: Did one of you yachtsmen build a folding or s-o-f with pine, spruce, fir or
: other in lieu of ash?
: I need lenghtwise stringers to hold the p-v-c skin, that mustn't rot or
: break.

No guarantee that Ash wouldn't rot or break, either.

Knots and Pine seem to go together, but if you can get wood that has minimal knot problems, then any of these materials should be fine.

Pine, spruce and fir are common white woods used interchangeably for building construction in my geographical area. Frequently they are advertised as s-p-f (spruce, fir, pine) when the dealer doesn't know exactly which species they are selling, and doesn't really care. Certain subspecies of these are very desireable, though. For example, Sitka Spruce is very strong for its weight, frequently available in clear lumber, and is used in the making of masts for sail boats, as well as aviation parts. This would be great for your stringers.

If the plans call for a hardwood piece of a certain size, and you are going to substitute a weaker wood, you may want to make the newer part a bit larger so it will be just as strong as the hardwood piece. This may mean that a 3/4 inch thick hardwood stringer is replaced by a 1 inch thick pine stringer.

You can solve the rot problem by giving each wood piece two coats of epoxy resin before assembly, or one coat before assembly and another coat after assembly. If you want to increase the strength of the wood, wrap it with glass cloth at the same time. You can laminate strips of glass cloth to the sides of a thin piece of pine to make it much stronger than any hardwood. In this case you might want to cut your pieces thinner or smaller than the plans call for, and build them up to size with several layers of glass cloth. While I'd probably use ordinary, plain-woven glass cloth, for stringers you could try some of the materials that have most of the glass fibers running in one direction. Or go with Carbon fiber tape.

Another thought is to round the edges of your stringers and cover them with 1 inch wide, or 2 inch wide, strips of glass cloth which are wrapped on at an angle. Saturate the glass cloth in a puddle of resin and start at one end and keep wrapping it untily ou finish at the other end. Overlap the material as you go and it will hold itself in place until the resin sets up.

Hope this helps.

PGJ