On another front, the Portabote guy said the limit for the outboard is not HP,
its 54 pounds. He didn't offer a suggestion (I didn't press) but preferred to
say that any small engine would get it up to 20 knots.

I called Portaboat and they suggested that the 4HP would do the same as the 5.
I'll probably go with the 4 and enjoy the built in tank option as well.

RB