"Mid Size Power Boats": A Guide for Discreminating Buyers - by David Pascoe


Preventing Rot in Encapsulated Wood Structures


This article is in response to Mikey at yachtingmag.com powerboat forum, where he asked what you can do to protect wood components of a boat from rotting. His request couldn't have come at a better time. A few hours before seeing his post I had just gotten off a ten year old Sea Ray 390 Express that had rotten structurals everywhere, including the hull stringers that were completely wasted.
Water through a stringer

This is water weeping through a stringer because the compartment behind the stringer has filled up with water due to a lack of drainage. The problem usually isn't discovered until it's too late. Shown above are the teltale signs of seepage.

This is a widespread problem that occurs in  large number of boats that, when the boat gets on in years, can result in serious degradation of the hull structures. In the case of our Sea Ray here, after only 11 years of life, the hull is shot. The cost to repair this kind of damage is too much for anyone to consider.

Unfortunately, when a boat is shabbily built with low quality materials, with some boats there isn't much you can do since the whole boat is in question. But in other cases, exposed wood may only appear in random instances where protecting the wood is possible. In this article, we'll take a look at both situations so that you can learn to identify them, and what you may or may not be able to do.

Stringer and plywood deck in bilge

Stringer and plywood deck in bilge. Conditions like this are waving a red flag. Note that the stringer here is only covered with chopped strand mat. The plywood deck at left is rotting because of water weeping though the stinger, which is also rotted.

There are several reasons why rot occurs in structures that are ostensibly "glassed over.". The first is what I call the GEICO SYNDROME (we all do dumb things) when the builder does something dumb like boxing the stringers like Sea Ray does. In this case, numerous  leaks in decks and windows caused the pockets formed by the stringers to fill up with water because there was no drainage-- and being boxed in, no on could see what was going on down there. Secondly, and this is a common problem, is that the "fiberglass" consisted of chopped strand mat is about as porous as a sea strainer, even when painted with gel coat. 

Next, builders routinely paint wood with gel coat to make it look nice, but looking nice doesn't help much because water goes right through gel coat and gel coat doesn't stick to wood so good. When the wood absorbs water, it swells and the gel coat cracks, allowing in even more water.

The third problem is when the builder does a sloppy job of encapsulating the wood framing system, water gets in and never leaves-- ye old osmosis where water goes one way but not the other-- you'll find things like limber holes and exposed wood in places. Many builders like to paint the wood with gel coat so that it looks like protected. As you know, water goes right through gel coat, the wood gets wet and swells, which is why you often see all those cracks when you open the hatches.

Another part of the problem is that the quality of wood being used is often akin to Home Depot lumber-- third rate stuff like sugar pine and the world's cheapest plywood. Often the builders claim that the wood is treated when it isn't -- otherwise, why would it rot so fast? If that's the case, there isn't much you can do to prevent deterioration except to buy a better quality boat next time around.

Exposed wood presents an obvious problem that needs to be addressed. This usually means incomplete glassing of parts as shown at right. Unfortunately correcting this after the boat has been built is not easy as such places are usually hard, if not impossible, to reach.

Exposed wood of plywood stringer

Here's a plywood stringer that wasn't completely glassed, leaving the wood exposed. Painting it with gel coat will only keep the water out temporarily.

Then builders, in order to save a few dollars and make more profit, have fallen in love with staple guns. check out some of the smaller Sea Rays and you'll see staple wounds all throughout the stringers and other plywood parts. Obviously water can enter the wood by these deep staple depressions in the wood. A good method of sealing these is our old friend 5200.

If you have a boat with a lot of gel coat painted wood, there isn't much you can do to stop water absorption. If you see exposed wood, the best thing is to clean it and coat it with dilute resin. I dilute it with a bit of acetone, catalyze the hell out of it,  and paint it on to roughed up wood so that the wood absorbs the resin well, and so that it won't just flake off again.

A rotary tool with a grinder head or sandpaper drum head is an extremely useful tool for dealing with this sort of thing. In order to seal the wood, you have to have a good, clean surface to make your sealant stick. It's a waste of time trying to seal dirty or wet wood, but getting old, oily wood clean often isn't possible. In that case, resorting to painting on a wood preservative may be the best solution.

People often ask what to do about glass over wet wood stringers. My Rx is to use a hole cutter and cut through the outer laminate and expose the wood to let it dry out. yes, that means drilling a lot of holes that have to later be glassed up, but that's the only way I know of getting the water out. If it's real wet, you can try pulling a vacuum with some poly film and tape. On really wet wood, that will pull a lot of water out fast.

When going through your boat, look carefully at all those places where a hole cutter was used to cut limber holes. Is there exposed plywood? See photo below. Gel coat painted over exposed plywood grain won't do the job of sealing. So, grind it off when it dries -- like around lay up time-- and be prepared to reseal it. Here you want to use a liquid that is thin enough that the plywood will soak it up, once again, diluted resin usually does the trick. When set, come back and recoat with undiluted resin.

Plywood painted with gel coat

This hole cut into frame leads into fuel tank cavity. Note that plywood is only painted with gel coat and that it is constantly wetted with bilge water.

Cut in plywood painted with  gel coat

Close up of view above. It's not hard to see why this piece of plywood would start to rot.

I use the same method for all exposed wood and especially plywood.

Flat Deck Surfaces. Most boats have plywood decks located deep in the hull. These, too, are often just painted with gelcoat. These also prove troublesome because things like pumps, water heaters, hoses and other equipment are screwed down to these decks. Because the decks tend to collect water, the water gets into the screw holes and starts the rot process in the plywood. Obviously, all screw mounting need to be bedded.

Also make sure that your water heater pressure relief valve has a drain hose on it, and that it isn't dumping water directly onto the deck on which it is mounted. Leaks in pumps and plumbing need to be corrected promptly.

If there is a tendency for water to collect on these decks, some provision for drainage needs to be made, Or, better yet, find out how the water is getting there and stop the leaks.

For reasons I'm not aware of, freshly condensed water will cause rot faster than anything. Like sweat from refrigerators and condensation from air conditioners. That means you'd best not route condensate drains into the bilge. Often there seems to be no alternative since water doesn't run uphill. Try routing these drains into a shower sump box which, in many cases is the  only way to eliminate the damage that condensate can cause.

Stringer covered with gel coat, with holes and screws

This stringer looks like it's glassed over but is actually covered with gel coat. It's also full of staple holes and screws, plus the untreated hole at bottom (red arrow). The blue hose at center is an A/C condensate drain. The brown spots are water weeping out of cracks in the gel coat. This stringer (and boat)  is surely going to have a rather short life.

Finally, many people ask why wood should be used at all in "fiberglass" boats.  The answer is that wood isn't the problem. If the windows and doors of your home were made of sugar pine, they'd rot away just as fast as your boat. The reason they don't is that good quality wood like cedar and fir is used which doesn't rot away in a heart beat. The essence of the problem is low grade wood being used. We routinely see 20 year old boats with exposed plywood structurals that has been soaking in water for nearly a quarter century, and yet it hasn't rotted. That's the difference between quality material and the junk.

When we talk about quality boats on this web site, it is under appreciated details such as these that make all the difference. After all, if you spend $30,000 for a boat, you expect something better than interior grade plywood.

Posted May 18, 2000

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David Pascoe is a second generation marine surveyor in his family who began his surveying career at age 16 as an apprentice in 1965 as the era of wooden boats was drawing to a close.

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Last reviewed October 5, 2016.