Repacing Rotten Deck Core, Part I

4 Feb

As I mentioned in this post one of the major repairs on my boat is a section of deck where the upper fiberglass laminate had cracked, allowing water into the balsa core and starting a cycle of rot and delamination which led to this:

Close-up of the worst damage. These cracks go all the way through the fiberglass.

I knew this project would take more than a few days so before starting I made a series of dams out of plywood so that I could protect it with a tarp and not have rainwater running into the repair area. This worked quite well except that the waterproof tape was a real chore to remove afterwards.

To gauge the extent of the problem I drilled 1/4″ holes outward from the cracks searching for dry core. You can glean quite a bit of information about the state of your deck from taking these ‘core samples’. The main thing you want to learn is where the edges of the damage are but samples can also tell you a bit more.
 With a balse-cored deck a dark or black pulp indicates advanced stages of rot while water welling out of the hole shows you’ve got lots of moisture coming in somewhere, more than might come in through just some poorly bedded fasteners. Balsa which is wet but has a nice fresh color and doesn’t feel rotten may mean that you can get away with just drying it out rather than replacing it and of course dry wood shows that you’ve found the edge of the damage.

Because the deck was cracked through there
was a lot of water in there.
This balsa is quite pulpy and saturated with water – not a good sign.

Dry balsa at last!
Once you’ve found the extent of the damage the next step is cutting open the deck so you have access to the core. This feels quite harrowing, particularly if like me you’ve never done it before! I used a circular saw which worked well. Be sure that you have the blade exposed only enough to cut through the top layer of glass and not the entire deck. 
It is often possible to re-use this top skin and save yourself a good deal of glasswork but unfortunately this was out of the question as the upper laminate was deeply cracked and quite warped. I ended up cutting it out in two stages, first the most damaged part and then out to the edges. There is no real practical advantage to this except that it allowed me a practice run to get a feel for what I was doing. With the first cut I tried to make rounded corners with a cut-off wheel on an angle grinder. As you can see this was not a success so for the final cut I just cut the corners square and rounded them later with a grinding wheel.
The practice run. Note the messy upper right corner.

 The rotting blasa made a strangely beautiful sight.
The final cut. 
The other mistake I made was to make my outboard cut too close to the edge of the deck. I actually cut into a layer of solid laminate, not the top skin. So I had to make another cut about an inch inboard. I didn’t have the blade set very deep so I don’t think my first cut did any real damage to this overbuilt boat making this just another case where I might have run into serious trouble if I was working on a new boat with a thinner layup.
Getting the top skin off ended up being much harder than I expected. Even though the core was thoroughly soaked and partially rotten some of the fiberglass still had a tenacious grip on the balsa wood. In the end my technique involved pounding a prybar in with a hammer and then ripping the deck skin off in chunks. I have no idea how one would remove a skin as well bonded as mine without destroying it.

In some places the bond between core and skin
was still so strong that the fiberglass layers ripped apart.
Next up was the slow process of cleaning and drying. I started by chiseling up all the wood that I could get out followed by a rough sanding to smooth things up. Then I let everything sit for a while so that any remaining moisture had a chance to work its way out. In this I was greatly aided by an insufferably hot New Orleans summer and a drought. At least this gave me some minor condolence as I was suffering through trying to work outdoors in the 110 degree heat!

Once everything is nice and dry it’s ready to be made into a deck again. But I’ll save that for next time. Here in New Orleans it is Mardi Gras season and I’ve got some all-important work to do on a costume!

This article was syndicated from Safe At Harbour But Meant For The Sea: DIY Sailing with Paul Calder


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