Hull-deck joint

The Columbia 43 used an H-channel hull deck joint ("HDJ"). This consists of a long aluminum extrusion shaped like an H in cross-section. The hull fits into the bottom part of the H, and the deck piece fits into the top. Here's a page that illustrates it from Boating Magazine, September 1971. The diagram there is:

 Mine is slightly different:

There is no rubber trim strip, and two SS strips cover the top and bottom of the joint. Here's how mine is built:

I suspect that a PO removed the rubber strip and ground off the lip that held it. Two long stainless steel strips run along the upper and lower edges of the H-channel, screwed into the channel with SS screws. They cover the pop rivet heads and extend just over the edge of the channel. There's a fillet of epoxy along the top and bottom of the channel, which I suppose was meant to seal it. It failed.

This photo shows the SS strips removed:

I cleaned off the surface oxidation with a wire wheel:

You can see the pop rivet heads and the epoxy fillet along the top. It looks like a PO also smeared epoxy over the whole H-channel, then painted it blue.

There are many places where the epoxy fillet failed. I dug around one:

The brown stuff is failed caulk from the Pre-Cambrian era. I could dig it out with a knife.

Here's a simplified sketch:

The epoxy fillet has cracked in many places, and water has worked through the cracks and through the ancient caulk. But I notice water coming down the inside of the hull from the lower part of the aluminum extrusion. It's not obvious where this is coming from, but it may be that water running down the outside of the hull seeps up into the lower part of the H, up around the top of the hull, and down the inside. Or maybe water gets in around the pop rivets or the screws that fasten the SS strips. The water fills the extrusion, then leaks out the lower pop rivets, and down the inside of the hull.

And no, the surveyor didn't catch any of this. The POs had carefully cleaned the interior water stains, and didn't mention the leaks at all. Mind you, the boat is 40 years old, so I was hardly surprised that it leaked.

So here's my two-part plan.

Plan A

I have a combination tool with a 1/16th wheel that I will use to grind out the ancient caulk without going down far enough to decapitate the pop rivets. I'll do this top and bottom, then fill with the thin version of Life-Calk (the one that penetrates into cracks more easily). To prevent the material from running out from the lower part of the H-channel, I'll run a strip of outdoor blue tape along right after my caulk gun to seal it off. I hope.

Next, I'll test with a garden hose and see if the seal works. If it does, terrific. I just replace the SS strips (with caulked screws). If it doesn't, then on to plan B.

Plan B

I will rout a groove into the fiberglass immediately above and below the H-channel. The groove will be about 1/4" wide and 1/8" deep. I'll lay wet strips of FG cloth over the H-channel, and tuck the edges into this groove. When I have three or four layers of cloth, I'll fill whatever is left of the grooves with epoxy putty.

I won't be able to fair this glass overlay to the hull, because that would be just way too much work -- and would ruin my beautiful Awl-grip paint job. So to cover the whole thing, I'll build a solid teak rubbing strake wide enough to cover from just above the upper groove to just below the lower one. I'll mill a recess in the back of the wood to accommodate the thickness of the aluminum H-channel plus my glass layup. I'll then fill the back of the rubbing strake with caulk, screw it in \with SS screws (caulked), and cover with plugs. It would look something like this:

I'm wondering, though. When I dig out the old caulk, if I avoid the rivets I will only be digging out about half of the caulk. Maybe I should drill out each pop rivet, then grind out the caulk most of the way down the H (my grinding tool isn't wide enough to reach the cross-bar of the H, so I won't cut through it). Then insert the new runny caulk, then immediately insert a new (caulked) pop rivet. I would work in sections of about 6 feet at a time, so I won't actually separate the entire deck from the hull all at once! But I don't know how long the pop rivet would have to be. I guess I could drill one out, then feel around with a wire to find out.

Does that sequence make sense? Or should I apply the runny caulk, let it set for a day or two, then insert the new rivets? Or not chop off the old rivets at all? I figure if I'm going to this much work, I may as well do as thorough a job as I can. And I really want to avoid plan B if possible (although that teak rubbing strake would look really nice!)