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-   -   Need help with replacing rotted plywood (http://www.sailnet.com/forums/gear-maintenance/57521-need-help-replacing-rotted-plywood.html)

BC2316 08-27-2009 10:36 PM

Need help with replacing rotted plywood
 
My Cal 24 has wet and rotted plywood in the ceiling of the cabin. I cut an inspection hole through the thin fiberglass covering the bottom side of the ceiling and found the plywood comes out in shreds. I pulled enough away to expose the fiberglass of the top deck. Can the plywood be replaced from inside the cabin or does the top deck need to be removed? I'm pretty competent with home repairs but I'm new to boat repair. Any advice on the best way to proceed with this repair would be appreciated.

WanderingStar 08-28-2009 10:08 AM

I read an article about this problem, but I don't remember details or where I read it. Working outside would be easier, but I think you should be able to do it either way. Would you be able to get the ply inside? You might also build the cabin top at home and then replace it whole. Before drastic action you might drill some more holes and see how extensive the rot is.

slocum2 08-28-2009 07:41 PM

Here's my experience with rotten plywood in an Offshore 33 cabin top. It all started by following a leak and ended up with me ripping the inner liner off. Numerous test holes found that almost the whole cabin top was wet and most of the wood soft and rotten. During the process I found that previous owners had tried to fix the problem by drilling holes from the top down and filling them with epoxy. The standard thought is that the epoxy will penetrate the plywood and firm it up. Epoxy doesn't get past water, all that happened was that there were many, many little epoxy stalagmites in the cabin top. Useless.

It was winter in New York and I couldn't see myself cutting the whole top of my cabin off, so I decided to work from the inside. To do this required a lot of patience and a set of extra hands. It also meant working against GRAVITY. I replaced the plywood with balsa and then two layers of fiberglass matting. Unless there was something I was missing, I could only put up relatively small pieces (maybe 2' x 2') at a time and then had to hold or brace the piece until the glue started to setup enough to hold it. Putting up the fiberglass mat was even worse. It took many hours, and was probably one of the worst jobs I've attempted.

I'm still finishing out the interior wood work so I can't give you a long term status report but I do feel the cabin is as strong as it ever was. A rotten cabin top core made the boat basically worthless, now I feel its marketable again if I ever decide to sell. I would like to sail it again first :)

TQA 08-28-2009 07:56 PM

Horrible job to do from inside.

Here is an off the wall thought.

Turn it upside down after removing the roof plywood the job then becomes simple. How? hmmm thats the rub. remove the mast lifelines etc get a load of old tyres some old sleepers, a few blokes some straps and an old fashioned dual crane wrecker. That should do it.

You might need to build a false floor to work from though the roof will be weak.

BC2316 08-28-2009 10:46 PM

Thank you all for the help! The rot covers about 1/3 of the ceiling and I want to have a wood finish rather than a painted ceiling when I'm done, so I will be replacing all the plywood to get a consistent stained finish when completed. Do I need to replace the fiberglass mesh that was below the plywood? I was thinking that if I left it uncovered it would dry out better if, God forbid I ever have a leak again.

slocum2 08-29-2009 09:53 AM

The cabin top construction: a layer of fiberglass, plywood or balsa, and another layer of fiberglass is all part of the structure that keeps the cabin top stiff and strong. I wouldn't (didn't) skip the interior layer of fiberglass.

I'm sure my prolems started with holes drilled ito the cabi top and then thigs screwed directly into the plywood. proper way is to drill out a biger hole, fill it with epoxy, and then screw into the epoxy. A little more work up front but saves $$$$ and times later.

Good Luck

slocum2 08-29-2009 09:56 AM

The cabin top construction: a layer of fiberglass, plywood or balsa, and another layer of fiberglass is all part of the structure that keeps the cabin top stiff and strong. I wouldn't (didn't) skip the interior layer of fiberglass.
I'm sure my problems started wit too many holes drilled into the cabin top. The correct way is to drill a bigger hole, fill it with epoxy, and then screw into the epoxy. A little more work up front but saves $$$$ and times later.

Good Luck

BC2316 08-29-2009 11:23 AM

Thank you slocum2. I was wondering why Cal had sandwiched plywood between two layers of fiberglass with no way for water to dissipate. Your idea of the larger holes filled with epoxy answers the question of how to keep the water from contacting the plywood in the first place. Even if the screw hole leaks, the water will travel into the cabin and not make contact with the plywood. Brilliant!

I'm headed to the boat to start work!

Thank you.

LookingForCruiser 08-29-2009 02:33 PM

For a stronger repair, rather than drilling an oversize hole, you should drill a normal size hole, and then use a router bit to carve out the core between the upper & lower skins.

Then duct-tape the hole from the bottom, paint the exposed core in the hole with epoxy, and fill with thickened epoxy (with colloidal silica - some of the other thickeners will wick water down thru the epoxy). You coat the exposed core with unthickened first because the wood will suck up epoxy otherwise, and end up sucking epoxy, leaving an epoxy-poor plug.

This results in a plug of thickened epoxy sandwiched between two fiberglass skins, so it can't pop up and out if there's a vertical load on whatever hardware you're attaching.

LookingForCruiser 08-29-2009 02:36 PM

Also, the method of repairing a cabin deck from the bottom results in messy dripping epoxy, and poor adhesion/voids, resulting in a weaker deck. Propping up core from beneath is never going to bond as well as letting gravity (and sand bags, weights, etc) do the job from the top. The previous owner of my boat fixed the coach roof using the from-the-inside method, and there are voids that I need to fix now.


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