Small rotten spot in stringer, advice needed

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Dougcole

Dougcole

Guru
Joined
Jan 21, 2008
Messages
2,337
Location
USA
Vessel Name
Morgan
Vessel Make
'05 Mainship 40T
Hi All,

Along with my fuel tank replacement, I found this little gift under the tree today, and I'm soliciting repair advice from the board.

This is in a notch that was cut for the forward engine mount in my port side, midship stringer. With the motor out for the tank replacement, I can see it and reach it easily, which would not have been the case with the engine in place. So I'm lucky, I guess? I have no idea why Mainship cut that notch, they built the height back up with aluminum plates under the mount, it's odd and they did not do it on the starboard engine. I strongly suspect that they cut the notch and did a poor job putting a new skin on top, resulting in the void. I'm stumped as to where the moisture came from though as this area never gets wet.

I dug back in with an oscillating saw until I got to solid wood core all around, it's about 2" high, 1" wide and 3/4" deep. It's still a little damp though. This is my plan, please let me know if you have suggestions.

1. Use a heat gun to dry it out as much as possible.
2. Paint unthickened epoxy in to soak the core.
3. While the epoxy is still wet, layer in thickened epoxy.
4. Push in a small block of plywood to fill the void, with more thickened epoxy on top.
5. Put a couple of layers of mat in to fill the hole.
6. lay a layer of cloth over the top, overlapping onto the sides of the stringer.
7. sand and paint.

Thanks.
Doug
stringer 1.jpg
stringer 2.jpg
 
Never use fiberglass mat with epoxy. At least not the mat you can get in the US. It's held together with a styrine solulable binder that's meant for polyester or vinylester resin. The binder must be dissolved to properly wet out and epoxy won't do it. I do a lot of glass work and use epoxy exclusively but never mat, I use straight cloth or mostly 1708 stitched biaxial. I would fill the void with wood bonded in with thickened epoxy then cover with 1708/epoxy. 1708 is 17oz cloth layered with .08oz mat. But there's no binder in the mat, it's all held together with rows of stitching every 1/4 inch or so. It does wet out properly with epoxy resin.
 
I'm not there, but I think if it's only 3/4" deep (it does look deeper somehow though?), I'd be tempted to skip the wood-block "filler" and just used thickened epoxy. Unless it's really hot, you can do a couple of 3/8" fills (slow hardener; do the next fill after it has kicked but while still in the green stage)). Then I'd probably put some biax over the top (I don't see a huge need for biaxmat, but if that's what you have I don't see an issue either).

Then again, probably no real issue with the wooden filler block. Or if you have any scraps of GRP board, that would be swell.

A caveat is I don't 100% understand the big picture of the whole structure you've got there. I might be tempted to remove a bit more paint and see if any areas might at least want to be epoxy coated. Maybe someone else has a better idea (especially someone else with your same boat).
 
jgwinks said:
Never use fiberglass mat with epoxy. At least not the mat you can get in the US. It's held together with a styrine solulable binder that's meant for polyester or vinylester resin. The binder must be dissolved to properly wet out and epoxy won't do it. I do a lot of glass work and use epoxy exclusively but never mat, I use straight cloth or mostly 1708 stitched biaxial. I would fill the void with wood bonded in with thickened epoxy then cover with 1708/epoxy. 1708 is 17oz cloth layered with .08oz mat. But there's no binder in the mat, it's all held together with rows of stitching every 1/4 inch or so. It does wet out properly with epoxy resin.
Click to expand...
I didn't know that, thanks. I'm at the boatyard, I bet they have some 1708 laying around.
How much would you wrap it around the sides? Will it make the bend around those corners?
 
Biaxial is pretty good at taking bends. As long as any corners are radiused (or filleted). Kind of like sewing fabric on the bias (stretchier).

PS: My mind is also still going to: What happened and why? In case that better informs the repair...
 
Here is another pic to add more perspective.
 

