Colletiere a Charavines continuing experiment

The archaeologists that first looked at the Colletiere a Charavines crossbow speculated it was a child's bow, as far as I can gather for no other reason than the stock was short and looked to be insubstantial.  Research since then has found that short tillers were normal in the early medieval period. Reconstructions in the past few years have shown the tiller can cope with bows of over 100lb, so the tiller was not necessarily for a youngster. 

I wondered what sort of weight would be expected from the tip fragment that was found near the tiller.  First I built an elm bow but I went much too short and didn't deal correctly with a row of pin knots that crossed on limb.  I saw over 100lb but then had to scrap the bow as the knots began to fail.

I have had another go with a better piece of elm, going slightly longer at 95cm tip to tip, including the recurves - 89cm working length - working backwards from the tip, tracing a correctly sized tip drawing of the tip and extending the bend the tip dimensions produced backwards to the centre.  I also experimented with making the proper recurves. The work was very revealing about why the tips are the shape they are (the triangular end is perfect for hanging a stringer on) but the bow was only 65lb.  I reckon a better bowyer should get a few extra pounds from it, maybe 80 to 100lb is reasonable.  Don't forget the bow is slightly stronger than it feels as the recurve levers will have an effect but they are very short, so the effect will be limited. 

I'm going to try again and see where I end up. 

I have spent a few hours roughing out an apple-wood bow to the same pattern but using a taper of 1mm per 2 inches, again starting at 11mm.  The draw weight has actually come out well below the elm, which at least confirmed that the tip fragment does not produce even a medium weight bow.  With careful tillering and going a couple of inches shorter than I did, and using a class 1 bow timber, such as elm, the tip might have belonged to a 100lb bow, but I don’t think even that is likely.  I can’t get the idea of sinew out of my head.  Sinew on wood crossbows is such a game changer, and a substantial layer of sinew could raise draw weight by 30 to 50 percent, so a 75lb elm bow would pass 100 with more energy storage and quicker cast than wood alone .  Would welcome comments and questions 

Andrew

No evidence of sinew in the archaeological fragments, if I remember correctly, but that could well be a victim of time.   

I say go for it.  As we know,  100# prod would be pretty much the minimum acceptable draw for anything more useful than bird and close-in small game hunting.

You are correct, there is no evidence for sinew, but then, what would that look like? EVERYTHING eats sinew from microbes and fungi to fish and rats.  Moreover sinew is fully recyclable, all bow makers soak it off a broken bow and use it on the next one.  The only possibility of evidence would be light scoring, but on bows like this, you’d only scuff the surface..You could perhaps argue that a (deliberately) very underpowered recurved  bow is possible evidence for sinewing

Well, I don’t know what happened but when I finished tillering the bow ended up with a draw weight of 41lb! I know that the energy stored would be much higher than that due to non-working recurved tips but performance was dreadful sending a 350 grain bolt at only 25 metre per second .  Still in for a penny  and all that.  One layer of sinew dried for a month, draw weight increase by 6lb but I lost all that as I had to correct the new tiller profile.  So, no change in draw weight at 41lb but bolt speed had increased to 32 meter/second.  Still not great, but kinetic energy has gone up 25 percent for no increase in draw weight. 
I think I’ll apply another couple of layers and see what happens.

"I saw over 100lb but then had to scrap the bow as the knots began to fail."


Failing like completely collapsing or failing like tiny lines going across the belly? In the case of the latter, a bow can still work for a while and be usable.
We are used to look at very strong and permanent bridles on museum crossbows, but those go with composites which are not expendable and they do add to an overall aesthetic.
It's quite possible wooden prods were seen as expendable and replaceable with very flimsy bridles, or hinged stirrup-bridle combo that can be seen in late medieval paintings from Italy and also on a crossbow from Glasgow. Nor do bridles have to be strong, not much stronger than the left fist of an archer. These early prods are in practice just very strong shortbows.
So getting chrysals would not automatically disqualify that prod in a medieval setting, it is something we obsess with nowadays because of our modern perspectives.


Regarding low final draw weight, I could never tiller a prod by feeling because it would always turn out too light. So I tiller by measuring depth of the central cross section, and just getting a smooth curve from there, like a full compass tiller. With enough experience I just know it will bend, despite it feeling ridiculously high. So I just push myself through it.
In practice so far, the most I can get away with is a bit less than 25mm depth for 90cm ntn prod drawn to 30cm for the absolute best wood that I can get. For a longbow-ish profile it turns out a bit above 100#, if made wider and flatbow-like it can get over 150#. I think that would be reasonable for hunting or unarmoured fighting/sharpshooting at unarmoured parts.


Tips are not really indicative, I can make a weak bow with massive tips and I can also make a stronger bow with tips quite delicate. Why should a 150# crossbow have tips any thicker than a 150# longbow? The crossbow's central cross section is actually smaller for the same final weight, and if proportions were followed tips would be smaller as well. We make strings too thick for these early periods, again modelling them on much later bows, and our prod ends too often follow dimensions of these inappropriate strings. Early crossbows probably used typical flemish bowstrings, not much thicker than the nocks of Mary Rose arrows.  

So what was the weight of the Charavines crossbow? I have no idea, just that it's limited by the quality of the wood and the dimensions of the prod socket on the tiller. And there could have been more than one prod for the same tiller.

The prod collapsed across the knots.   Looking back at the work I did, I used completely the wrong length and completely the wrong cross section. I used a modern flat belly approach, and in my defence had very little reason not to,  but, I recently discovered the Irish crossbow/short bow remains (Halpin ).  The National Museum of Ireland has a bow that would almost drop straight into the CC tiller and it is something like 67cm long, 25mm wide and 20mm thick with a possibility of a reflexed tip and made from a reflexed yew stave… I will post a new thread on it soon.