The following images and notes are based on the detailed repair that "Geitz" the owner of a Sun Odyssey 36.2 originally detailed here on the Jeanneau Owners Forum. Also there is an associated Hints and Tips article on how to reduce the risk of water ingress at the mast foot.
This winter's project: re-core the mast step as the original wood core has gone soft.
This is something that the surveyor, and myself, missed when we bought the boat in 2015. The mast step has been pushing into the deck under the step. We had already confirmed that the compression post wasn’t sinking into the hull timber.
Taking out one of the deck wire glands revealed wet core and the deck under the mast step has become slightly concave. We un-stepped the mast and I committed myself to the project by making cuts with a circular saw, just through the laminate. About a 2-3” area of the original deck laminate was left intact to laminate onto.
It looks like Jeanneau used whatever scrap wood was laying around that day.
Once the wet wood was chopped out and the area cleaned, there were several options for rebuilding. It would be possible to fill the entire area with thickened epoxy with chopped fiberglass for strength, but my boatyard mentors thought that much epoxy would get too hot when it starts to harden and might lose some strength. So, we chose to use G10 as the new core. G10 is a solid fiberglass/epoxy material that is very dense, very strong, and won’t absorb water. G10 Specifications
Below is the result of grinding the existing FRP and fitting the G10. The hole on the upper right is where I leaned on the existing laminate. It turned out to be mostly resin and shattered. No problem, West System epoxy and new fiberglass will take care of it.
1/2" G10 all dry fitted. There's at least a 1/4" space above the G10 to make up, so I've ordered another piece of 1/4" G10. When it arrives, I'll cut and dry-fit, then start cutting out the fiberglass schedules (layers).
The ¼” G10 arrived but it turned out that it was too thick to fill the rest of the void, so I laminated a 3/16" thick epoxy/fiberglass filler piece at home. Once it was cured, I washed it to remove the amine blush, let it dry, then cut and dry-fitted the pieces. These were cut into rectangles that overlap the joints in the underlying G10. I numbered all the pieces in the order to install and traced the outlines of each so that once we began pushing them into the epoxy, we'd know that we got them in all the way.
Large amounts of thickened West System epoxy were used to glue in the G10 and the homemade glass filler. We pushed as much of the mixture as possible into the surrounding areas under the existing deck, then started to push in the G10 pieces. The epoxy oozes out as you push the pieces in and hopefully fills the voids that are hiding inside.
During dry-fitting, we cut all the fiberglass laminations and stacked them up in the proper order. The three topmost overlapping laminations were traced onto the existing FRP to help us place them in the correct order and location.
Numbering and making outlines help you stay organized as you laminate. It's easy to forget where everything goes. My friend did all the epoxy mixing while I did the laminating. He mixed up small batches to help prevent the epoxy kicking off in the pot to quickly.
One by one, the laminations are applied. The irregular area in the corner was inadvertently created when I accidentally leaned on the edge. It was a very resin-rich area and just snapped off. No problem, we filled it with glass.
Once all the laminating was completed, we covered it with polyethylene and placed a piece of scrap drywall on top, followed by a very heavy piece of granite that my friend had.
I won't be able to finish the gelcoat stage on the mast step until warmer weather, so the project is on hold for now.
Geitz - Sun Odyssey 36.2