Glen,
I had an elegant reply written, but it escaped into the ether when I attached too large a scan
Basically it sed: Isn't it curious that a conventional cruiser like Clementine (is she made of herring boxes; without topses?)
should give you grief, and I'm willling to bet that Lialeelta will sail like a piece of trouble-free swansdown
Clementine - I gather that you're working on the solution - in which case ignore the following.
I'm not an expert powerboatie, but it seems to me from your pics that:
Prop is small compared with thickness of keel
You could try a larger diameter prop - it would get a better grip on fresh water without keel interference
Bigger diameter would also reduce speed and (might) reduce the sucking in of surface air that might be a factor
Reduce pitch if necessary to keep amps under control
You should try to give the prop more free space between keel TE and rudder
Even if you have epoxied the proptube in place, you might be able to pull out the tail bearing, shorten the tube and press it back!
But I would suggest just cutting back the keel woodwork to give the prop some clearance - one prop diameter (1D) if possible
Sketch attached
If you are sorted - ignore all the above
andrew