Originally Posted by Trinidoc
Ok you have sufficiently worried me to redo the project. I'm still out the water and only rushed to get the stern tube down so that my new engine can go in. I will pull it apart tomorrow and start over. Thanks.
The good news is that the tube will be easy to remove once you get the bolts off as the butyl rubber is pretty easy to remove.
Once you get the stern tube removed. Rough both the bottom it's flange and the hull interior with 100 grit sand paper, soak a rag with rubbing alcohol to remove the sanding dust, and then follow it with a thorough cleaning with an acetone soaked rag. Allow the acetone to fully evaporate (pretty quick) before bonding. Butter both the stern tube flange and hull with the adhesive (Sikaflex 291 or 3M 4200) about 1/8" thick, position the flange onto the hull, bolt tight an clean excess adhesive. Run your finger along the edge of the flange and hull to get a nice smooth transition of the adhesive. Go under boat and remove excess adhesive that may have squeezed out into the cylinder where the prop shaft will go. Should take no more than 30 min start to finish.
I know that I lot of folks on this forum think the butyl rubber is the best bedding agent because it flows well, is sticky, and is easy to apply and remove. But because it really has no adhesive properties nor structural strength, it isn't a good material for thru hulls and other below waterline fittings nor as the primary means of fastening something to another surface. It is also very easily degraded by organics like diesel, oil, and gas. "
Folks have to remember that adhesive doesn't necessarily mean something that is tacky/sticky. Honey and syrup are sticky/tacky, but have no significant structural adhesive properties. The best way to think of Butyl Rubber is a soft and deformable "o-ring and use it in in areas where a item like that would be okay.