Ash makes superb longbows. It was used for longbows in the middle ages, and they would have been very high draw weights. You could (if you wanted to) get at least 150# out of a typically shaped ash longbow.
Keep it as long as possible, about 35mm wide and 30mm deep. Keep it full width to just before mid limb, taper to 1" wide tips, then taper the tips more steeply as you tiller.
Round the edges of the back quite a lot, almost trapping it. Heat treat the belly if you want some real punch. Don't let the tiller get out of hand - you want to make sure you never pull the bow if it has even the slightest hint of a hinge. Ash will chrysal and fret if it's even slightly hinged.
Keep it as dry as possible. If you take a long time tillering, keep it dry and hot between sessions. Once it's done, lightly toast the belly then seal the bow with a good waterproof finish.