Thanks guys! The way I do underlays is to tiller the bow out to 24 or 25 inches and then use a belt sander to remove some wood on both tips for about 10 or 11 inches. Then I glue in a new piece and shape it to fill in all the wood I removed. The trick is to make them long so they can safely bend with the bow. You can use them to help keep the tips stiff, but in the case of this one the proflie looks the same as it did before I put them on. I just put them on to look nice and cover up a mistake where I let the rasp tare into the edge grain core a little.
I've done a couple with walnut cores, like you said not the best core wood out there but it can work nice if you use a really nice piece of walnut for the job. Oh ya, and the bow is 63" ntn.
Dan