...First, I have a piece of FREE mahogany that is almost 17'
: and 14.5" wide and CLEAR. It is a full one inch thick and since the
: price is right seems like the right board to use to cut my stringers,
: gunwales etc. - I know it will be heavy but since the price is right
: thought it would be worth it -especially for my first one!

Well, you're right that this would make for a heavy frame, but it would work.
Personally, I doubt I could ever justify wasting what sounds like a treasure on something like a kayak frame, which would hide the wood almost completely. What a tragedy.
Depending on your locale, the wood for the frame - Cedar, Redwood or whatnot - can be had for around $50 or so. That's not free, but it's a trifle, really, when compared to the materials cost of most boats.
And, since nominal 1x boards are already 3/4" thick, you'd be saving yourself a number of long, wasteful, saw cuts when it comes to the stringers and gunwales.

Anyway, I wouldn't use your mohagany for a kayak frame - but that's just me. Good luck and enjoy!

Dave