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Double post. Sorry.
 
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It’s actually about 1 1/4 inches deep.
 
Frosty said:
PS: My mind is also still going to: What happened and why? In case that better informs the repair...
Click to expand...
Hard to say, exactly. I think Mainship cut that notch in the stringer then put a very thin layer of glass on the cut section. I pushed my finger right through it.

I can’t find any other soft spots in the stringers, and they sound good to the knocking test. This spot, as long as it doesn’t get worse, is not really weakening the stringer, so I mainly want to stop any further rot.
 
So what happened? Possibly change in engine ordered, possible rookie cut too deep, possible ran into bad wood.
 
Bi-axial cloth is what you want to use. Forget the plywood. Coosa board or a hard wood is what you want to use. Screw and epoxy them down. I would sand the paint off both sides and wrap 2 layers of cloth across the top and down the sides. Fair the wood before wrapping the cloth.

All of the above is a waste of time if the wood isn't thoroughly dry. In the old days, a 150 watt spotlight from about a foot away would heat the wood and dry it out over several days.

Ted
 
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For the soft wood, after it is dry, I would use an injectable epoxy to make the wood solid again. Using regular epoxy will not penetrate much if at all. The penetrating epoxy is very thin and you drill a series of small holes and use a syringe to inject the epoxy. You keep injecting it until the wood won’t accept any more. When it goes off, takes a while, the wood will be solid again. Total Boat makes it as well as Smith and Company. Then I would coat it with unthickened epoxy to seal it up and proceed with the repair.
 
Here's how I would go about this repair:
  1. Clear out all the soft and wet wood you can (sounds like this was done).
  2. Cut a small cap for the end out of some 1/2" ply with a bunch of 1/4" holes in it and tape or hot melt glue it in place.
  3. Put a vacuum bag over the area with breather cloth over the temporary plywood end (to prevent the bag from inverting), use mastic to seal the bag around the periphery of the area. You can snake in a 1/4" poly line for the vacuum in the edge of the bag and double up the mastic.
  4. Put it under vacuum for a couple days, this will draw all the moisture out of the area, add a heat lamp if you can to speed it up.
  5. Cut and bond a stack of ply, coosa, or Divinycell to fill the void leaving about 1/8" gap all around.
  6. Make up a batch of epoxy with colloidal silica and some 1/4" chopped strand mixed in if you have it. The consistency should be like Vaseline.
  7. Wet out the ply block with unfilled epoxy and paint around the inside as best as possible.
  8. Slather up the ply block really well with the filled epoxy and insert.
  9. After curing, trim and sand back as needed.
  10. Prep surrounding area back to raw glass 2" or so and get a 1/4" radius minimum where the glass takes a bend.
  11. Apply 2-3 layers of Biax cloth and epoxy.
  12. Cover with peel-ply and work out any bubbles, allow to cure.
  13. Remove peel-ply, sand and paint with Bilgekote.
  14. Have a beer and admire your work.
 
Thanks for all of the replies. A couple of things;
1. I have to have this repair finished by tomorrow afternoon as I have to leave here for work. The yard will put the tank and engine back in place next week while I am gone.
2. I could possibly get the yard to do it, but it would cost a fortune, I am already spending what budget I have for the tank repair, so I need to do this myself,.
3. I don’t have access to a vacuum pump, though I love that idea.

Maybe I should sacrifice the existing top skin in that area? I could cut away the skin on all three sides back until I reach totally dry wood, then laminate on a new piece of core then layers on a new top skin with epoxy and 1708?
 
I have a hard time believing that the top of the stringer is wet and rotten but the bottom where it meets the hull is not. I can certainly sympathize with the time and financial crunch; most of us have likely been there. But I worry a repair as you are wanting to approach it may not be addressing the whole potential issue.
 
