Yew and sinew questions

[stuckinthemud]

I am building up to copying this bow.  It is 78cm ttt, 70cm ntn, 2 inches wide and tall at the centre, 1.5 inches tall and wide immediately before the nocks. It is the only example of a totally undocumented and forgotten type, a yew and sinew bow.  I imagine the original bowyer took off the sapwood and smoothed out the lumps and bumps. Do you think the sinew was laid on the back only, or taken around onto the sides, or was the wood notched on the sides and the sinew wrapped around the sides and laid into thr rebates

[bradsmith2010]

not sure what im looking at, the orientation,,the bow is 27 inches long?

[PatM]

Are you sure there's not vertically stacked horn in the core?

[stuckinthemud]

Yeh, sorry had trouble posting the images, hopefully my edit has sorted things.  The images show the whole thing, a close up of the tip (which proves its not a horn composite), and a close up of the inner third, which is partially covered in decorated parchment

[stuckinthemud]

Are you sure there's not vertically stacked horn in the core?

Fairly sure, not 100 percent. The bow was examined by Micke Dahlstroem who posted here http://www.vikingsword.com/vb/showthread.php?t=7516&page=5&pp=30&highlight=crossbow
Also, there is no horn at the tip and the belly is solid yew, so balance of probabilty is no horn

[bownarra]

Sinew just the back.
Crown it as it appears to be on this example.
Yes remove spawood and only use a flawless piece with no knots and good density.
The key with getting the best bond with yew and collagen glues is sizing it properly with very thin glue 5% or less until you have a glossy coat. As though you were doing a horn to wood joint. Heating it up on the first few coats is a good idea to increase porosity of the wood.
Also that thing has a whole load of sinew on it! Better get shredding!
And I bet it had some significant draw weight with those dimensions.....Do you mean it is as thick as it is wide???

[stuckinthemud]

Quite something isn't it?  For winter hunting in 16th century northern Europe,  this would be the weapon of choice. Steel bows had  a nasty habit of snapping while horn composites lost significant performance in the cold and the wet.  As far as draw weight goes, imagine adding a half inch all round to a 150lb war bow, piking it by 40 inches but then only having a 9 inch draw

[bownarra]

Indeed!

[lonbow]

Thatīs a nice late gothic crossbow. I would expect that this bow has more than 500 kg draw weight!

[stuckinthemud]

Not sure about 500kg, my first yew and sinew to this length and height was 12mm thick at mid limb and draws 70lb at 9 inches but 105lb at 12 inch draw, so, if this bow is 4 times thicker, does that make it 8 times more powerful? Also, using a rough rule of thumb of 10 lb per mm of thickness, I'd  guess draw weight would be 500 lb.  If you could draw it to 12 inches, weight would go up to nearly double that but that s why I need to do ths project, nobody knows for sure what it can do.

[mmattockx]

Not sure about 500kg, my first yew and sinew to this length and height was 12mm thick at mid limb and draws 70lb at 9 inches but 105lb at 12 inch draw, so, if this bow is 4 times thicker, does that make it 8 times more powerful?

Limb stiffness is proportional to the thickness cubed. If this bow is 4x thicker than your reference it will be ~64x as stiff, all else being equal. It certainly looks hell for stout, I can't imagine being able to draw it by hand, even to just 9".


Mark

[lonbow]

I thought that the draw weight is that high because a friend of mine has built a late medieval crossbow with a draw weight of 1200 lbs (with a composite prod). I think that the prod is a bit wider but flatter than the example you have shown. Your example has a higher cross section, so I think that the draw weight will be higher than 1200 lbs.

Iīm just wonderning how the yew wood can withstand the strain.

[stuckinthemud]

These were drawn by winches .   Many, many people have expressed the opinion that bows like this couldn't possibly have actually worked, just been fancy wall hangings.  Its going to be fun finding out.

64 times?  We're gonna need a bigger winch......

[PatM]

I can't imagine why people would think they didn't work.  Nobody would put that much effort into the prod for looks.

[stuckinthemud]

I know,  I corresponded with a leading warbow expert who didn't proceed with copying the Glasgow museum yew crossbow because one of the UK leading (medieval period ) experts believed it couldnt have possibly been a working bow. Afraid I have a really really really low opinion on theoretical academics.  The Berkhamsted bow was reckoned to have a draw weight of about 150lb even though it is 2 inch by 1.5 inch at centre and 48 inches ttt........