I modified my Bolger Diablo along those lines - It had a 20" transom, and I
wanted to run a "short shaft" outboard. It worked ok 90% of the time, but
ventilated like h3!! when accelerating. So I cut a notch out of the
bottom - maybe 2 or 3" high into the transom, 16 " forward into the bottom
panel, and 8" wide. Sort of a mini-tunnel. I'm sure this causes a small
degradation in performance, but gains me a few inches of draft.

I'm sure you could take any skiff design, with a flat bottom, and play
around with this. I would start with a really deep tunnel, and high
transom. If that seems too much, just cut down the transom (and glue foam
into the top of the tunnel) until you get the desired performance. You
might be surprised.

But, if you are going to cut a big notch in the bottom, make sure there is
enough bottom left at the transom to provide lift. Maybe extend the panels
out beyond the transom, on either side of the tunnel - you can always cut
them off. If there is a good high-speed tunnel design for home-builders, I
haven't seen it. Atkin had a bunch of low-speed tunnels, you might take a
look at the Atkinboatplans.com website for ideas.

Sal's Dad

I'm not sure what you mean by "semi tunnel" but I built an Atkin Rescue
Minor with fully protected prop and rudder. Or you can modify the
transom of any skiff to give you a bit of a tunnel effect with an
outboard - at the loss of some "lift".

There are some "tunnel" designs for running an outboard a few inches
higher that just have a small tunnel at the rear of the boat. When I was
really reading a lot about boat building a few years ago I recall reading
about some of them.