3rdOrderEffect said:
I have a hard time believing that the top of the stringer is wet and rotten but the bottom where it meets the hull is not. I can certainly sympathize with the time and financial crunch; most of us have likely been there. But I worry a repair as you are wanting to approach it may not be addressing the whole potential issue.
Click to expand...
I understand what you are saying, but I just can’t cut the entire stringer out of the boat. It’s just not realistic right now. The wood core is solid below where I dug out the soft core and it is solid forward of it as well. I can’t jam a Phillips head screwdriver into it and it is binding up the plunge blade on my oscillating saw. I really don’t think the entire stringer is rotten.
 
Dougcole said:
I understand what you are saying, but I just can’t cut the entire stringer out of the boat. It’s just not realistic right now. The wood core is solid below where I dug out the soft core and it is solid forward of it as well. I can’t jam a Phillips head screwdriver into it and it is binding up the plunge blade on my oscillating saw. I really don’t think the entire stringer is rotten.
Click to expand...
The rest of the stringer may not be rotten which is a good thing. There is a good chance that there is moisture in the stringer. That needs to be removed to prevent the rot from spreading. Your proposed repair may remove what is rotted now but will not prevent the rot from spreading if there is moisture in the stringer. Right now you are in a position to make a good permanent repair. You are spending a lot of money now for fuel tank replacement. If you plan on keeping the boat I would do a full inspection of the stringer now. Confirm it is completely dry by doing core inspections. When you find dry wood you know that is how far the moisture has spread. Dry the stringer completely and treat it with epoxy to seal the wood grains. I know this can not be done in a day. If there is moisture in the stringer now it isn't going to go away. If you keep the boat you will have to deal with it in the future. If you sell the boat this issue will be found when a survey is done and you will deal with it then too. Or have an issue selling the boat. My opinion step back and reevaluate the repair.
Brian
 
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SteveK said:
No one would consider filling with Bondo?
Click to expand...
I would but thats only because i plan on my repairs to last 20 years. After that my kids will have to deal with it :)
 
The first step is to buy or borrow a Moisture Meter and check the area for water intrusion. I would also check the area where the hole was drilled in the stringer for the hose to go through (not sure why someone would drill a hole in a stringer to run a hose.....that is a no-no). Anyway, the moisure meter would give you a good feel for the extent of the wet area. With any luck, it is just contained to the area you have stated
 
A filler material is one thing for a small area like that once it's dried out, but Bondo specifically isn't water resistant enough to be a good choice in my mind.

Taras said:
not sure why someone would drill a hole in a stringer to run a hose.....that is a no-no
Click to expand...
One would think, but it was done right from the factory on my boat in one spot. Fortunately in a hollow fiberglass stringer so no moisture intrusion concerns.
 
rslifkin said:
A filler material is one thing for a small area like that once it's dried out, but Bondo specifically isn't water resistant enough to be a good choice in my mind.
Click to expand...
Is the wood stringer more water resistant than fibreglass laced bondo? I would fill, sand and then lay on mat for finish.
 
SteveK said:
Is the wood stringer more water resistant than fibreglass laced bondo? I would fill, sand and then lay on mat for finish.
Click to expand...
That's a good point, but my thought is that it would be the same effort to use a better filler than bondo, so...
 
SteveK said:
Is the wood stringer more water resistant than fibreglass laced bondo? I would fill, sand and then lay on mat for finish.
Click to expand...
Bondo has no structural strength. IMO drying the exposed wood and then sealing it with epoxy would be a better repair. If bondo or filler is used it serves only one purpose cosmetic. There are right ways to make repairs and wrong ways. The right ways are expensive and last forever. The wrong ways are bandaids that lead to further issues. In the 70's,80's and 90's wet stringers were a common problem. I was involved in several stringer repairs while working as a marine technician. Digging out the rot and filling the void with filler is a bandaid. The wood needs to be dried and sealed. The fiberglass needs to be cut away to expose the wood so it can dry. All rot needs to be removed and new wood needs to be laced into the existing wood that is in good condition. Once the stringer is dried and new wood is laced in replacing the rotted wood it should sealed with an epoxy resin. The final step would be to wrap the stringer with fiberglass material replicating the original layup.

