be tillered out to avoid hinging at that spot thanks for pointing this out !

[DC]

If you don't mind could you keep us posted as to the results? Even if it fails. Well, actually, especially if it fails.

I'm still thinkin' PatM. Trying to understand sinew makes my brain hurt.

[2nocks]

.. Yes of course I'll definitely keep this thread up to date with results - even if it fails !
 

I'd love to at least get this thing strung up before it breaks.
I have no problem if it shoots poorly or if it only get a 16- 18" draw -actually that would be a fantastic success !

I'd love to see what a more experience builder can do with this.
Happy to share more info if ever anyone is interested.

These were collected by Captain Vancouver in the 1790's. The origin or location, however, is a bit unclear.
I can also pretty safely stylistically link these two bows to another two bows in the Smithsonian collection, but by then -the Wilkes expedition- a hundred years later they had lost the extreme U-shape recurve.

Edited Sorry I'm wrong on that date -It was Captain Vancouver in 1790's and Cook in 1770's - these bows do indeed come from Vancouver 1790's

[bradsmith2010]

I have some bows that draw 20 inches and they shoot well at close range with the right arrow,, I think it will shoot well at the 18 inches,, you not gonna win the olympics with it,, but for what it is ,, it should shoot nice,,
and probably pretty far with a light 18 inch arrow,, it may take you more than one try to get a shooter,, buts thats normal when learning a new skill set,,