We've done six kayaks out of old power poles and just got a few more poles that were being replaced. The large ones on either side of a river have the most clear although I don't worry about small tight knots. The last ones were 40 to 45 feet long so enough to get quite a bit of good material out of. We have also started putting patterns into our boats and consequently need very few full length pieces. The stuff that isn't good enough gets turned into fences, rough siding etc. Not a bad way of doing things if you like scrounging.
good luck
don