This is a lot of work. If the OP does not have time or want to spend the money to do it right. The best thing to do is leave the opening alone give it a chance to dry and once it is dried seal it with epoxy and leave the opening open. The bondo will do nothing except cover up a future problem.
Brian
 
@BB-marine You must be kidding or did not read that it is 1-1/4 inch void to fill due the exposed wood end which is already surrounded by firm glass. There is no structural involved. By all means seal dried wood. we agree on this.
"Once the stringer is dried and new wood is laced in replacing the rotted wood it should sealed with an epoxy resin. The final step would be to wrap the stringer with fiberglass material replicating the original layup."
 
Thanks for your help everyone.

The yard manager and the head glass guy from the yard came down and looked at it this morning. They felt that there is sound wood in that spot and that it was pretty dry.

On the advice of the forum, while they were here, I pulled that stupid plastic sleeve out of the hole that the raw water pick up hose runs through. Unbelievably, Mainship did not coat, glass or seal that intrusion at all, other than the plastic sleeve. That hole is big enough to see down into and feel around. There is a lot of wood left there but it is pretty wet.

We sounded the stringer with a hammer and drilled some exploratory holes and found mostly all dry core other than around that area.

Their advice, which I took, was to fill the void on the top of the stringer with thickened epoxy reinforced with chop strand, then wrap it in 1708. They feel the stringer is still very strong and not a safety risk, so we can use the boat.

They want me to leave the hose hole open (I will reroute the hose) with a fan on it or possibly a vacuum as suggested, for a few months to let it dry. After that the glass guy, who does after hours work at a more reasonable rate, is going to come to my slip to investigate the best way to do a bigger repair. Maybe injecting epoxy, but likely cutting out the inside skin of the stringer where needed, putting in new core, and glassing on a new skin.
 
SteveK said:
@BB-marine You must be kidding or did not read that it is 1-1/4 inch void to fill due the exposed wood end which is already surrounded by firm glass. There is no structural involved. By all means seal dried wood. we agree on this.
"Once the stringer is dried and new wood is laced in replacing the rotted wood it should sealed with an epoxy resin. The final step would be to wrap the stringer with fiberglass material replicating the original layup."
Click to expand...
The quoted comment that you posted is a procedure that is done if the stringer is found to have rot in it. I find it hard to believe that the only damage to the stringer is a 1 1/4" section. The nature of wood is it absorbs water. Capillary action ignores gravity. Most water damage found on the top of a stringer comes from the bottom of the stringer. That may not be the case in the OP's issue. It may be concentrated in the 1 1/4" area. I don't know because I did not survey the stringer. I have a picture to look at. If the area is contained to the 1 1/4" area and the absorption of water was contained in that small area then as I stated in the previous post #17. The wood needs to be completely dry then sealed with epoxy. It is rare to see a stringer rotted in a small area and the rest of the stringer dry with no migrating damage. It is possible though. The only way to know that is to core sample it or remove some of the fiberglass that is encapsulating the wood. Either one of these practices are done to inspect, dry and repair stringers. In either case this would involve some future fiberglass repair in the final steps of the repair. Not bondo. If this is done and the stringer is found to be in good condition and is adequately dried then seal the wood with epoxy resin and repair the areas where the core samples were taken or replace the fiberglass that was removed that was encapsulating the wood. If there is more damage to the wood it should be removed until there is solid wood available. At this point a new portion of plywood could be laced into the existing stringer of solid wood sealed then encapsulated with mat and resin. This is a common practice when repairing stringers.

