Repairing and/or drying GRP covered stringers and bulkheads

Hi all, I recently purchased a Southwind 655 21.5ft) 2003 model with a Yamaha 200 four stroke at a bargain price knowing I had water ingress issues to deal with.
I have two reasonable options. 1. Ignore things and use the boat until the water causes too much damage or 2. Attempt some reasonable repairs to prolong the life of the boat (hopefully significantly).
The later model Southwind appears to be a reasonably well made boat with a decent layer of glass over the stringers 3-5mm per side and 2-4mm on bulkheads. The outer part of the transom is solid glass (I presume) (and dry) around the outer corners leaving just the inner easily accessible engine mounting area as ply. There is one ply stringer each side (roughly 20mm) with 3-5mm of glass either side. These stringers are under the moulded deck are glassed to the deck (but are not boxed in) the forward and aftmost parts of the stringers are boxed in with a piece of GRP over ply horizontally.
(Please see the layout diagram attached.)
Access to the outer side of the stringers is now easy given that the fuel tank came out with NO GLUE !!! (Super happy about that) (see photo.)
The problem
I removed some of the grp with a hole saw over various parts of the structure and took a few core samples as well. It appears as though fresh water has come in from rain over time and caused a degree of rotting in the bottom of the forward bulkhead and various parts of the forward parts of the stringers.
The rear parts of the stringers, rear bulkhead and transom are damp (from sea water I think) but no rot from what I can see (had to take the engine off so was able to inspect the engine mount holes). See an attached guide as to where the problems are (note where I’ve indicated rot is only sporadic not indemic)
To dry and repair or replace.
Originally I had intended to try to evacuate water with a vaccum pump (which works well btw) then airate the stringers/bulkheads by removing the grp with a hole saw (not drilling into the timber) at strategic locations (see example in my attached photo), covering the boat in insulation and putting a heater and dehumidifier and leaving it over the winter. Then using something like west system on the rotted parts.
Now given the accessibility I’m thinking that I could do that dehumidifing idea for the damp parts but perhaps I’d be better off stripping off the outside (exposed part of glass on the stringers and bulkheads and replace the wood and outer glass very much like you would do a transom.
My questions;
1. Given the accessibility of the stringers and bulkheads is my replacement idea practical?
2. Given that the rear areas are damp but not rotted but more difficult to access is my opening up and dehumidification idea practical?
Appreciate you wisdom guys.
Cheers

 

You are facing a rather impossible battle. The wood saturates and that saturation is protected by the resins and coatings. It is unlikely you would be able to dry the boat out as you wish as described.

The only way to truly fix the boat is to remove the resins and coatings until you get to dry wood. And remove rotten wood that is soft back to no wood or solid wood.

Then you dry it out in the fashion you described. You do not need heat. A dehumidifier in a tent provides quite a bit of heat if the top does not lose it.

The timbers dry in a strange way. They surface dry and then they resaturate from the wet wood underneath. The process is slow and rather bizarre. One day you'll read 12 percent moisture and the next day you'll read 15 and this process goes on for quite awhile. I have seen the phenomena in wood drying when drying too fast.

Using anymore heat than the dehumidifier can result in steam. Steam is very bad as the moisture travels throughout the hull and will likely cause blistering.

Anyhow, you dry down to 7-8% for a minimum of about 30 days; then you can repair the areas of removed resins.

There is really little reason to try to dry it out without fully understanding ingress sources.

If the boat has a balsa core. Kiss it goodbye. Way too much work to decore it..

 

There is an easy repair. Grind the vertical surface of the existing stringer and 5-6 inches of the bottom. Put two layers of 1.5oz mat on the vertical surface and screw a 1/2" of plywood to it. Laminate over the whole thing.

 

Over wet core? His hull is all wet, too, see pics.

 

The hull is solid glass. There is no balsa or anything like that. The only timber is in the stringers, bulkhead and transom.

 

I did consider this Gonzo however I was thinking that if I don’t deal with the existing rot and dampness it will eventually infect all of the existing structure (stringers and bulkhead). It is definitely another option though.

