: - Short length, lots of rocker, U'd bow and flat bottom aft should all mean
: very manoeverable.
: - Short length = light and easy to handle out of the water. Plus, wooden
: constrution is almost bound to be lighter than the polywotsit commercial
: alternatives anyway.
: - Will probably track positively as is, with the hard chines in the aft half
: compared to the U'd front half giving some directional stability, but I
: would also add a retractable skeg to increase tracking further.
: - Fairly wide, and flat bottom, will give high primary stability.

My thought is that it will have such high primary stability that it will
be difficult to lean quickly to cope with beam waves. Once leaned, the
relative lack of flare will give a lower secondary stability and the sharp
rails will mean that it will catch and capsize rather more readily than
perhaps you want. As both stem and stern will disengage in waves, the boat
will be turned very easily and may be impossible to keep on track without
some quite athletic paddle strokes. But that's what whitewater boats do, of
course, and we use those in some big waves :-) If the cockpit is a tight fit
and your aunt can drive the boat from one edge to the other quickly, it ought
to cope ...

It also looks to have a lot more windage forward than aft, which would lead to
lee helm, which would make it very hard in wind - adding a skeg may improve
tracking in some circumstances, but it makes lee helm worse.

The rocker will contribute greatly to stability (both primary and secondary),
so I suspect you could afford to make it a little less flat-bottomed, and more
flared at the sides.

: I've been tweaking the design for a while and have pretty
: much settled on it as where I was trying to get to. Decision time: do I
: build it or not?

I'm sure you should build it - but accept that it will be something of an
experiment, and may not be the boat your aunt wants. Paddling it will give
you a great insight into what does and doesn't work in this sort of design,
and may lead you to another iteration ... Meanwhile, in some situations,
I'm sure this boat will provide a lot of fun. Penrhyn Mawr in an offshore
wind could be interesting ...

Andy