Just to add a little to this thread, my former boat, a C&C 26, had a folding prop, not Martec (Michigan?). It would not open unless I put the transmission in reverse and gave the engine a rev-up before shifting into forward. This was the case when leaving the dock, or when starting the engine at the end of a sail, while still moving at a good clip.
The other day, I experienced the vibration on my J-30 for the first time, and decided to try to same tactic -- it worked! Being an engineer, I have never been able to figure out the dynamics of how or why this works, but I pass it along as something to try before tearing into linkages, transmissions, etc.
