Steve, I believe that your link question is really for Glyn...
I do have a comment about the Yamaha OB. We put a Yamaha "Hi Thrust" 4-stroke 10 hp on our prior 26 foot sloop. It burned only a half gallon per hour at 6 kts. and was quiet. Large 11 3/4 " three blade prop and only came in a 25" leg model. Three (!) to one reduction gear.
Years later, our YC has replaced both old 60 hp two-stroke OB's with new Yamaha 25 hp Hi Thrust 4-stroke engines and we are VERY pleased with the result on both of the work boats.
This little portion of the Yamaha engine line up is aimed at displacement speed hulls, and fully lives up to the promises, IMHO.
As to what shaft length is "right" this will always be a compromise. Our engine would indeed drag the prop a little when we got over 8 kts on a reach or run, and start spinning the prop... At that speed, this was not an inconvenience. Like the E-27, we had a fixed mount via a cutout in the transom.
Note that there will (!) be times when you have to motor into a short chop in a narrow channel and will want all the prop depth you can get!
"Choices and Options" as someone once said....
Loren