It's been interesting doing the patch. Having rasped off about 1/8" from the belly I thought I'd got past the chrysal, but when I cleaned it up on the belt sander to get an nice even flat area I could still see it. In the end I went down about 4mm and there was just a hint of the main one still visible. The slat of Ash (7mm thick) was heat treated and I could see the heat had penetrated right through it so it should provide a good solid belly (see the vertical pic which shows the underside)
The close up (right) shows how the patch is quarter sawn with the grain (growth rings) running back to belly. The term 'grain' gets a bit confusing, obviously its running along the bow too, but the growth rings in the main bow follow the back of the bow. E.G the back of the bow is simply the under bark surface. Whereas in the patch the rings run at 90 degrees to that. One of the guys asked me what effect that would have. The honest answer is 'I dunno' but one could argue that any discrepancy in strength between the various rings will be better averaged out with it done like this. Laminated bows often have the laminations cut this way and self bows can be made like this too.
The patch goes almost the full length of the limb, extending into the stiffer handle area and up towards the tip where there is less bend (due to the lower leverage near the tips). It's almost a complete belly lamination. The actual chrysals had spread over about an 6" length so it seemed wise to do a nice long patch.
I will need to be careful that I don't have the lower limb too stiff and over strain the upper and make that chrysal! The last pic shows where I've started work blending the patch into the bow... the danger here is you blend it in exactly as it was before and end up with exactly the same problems. I need to get it roughly to size then get it on the tiller for some cautious flexing.