"How deep would I want the prop to be below the waterline, so I know where to mount the bracket?"
The mount position is a little tricky depending upon what your goal is. If you have a mount with a lot of range you may consider setting it up so with it all the way up the engine
is just barely out of the water
. Garelick has mounts with 15" of range so you would then be putting it down 15" from there. If you want it as deep as possible you would want it setup so that with the mount all the way up and the engine
tilted you are completely out of the water
. A little more work raising/lowering the engine this way but then you have the prop in deeper in case you are motoring in bad chop.
"If I have the adjustable mount, does short shaft or long shaft matter?""
Yes. The extra leg length lets the controls be closer to you and less likely to be dunked (for same depth
prop) or at the same control length the prop is 5" or 10"s deeper. Also the 25" shafts tend to come with high thrust props and charging
circuits which you would want.
BTW, 15hp is probably more than you need. There are a few Alberg
30s out there with 6hp Tohatsu sailpros and they move them pretty well. Great little engine, I had one on my old Compac 23. The 25" Tohatsu 9.8hp looks like a great engine too and should be smoother than the single
Trying to work through my diesel
issues too. Otherwise an outboard
is in my future as well.