The Native American way for small dugouts was the long "dough dish" shape, kinda squarish in cross section from end to end. The breadth in the ends gives back some stability and load carrying that a canoe loses in it's skinny ends. I can stand up in mine but can't balance with a stranger aboard. It's 22" wide on the bottom, tapering to nothing at each end of it's 16ft.   You can see it here.

If you're only going to have 15" of beam at the widest, you may have to go to the next step dugout builders used to use, raising the shear with an extra plank on each side. I suppose we won't know on that score till the spring thaw ;^)