Sapling, well the operation was a success but the patient died!
I'm really impressed with hollow tillering, but also had a lot to learn. It was going great for awhile, then some important points started cropping up and things went downhill until I got a big hinge and a buckled limb:
1.) the edges of a hollowed limb should be relatively sharp, not rounded.
2.) A very wide limb will create a LOT of camber -- there's a limit on how much camber the hollow tiller method can deal with. Beyond a certain point the limb edges will flap outwards and the back will buckle. I had a 2-3/4 limb width from a 3" dia sapling -- in other words nearly 1 to 1. It might have worked much better on a 4" sapling -- or if the wood was of a species and quality that the bow was say 2" max width on the 3" sapling.
3.) There was a definite obvious cause to the hinge -- a central knot with too much thickness -- I should have cut much more of that away.
4.) The failure was definitely a compression failure -- and a weird one because of the hollow. The bow buckled -- and then returned to it's original shape -- with only some severe frets showing on the underside where it had buckled. I mean it really let go hard on the tiller. Took it off and it looked like a bow again.
5.) Another error -- I thinned and narrowed the tips way too soon -- that was before I decided to hollow tiller, and I was thinking of it in the conventional way of flattening.
So a lot to learn -- but it really did work well for awhile, and I think if I do it again, I'd have a bow.
Much easier however to make a cottonwood bow from a board or relatively flat stave than a 3" sapling -- I think I came close, but not going to repeat it.
The bow after the break, first photo:
2wnd photo: damaged area. The edges should be sharper, and the knot dug out. I was just tillering lightly at 15 lbs and roughing out the hollow in the blank.
