I bought a Beta 28 to replace an aging Atomic 4 on my 1965 35ft Allied Seabreeze.  The prop shaft diameter for the Atomic was 7/8, and the Beta 28 specifies a 1 diameter shaft.  My initial inspection of the shaft log protruding into the prop cove showed this would not be an issue.  Now that Im ready to start the job, Ive determined Allied used two separate fiberglass tubes to build their shaft log.  The two pieces butt end-to-end in the deadwood area of the hull, with the interior tube heavily fiberglassed to the hull in the narrow crevice.    

The problem The interior tube has a smaller inside diameter than the exterior tube used to house the cutless bearing.  In addition, the interior tube has signs of wear elongation due to poor engine alignment. My fear is the new diesel sitting on its rubber isolation mounts with a 1 shaft will vibrate sufficiently when running resulting in the shaft hitting the interior shaft log.  The interior tube ID is 1.125, static clearance with 1 shaft and perfect engine alignment is .0625.  In an earlier forum (several years ago when I bought the engine) you recommended I use a 13x10 three blade prop with my new Beta.

Questions- What is the recommended clearance between the prop shaft and the shaft log? Would it be safe to use a 7/8 diameter shaft with a 13x10 three blade prop?  The length of the shaft is 34 inches.  Im really trying to avoid replacing the interior tube, and opening the clearance by drilling and reducing the tubes wall thickness I dont consider a safe option.  I guess another option would be to sacrifice hull speed and use a smaller diameter prop or go to a 2-blade, thus reducing load on the shaft?

Any advice/input is greatly appreciated.  Thanks!