Mark, I am surprised that nobody has responded to your question yet!
Like you I have an epoxy/ply BR20. On my boat there is a drain hole in the bottom of the mizzen mast receiver box that simply allows any water that enters through the top opening to drain down directly into the outboard well and away. Without this as you say there is a strong possibility of rot in the wooden structure of the receiver box.
A bit off topic but word to the wise FWIW: I have had 2 problems with this area of my boat!
1. The previous owner did not tell me that he had broken through the bottom of the mast receiver when dropping the mast into it. The damage only became apparent after I got the boat home and rigged it fully 😬. I think this is a weak spot in either the design or the construction of the boat as it would appear (though hard to confirm) that the receiver is an open ended square tube set into the after deck relying on only the thin ply on the underside of the afterdeck to close its bottom end and prevent the mast from dropping into the outboard well. To repair I poured epoxy into the box and when it started seeping out of the cracks in the broken flap I closed the flap up with a car jack set in the outboard well and some battens to spread the upwards load. I then poured some more epoxy into the receiver to create a hard plug in the bottom and hopefully prevent a repeat of the damage (NB I took care to ensure that there is still a drain hole through the epoxy to take water ingress away). This seems to have worked to date but…
2. Recently I smashed up the receiver box completely when I inadvertently caught the mizzen mast on an overhanging branch while reversing down a boatramp. The mast acted like a lever over the fulcrum of the mast support (gallows) at the front of the box and smashed open the rear side of the box inside the afterdeck. The boat is currently in for repair. There is no access into the afterdeck compartment so an access hole will be drilled through the bottom skin (i.e. upwards in the outboard well) to assess the damage and see if a repair is possible from there. The access hole will not be visible from inside the cockpit unless you go looking and it will eventually be sealed up with a watertight hatch to maintain buoyancy. A repair in this case might include pressing the broken box wall back into place & shape with epoxy to glue it back together and then fibreglassing over it to impart strength. The other alternative that i have discussed with the boatbuilders is to replace the mizzen mast with a RDM (reduced diameter mast) windsurfer mast, assuming a suitable one can be sourced, which would mean that an appropriately sized sleeve that will accept the base of an RDM mast could be inserted inside the broken mast receiver with epoxy pumped in around it to hold the sleeve in place and seal up and set the damage to the box. If this sleeve solution ends up being the one we adopt but a suitable RDM mast is not available then a RDM “foot” piece could instead be glued into the base of the existing mast to allow the mizzen mast to still be inserted into the sleeved receiver box… )
Moral of the story: don’t mess with your mizzen mast receiver! You definitely don’t want to let rot get in there and double definitely don’t catch the mizzen on any branches when reversing because there is no easy fix if you do!