 

If the rest is already wet, you won't be making anything worse. If they are dry, the repair won't affect them. You will be adding some weight, plus whatever the absorbed water has. It won't be enough to make a huge difference though. If you decide to remove the cap and do a complete job later, it will require grinding a bit more.

 

Ignore the core. He would build a new stringer next to the wet ones.

 

Oh so much easier to sister in than scarf in a repair. If the fuel tank will fit between the sistered stringers.

 

Oh, sorry guys, I thought the stringer was hullsides.

I shudda looked up eh?

Personally, I tackle the transom first.

see where that leads you

 

Thanks gonzo ... that is a serious (and much easier) option.

What do u think about the drying out option I mentioned?

 

It would take years, literally. You would be air drying semi-encapsulated wood. Anyway, dry rotted wood is lighter but structurally just as weak.

 

Just to concur with Gonzo, I have tried many different ways to dry out laminated sections with very little success!
Sistering on to the existing stringers May buy some years of service, but in order to be continuous, would require cutting out bulkheads and possibly some floor down the center, but still easier than a total strip out.
Don’t forget to include some limber holes to allow drainage of all compartments to the bilge.
The lack of drainage is often the root of the problem in the first place.

 

Thanks for your advice guys. I hit the project today and started investigating things more earnestly. They say in boats that water intrusion is always worse than what it seems. My boat definitely meets that description.
Gonzo whilst I liked your suggestion but after thinking about it and considering the developments I decided to do it properly.
I ended up cutting the outside face off the stringers and the rear bulkhead. It was pretty rotten. The good thing is by being a bit nifty and making a powered chisel with a rotary hammer drill I got it all done in 5 hours. Still got a bit of clean up in morning and will attack the damaged parts in the cabin tomorrow. I’ve been able to preserve the outward facing glass (peeked it all off) and the inward glass is still intact so I think the rebuild will be a bit like a transom fixing the new stringer to the existing glass and tabbing it all in. Pretty happy with it so far.
question; what are some decent waterproof alternatives to just marine ply as a replacement for stringers and bulkheads.
Thanks

 

Why are you so worried about the core(s)? The core material of a stringer, or a floor (what you are calling a bulkhead) adds no strength to the structure. The core is there to give the fiberglass support as the layup is done and the glass/resin cures. Back in 2009 I was rebuilding and installing new engine stringers and the floor (transverse bulkhead) under the rear cabin wall in this 1973 Silverton sedan I was restoring. Like you, I thought the core was important to the strength of the structure. I learned that this is largely not true.
The cores of the engine stringers were completely deteriorated, as was the bulkhead. But that was really a minor issue. The fiberglass that made up the stringer/bulkhead wasn't up to the task. The laminate was too thin and weak. Over the years the engine settled from gravity as did the floor under the rear cabin wall. This caused all sorts of problems as you can imagine.
What I learned was that the core of a stringer or floor is only important if you are going to attach something to it and need strength in compression. Clamping strength. Otherwise the stringer/floor gets it's strength entirely from the fiberglass that is draped over it and attached to the hull.
When I rebuilt the engine stringers I used Owens Corning Formular 250 as a core material. Just 2" thick insulation board, right out of Home Depot. I won't go into the whole story of how these structures were installed and aligned. The point is that my engine and drive system has been sitting on those foam cored stringers for the past 11 years and hasn't moved a bit. You'll see in the photos below that the stringers/floor are foam with some wood inserts built in. The inserts take the clamping loads (compression) from the aluminum engine mounts that the foam would not bear.
Forward of the engine compartment, I never rebuilt the stringers or floors. The cores were/are somewhat deteriorated. However they are strong enough to do their job and haven't moved or caused any problems. I've been using my boat for the past 6 years and have logged hundreds of hours traveling from eastern New York to the Great Lakes and Canada.
If you see evidence of structural failure of the stringers or bulkheads then by all means address the problem(s). I would not worry about the cores being wet/damp provided the fiberglass laminate is providing sufficient support to whatever it's supposed to be supporting.
You can find a good discussion about stringers/cores and laminate structures in Dave Gerr's book The Elements of Boat Strength.