Here is something kinda on/off target...
We had a Rollercoaster at one of the places I worked at and depending on the temp out we would go from 10wt to 40wt. And in mid summer we would switch to grease. It was all about adjusting speed. Oil did make an awful mess. I have no experience with boat...
Yes. All new shafts, bearings etc.... just about everything. Would work until I torqued cap. If it was slightly loose it would operate. Removed 1 of 2 shims and been good ever since. 3 maybe 4 seasons so far with 1 water pump change so far
I had a kinda similar problem. I did a full rebuild on my drive including all bearings,races and all the drive shafts. Put it all together and would not turn. There are (atleast on mine) shims in the very top cap where the center post is. I removed .005 shim and freed up. Not your problem but it...
Try the above mentioned bevel/chamfer but before attaching whatever hardware take a screw, add a touch of lube and install just the screw going back and forth to cut threads just like a tap. That way your not cutting and pulling at the same time.
Bowmalock or also called bowmagrip. It's a crush nut. As long as you have threads protruding it won't come off. I have also machined them to about half their thickness and they still hold.
If there is structure under deck, stringers and bulkheads, 2lb foam. Flotation only. If there is no structure your foam needs to be structural, 4lb foam.
