Thank you all for your helpful responses. I have a two-bladed Martec prop, which is in good condition. I had the prop rebalanced as part of recent routine maintenance.
Since I pulled the engine in the water and have a face seal rather than a stuffing box, I thoroughly braced the shaft in position before pulling the engine so that the shaft would move forward. I didn't check to see that the shaft was dead center in the center of the log. I will do so when i check the mounts later today
These props are notorious for causing shaft whip. I have owned them and know it all too well. If you have a longish shaft between the cutlass bearing and the gear box output flange these two blade props can literally turn the shaft into a jump rope between the two bearings. If your shaft was not 100% centered in the shaft log this "whip" can cause the shaft to thump the shaft log at certain RPM.
Folding or many feathering props move the blade center considerably further aft of the cutlass bearing. This coupled with the fact that it is a two blade prop, non geared etc. can lead to vibration and shaft whip issues.
I am personally currently dealing with this on our own boat with a Flex-O-Fold two blade prop. See; FOLDING PROP REPORT
The calculations put her on the borderline for whip, and I took the chance... Missed....
The shaft is just flat out too long for a folding two blade and it causes MASSIVE vibrations at certain RPM. This despite the shaft being true to .001" and the alignment also to .001" and the prop having been run through a Prop Scan tool to confirm balance.. The prop is coming off this week and the three blade will go back on..
Blades further aft cause more vibration:
Here's the off set on a fixed prop and an exaggerated illustration of what shaft whip looks like.
Here's a photo of a customers boat that was experiencing massive shaft whip thumping. We were able to get rid of the thumping and get everything to .001" including alignment and shaft tolerance.
Notice the position of the shaft passing through the shaft log.