Welcome to my world
It's a good quality break
What I mean is the whole thing has gone bang, not just one obvious weak point. Without carefully inspecting the bits, and decent video it's impossible to tell, even then it may not be obvious.
It did look like it was taking some set, ( nope... the stave was deflexed to start with) was maybe a tad short and I didn't like the raised grip, it can be a lot more subtle than that, but to be fair, that's not where it failed. But a V gentle (or non existent) fade maximises the working limb. Ok I see the big knot... yeah gotta have that at or near the grip.
You could argue "Gee it's got hugely long limbs, so how come it's too short?"... but they are V highly stressed due to being narrow and thick.
I'll check back on the early parts to see what you do with the sapwood as I think I was watching the wrong guys vid!!!!
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The back is usually the key. I think I missed how you worked the back. I'd have been tempted to just take of the outer bark and leave it pristine, as it looked fairly this sapwood to start with. Yup I see you left the sapwood alone. I think you pretty much did it all right... these things happen.
Ok it's good to mention Yews toxicity but I don't bother with a mask except maybe when scraping off the cambium (that stuff is horrible) but I do have a dust extractor on the bandsaw.
66" inches is pushing it for a relatively high draw weight. Maybe better going for 40# or allowing the knot to be say an inch outside the grip, keep full width of the stave and so just gain an extra inch or two. maybe with big knots, a patch of sapwood over the belly area adds some extra security.
Let's see the bit of wood with the knot through the back so we can see if that failed.
I've had a closer look. Forgive me if I'm wrong. but it looks like that big knot didn't go right through the sapwood. If that's right, you shouldn't IMO drill right through, just clean out all the crud with a needle file ground to a chisel point and plug it from the belly (making sure there are no air pockets left inside) that would have left the back stronger. You could maybe had had that knot just above the grip and thus gained a few inches extra length.
E.G With 33" above the knot, the knot 1" above the arrow pass, the arrow pass 1" above center that gives the upper limb 35" tip to centreline, so the overall bow length becomes 70" which is a good bit better than 66" !. For security you could leave the stave a tad wider where the knot was.
Anyhow... shame it blew... and bear in mind I could be talking out my backside as I didn't have the stave in my hand.
Better luck with the good stave.
Del