: Ahh I probably should have specified I am building skin on frame.
: Going to see if I can make an under 20 pound 16' kayak to go
: with my 15lb 11' canoe.

Try these simple tests. Rip two strips from one of your boards. One form the sapwood side of the board and the other from the heartwood side.

You might as well make these the same width and thickness as you'll use in your kayak.

Hold them together at one end and see which one flexes more. Support them on benches or sawhorses a few feet apart and put some weight on them. See which one bends more.

Working on just one of the strips at a time, add weight until the strip breaks. Then see how much weight the other one can handle.

Weigh them. I'm guessing the sapwood will have a bit higher moisture content and be slightly heavier. I'm also guessing the heartwood will be stiffer and stronger.

If you find any significant structural differences you can do all sorts of things to exploit these traits. For example: you can scarf a 4' piece of flexible wood to each end of a 7' piece of the stronger wood. That would give you a 15' chine which was stiff in the middle but flexible at the ends, where it might need to curve more for the design, or flex to absorb the impact of a collision.
These tests might encourage you to go with thinner and lighter strips.

Just some thoughts

PGJ