What advice would you give for this question?
"Wooden Boat suggests using clear spruce for the bottom frames. Sitka spruce is definitely clear, but is so expensive, especially for "only" frames. How 'bout using lumberyard spruce/pine/fir lumber, carefully
chose for dryness and no more than pin-knots? Any other suggestions?"
offered great advice by suggesting the use of clear spruce for the bottom frames. Certainly, there is no reason why you can't use Eastern Spruce rather than Sitka. It's almost as light and just about as strong. The problem with using Kiln Dried (KD) Lumberyard building studs is that the kiln drying process is too fast and at too high a temperature which affects the strength and natural rot resistance of the wood. I wouldn't say not to use it, but be careful in your selection.
If KD spruce building studs are used as your source for frame stock, it might be a good idea to encapsulate the frames in epoxy resin (three coats).
You can certainly use Douglas Fir for the bottom frames, but you should also understand that this materials is much heavier than spruce. Stay away from using pine as the lighter species are too weak and too soft, and the heavier species (although stronger and harder) are just too heavy.
If you can't find clear spruce stock in your area, I would suggest ripping whatever stock you can find into thin strips. Pick out the clear strips, and laminate them together with thickened epoxy resin (such as MAS F.L.A.G. resin thickened with Cab-O-Sil and MAS medium hardener. You can then cut out the frames from the laminated stock. Although slightly heavier than solid spruce, it will be much stronger, plus it's a viable alternative in places where clear wood is hard to come by.
Paul J. Bennett
Marine Engineer, Naval Architect
Bennett Marine Enterprises