Companionway hatch cover rebuild

[walterg]

The cover over the sliding companionway hatch on my 1987 Mull 28 sloop had become soft and punky to the point where I avoided stepping on it, so its remediation was added it to my winter project list. After removing the 7 screws holding it to the deck, pulling the cover off the boat and flipping it over on some sawhorses, I could see the dark spots of rotted balsa core through the fiberglass skin. The skin where the sections of the cover joined had delaminated to where I could pull it apart with my fingernails.

Hatch cover underside before disassembly

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Hatch cover delamination

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My first approach to taking the cover apart for a rebuild was to use a circular saw with the blade set just deep enough to cut through the skin, in the hope that I could pry the old skin off in one piece, replace the balsa core, and then re-bond the old skin back on. This turned out to be wishful thinking as there were still too many areas where the skin was firmly attached and I couldn’t get it apart. So, I resigned myself to cutting away the skin and digging out all the rotted core

Rotted balsa core

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The next step was to remove and replace the balsa core.

[walterg]

The worst sections of the core were so far gone that I could dig them out by hand. After I got past that, the only tool I needed was an oscillating multi-tool with a carbide blade. This served all the purposes of cutting away the skin, gouging out the old balsa core, and scraping the interior of the cover back down to the fiberglass. With all of the interior structure removed, the upper section of the cover was floppy, so I worked carefully over 3 hours to dig everything out without causing any more damage. There wasn’t much more than a few inches of dry core left, so I cut it away back to the wide line of tape in the picture below.

Hatch cover scraped clean

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Next step was to cut the new balsa core to fit and bond it in with epoxy.

New balsa core installed

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It was not hard to envision how the damage occurred. When the original cover had the holes drilled for the attachment screws, the balsa core was left exposed. Water then seeped in around the screws, wicked into the core and, helped by freeze/thaw cycles over the years, waterlogged and destroyed the core and delaminated the cover. To prevent this from happening again I did something that, had it been done at the TPI factory, might well have made all of this unnecessary. I bored out oversized holes in the balsa for the attachment screws and filled them with “plugs” of epoxy thickened with silica. When I re-drilled the holes for the screws, there would be a barrier of epoxy around the screws that would insulate the balsa core from any more water intrusion. Next I beveled or “scalloped” 1” or so of the surface of the remaining skin (bottom of the photo above) with a grinder so I could taper the new skin onto it and get a solid bond.

The new skin was laminated from two layers of 17oz biaxial woven cloth and epoxy.

New skin added

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[walterg]

The rebuilt cover is very stiff and I can jump up and down on it without worries.

Rebuilt hatch cover

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Time and materials – as usual I hope that I could do this in 2/3 of the time if I had to do it again. I also hope I never have to.

Time:
Cut away skin on underside of cover and scrap out old core: 4 hours
Fit and bond new balsa core: 3 hours
Laminate new skin: 6 hours (done in 2 steps to avoid sanding in between)
Drill new holes, finish and fit: 1 hour
Total: 14 hours

Materials (all from Jamestown Distributors, http://www.jamestowndistributors.com):
2’ x 4’ x 3/8” balsa core, $36
2 yards 17oz biaxial cloth with mat backing, $25
WEST System 105/205 epoxy, about a quart, $90
Miscellaneous fillers and consumables, $30
Total: $181

[mike cunningham]

We'll, this one is going in my archive file. That's a two thousand dollar job! Beautiful.

I have a 30 and am sometimes surprised FYI did some el cheapo installs on the boat with unprotected or silicon sealed screws into cored areas. Why they would do this on an otherwise well built boat is a mystery.

[gamayun]

Ditto what Mike said! This was a most excellently executed and documented project. I've been meaning to pull the cover over my companionway hatch for a while now because I plan to mount a life raft. Every bolt that I've pulled over the years has been over drilled and filled with epoxy before getting rebedded with 4200. Everyone should be doing this on their Freedoms.