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Skin-on-Frame: Douglas fir dimensions

Hello,

My friend gave me a couple old growth Douglas fir beams from a re-model of his
1912 house.

I'd like to try and make a baidarka, as described by Zimmerley and Brinck,
with this wood.

Fir, I've read, is stronger and heavier than spruce. Can it be then cut
thinner? If so, then how much? Is there some proportion or formula?

Zimmerley specifies the gunnels at 2.2 x 3.5 cm. (about 7/8 x 1 3/8) and
Brinck calls for 3/4 x 1 3/4. I probably wouldn't want to go thinner than
3/4 so as not to weaken the rim mortises. Could the width be reduced to,
say, 1 1/2 or 1 3/8?

Thanks for any help.

Kirill