What is the appropriate range of distances from aft end of the bronze shaft bearing housing to the forward shoulder of the prop?

I purchased an Autoprop some time ago, to replace my 3-blade Michigan fixed prop. I don't recall if I have ever tried on for size or not. But, as I do so now, I find it sits too far back in the prop opening to rotate the blades fore and aft, without hitting the rudder.

One of the projects I did over the summer was remove and recondition my Perkins 4.108. As part of that process, I installed two 1/2" x 4" x 36" aluminum plates as a mounting platform for the new engine mounts. The Perkins was not the original engine, but installed new in 1987. In order to make it fit, the smaller engine pan was cut up considerable more than necessary, thereby weakening the resulting mounting platform. When the engine was reinstalled, I tried to get the mounts in exactly the same places, using side to side, fore-aft, and cross measurements. I seem to have gotten pretty close, because the engine set right on all four mounts, without adjustment. I did notice, however, that I needed to move the shoulder for the dripless shaft about 7/8" further back, suggesting that the engine might be sitting about the distance further aft. It certainly doesn't seem to be so, however.

When I slide the Michigan 3-blade in place, it sits right in the middle of the opening. The Autoprop, however, has a slightly longer mount and it sits back far enough that it cannot rotate without hitting the rudder. Both props have about the same distance between the forward edge of the prop base to the aft edge of the shaft bearing housing (2" for the Michigan, and 2-1/8" for the Autoprop.

I don't know how critical this clearance might be, with regard to cavitation, etc. But, I was wondering if I could shorten the forward end of the shaft slightly, to allow clearance for the Autoprop. Ideally, I'd like both props to fit, so I have a good backup. Shortening by 1" would probably do it. 1-1/4" to 1-1/2" would give me a safer clearance for the Autoprop and would would still leave 1/2" - 3/4" between the shaft bearing housing and the prop?

I have found it is usually necessary to shorten a shaft when installing a
feathering prop, as the blades extend well back when feathered, especially
with a Maxprop, and can interfere with turning the rudder.

The usual recommendation is to allow 1 to 1-1/2 shaft diameters between the
end of the bearing and the front of the prop hub for a fixed prop. This to
minimize wear of the bearing.

If the forward face of the prop is too close to the hull, or the tips of the
blades too close to the rudder opening, this will cause cavitation, noise,
and an excessive turbulence of water when powering. The tip clearance
should be a minimum of 1", but more is better.

A simple solution is to shorten the shaft as necessary for the feathering
prop. If you have to install a fixed prop on this shortened shaft, then
install an extra drivesaver between the transmission and shaft flanges to
move the fixed prop back about 1-1/4", which is the thickness of most
drivesavers. The drivesavers are also sometimes called a flexible coupling.

Bud: Thanks for your reply. I looked everywhere and could not find any recommendations with hard numbers. I did want to have the fixed prop as a backup, but it never occurred to me to add a spacer (drivesavers). That is an excellent suggestion.