[Uhehner]

I did see a coupling on a Cabo that had a large bolt in the center of the prop shaft holding the coupling to the shaft on the engine end. This to prevent the shaft from pulling out of the stuffing box in a hard grounding. If the shaft pulls out you have a 1-1/2" hole allowing water to fill the engine room ( this did happen to a friend).
Usually we separate the coupling, insert a large nut or socket between the halves, bolt them back together and tighten the bolts. Maybe then use some heat. Good luck.  Capt. George