The Rot Doctor


Subject: Sealing planked over plywood hull
Date: Wed, 08 Nov 2000

On the CCABC listbot, a recent topic addressed the use of CPES on double hulls (planked over plywood) which left me confused. I am preparing to strip, refasten the bottom and refinish my 57 CC Sportsman. Using the CPES on topsides prior to refinishing seems like a no-brainer. My question is the best way to seal the bottom. This boat when not being used sits in a lift, if I don't use it for several weeks, it leaks until it swells up (about a day). I am not ready to replace the entire bottom. How might I proceed with sealing this bottom such that it continues to swell so it won't leak but will receive the benefit of CPES long term protection? Unfortunately, this will be a several month long process so I don't think I can pull it and seal it before it shrinks up. Am I stuck with one extreme or the other?
Thanks, Chas


You're confused because it's a confusing situation.

Back in the 50's boat lifts were not generally used, so the builders of these boats were not overly concerned about total out-of-water water-tightness. I believe they used a canvas interface between the ply and the planks, usually bedded in an oil-based sealer compound. This situation worked when new but will gradually degrade over time...and from the 50's to now is a long time. Now of course there are all kinds of urethane sealants available that can make a boat watertight.

CPES will retard the ability of the bottom wood to absorb moisture, so you could expect that if you simply coated the bottom bare wood with CPES it would actually slow somewhat the ability of the wood to swell closed again. It might take it a couple of days to swell shut again after going back in the water. True, the CPES would help protect the wood from deterioration, but, as noted, it will also retard water absorption time.

The ultimate solution would be of course the rip off all the outside planking, seal the bottom in a urethane, and re-plank. Not practical, though, for obvious reasons. The next best thing would be to reef out all the bottom seams to good clean wood, treat all the planking (including the seams) with CPES, and then re-caulk the seams with a polyurethane caulk, such as 3-M's 4200 slow-cure or one made by Sitka. If done carefully this should stop leaking, and still allow the seam flexibility for the wood to expand. If you do this, be careful of sealants like 3-M's 5200 (which is very popular right now) and the 4200 Fast Cure, because they are relatively stiff when cured. They MAY allow sufficient room for expansion, but they make me nervous. Given my druthers I'd go with the Sitka polyurethane. We're testing some of our own polyurethane sealants right now and will have them for sale a 6 months or so.

Polyurethane sealants are tough and stick well to cured CPES-treated surfaces.

I'll conclude this by saying that even this approach is not guaranteed to produce total water-tightness. It'll surely cut the leaking down substantially, but getting total water-tightness in seams is not easy.

Glassing the bottom, even with cloth and prime epoxy resins, produces its own set of special problems, and we don't recommend it except under very special circumstances.

I hope this has been of some help. Come on back if you have any additional questions. We'll help you if we can.