This an my opinion of how I would approach the repair. If patching the hole is thought to be a good solution there is nothing wrong with that. It is just not approach I would take if it were my boat. We all have opinions.
Brian
 
BB-marine said:
The quoted comment that you posted is a procedure that is done if the stringer is found to have rot in it. I find it hard to believe that the only damage to the stringer is a 1 1/4" section. The nature of wood is it absorbs water. Capillary action ignores gravity. Most water damage found on the top of a stringer comes from the bottom of the stringer. That may not be the case in the OP's issue. It may be concentrated in the 1 1/4" area. I don't know because I did not survey the stringer. I have a picture to look at. If the area is contained to the 1 1/4" area and the absorption of water was contained in that small area then as I stated in the previous post #17. The wood needs to be completely dry then sealed with epoxy. It is rare to see a stringer rotted in a small area and the rest of the stringer dry with no migrating damage. It is possible though. The only way to know that is to core sample it or remove some of the fiberglass that is encapsulating the wood. Either one of these practices are done to inspect, dry and repair stringers. In either case this would involve some future fiberglass repair in the final steps of the repair. Not bondo. If this is done and the stringer is found to be in good condition and is adequately dried then seal the wood with epoxy resin and repair the areas where the core samples were taken or replace the fiberglass that was removed that was encapsulating the wood. If there is more damage to the wood it should be removed until there is solid wood available. At this point a new portion of plywood could be laced into the existing stringer of solid wood sealed then encapsulated with mat and resin. This is a common practice when repairing stringers.

This an my opinion of how I would approach the repair. If patching the hole is thought to be a good solution there is nothing wrong with that. It is just not approach I would take if it were my boat. We all have opinions.
Brian
Click to expand...
The opinion of the two glass guys who looked at it, including drilling out some test holes, is that the damage on the top of the stringer is confined to the spot that I repaired. There is also some wet core at the bottom of the stringer below and a little above, the hole that was cut through for the hose. It may very well have migrated UP to the damaged spot on top, but that doesn't appear to be the case as the stringer is dry BELOW the top damage and above the hose hole. Also, there was a crack in the skin right where the top damage was, and the skin was very thin there, it certainly could have allowed moisture ingress independently from the hose hole. Mainship notched the stringer there for some reason and did a poor job sealing it.
To be clear I 100% plan on addressing the rot around the hose hole as well, in whatever way is necessary. But in the meantime it is safe to use the boat and I can leave the hose hole open to let it dry for months if needed rather than hours if I stay in the yard. I drilled a couple of holes below it as well, to see if it drains.
This method allows me to remount the engine and get out of the yard, it buys me time to get a proper repair done when there is time and my budget recovers some. I can also hire the yards glass guy individually for half the hourly rate as I would pay the yard. He estimated 10 to 15 hours of labor for the repair.
 
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Dougcole said:
The opinion of the two glass guys who looked at it, including drilling out some test holes, is that the damage on the top of the stringer is confined to the spot that I repaired. There is also some wet core at the bottom of the stringer below and a little above, the hole that was cut through for the hose. It may very well have migrated UP to the damaged spot on top, but that doesn't appear to be the case as the stringer is dry BELOW the top damage and above the hose hole. Also, there was a crack in the skin right where the top damage was, and the skin was very thin there, it certainly could have allowed moisture ingress independently from the hose hole. Mainship notched the stringer there for some reason and did a poor job sealing it.
To be clear I 100% plan on addressing the rot around the hose hole as well, in whatever way is necessary. But in the meantime it is safe to use the boat and I can leave the hose hole open to let it dry for months if needed rather than hours if I stay in the yard. I drilled a couple of holes below it as well, to see if it drains.
This method allows me to remount the engine and get out of the yard, it buys me time to get a proper repair done when there is time and my budget recovers some. I can also hire the yards glass guy individually for half the hourly rate as I would pay the yard. He estimated 10 to 15 hours of labor for the repair.
Click to expand...
Sounds like you have a good handle on it. This sort of thing will have 30 solutions from 30 different people, all of whom haven’t laid eyes on it. If your local guys feel confident, and know you want a proper repair in the end, move ahead.
 
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