I think that mahogany would be perfectly fine for gunwales, other than the weight issue. Thinning the gunwales is acceptable, and 5/8" thick would certainly be ok. I've seen at least one survey of a Greenland kayak that had 1/2" thick gunwales, but that seems like it would create new problems (i.e. delicate rib mortise walls). Reducing the height of the gunwales is also a viable option if your design will allow it.

As for fir and pine for ribs, you will likely find they do not steam bend very well. Mahogany may steam bend better...

: This is not the first time I have asked this question, but I want to revisit
: it- I would like to know if anyone has built a SOF using mahogany for the
: gunwales and keelson. I have access to smaller lengths of fir and pine for
: the ribs and shorter pieces. It's the long ones that are killing me to
: find in good lumber. Where I live mahogany is readily available in very
: clear grain, and long lengths. Besides, imagine how beautiful she would
: look with Mahogany gunwales, even if you don't see much of them!

: My research on the web shows that it is about 33% heavier than spruce and
: about 15% heavier than fir. If I use mahogany for the gunwales and
: keelson, and fir for the rest of the boat, is it really going to add that
: much more weight? The cost of (a) purchasing good clear spruce and (b)
: shipping the good clear spruce to me is just too much. If mahogany is only
: going to add 3 or 4 pounds then what the heck. I can lose 3 or 4 pounds of
: body fat to make up the difference!!!

: I also would like to ask if anyone with some experience in building a SOF in
: traditional methods would like to comment on the idea of shaving 1/8"
: off the gunwales, keelson, and maybe stringers if they were made from
: mahogany. My thought is the mahogany is much stronger, and much less
: likely to crack around the mortices, so I could go a tiny bit thinner with
: the gunwales to make up for the difference in weight. The mahogany I have
: access to is very dense, very tight-grained, and would be quarter-sawn.