Small rotten spot in stringer, advice needed

[BB-marine]

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.

Sounds like you have a good handle on the issue and a good plan forward. Good luck with the fuel tank install and the stringer repair.
Brian

 

[Dougcole]

Sounds like you have a good handle on the issue and a good plan forward. Good luck with the fuel tank install and the stringer repair.
Brian

Thanks Brian, I appreciate your advice, it helped me a lot. I feel pretty good about my plan now, it helps me a lot when I can take my time and think things through.

The fuel tank repair is progressing well. The new tank should be finished on Monday and ready to pick up. I will be out of town, so the yard will put it in place.
I also took the opportunity to do a bunch of work to the port motor while it is out and I can reach everything on the port side so easily; new hoses, coolant change, water pump impeller, new zincs, degrease, wire brush/sand/ospho any surface rust I could find then prime and paint. It's looking brand new.
As you can maybe see in the stringer pics, I degreased, scrubbed, sanded and painted all of the ER areas under the motor, behind/under the tank etc. Tomorrow I am going to overdrill all of the screw holes in both stringers (wire ties, support bases) and fill them with thickened epoxy, to hopefully prevent any further rot.
After the tank is back in the yard will drop the motor in, then I have to reinstall it with my mechanic buddy, then he will align it for me.
Next put the floor back together with aluminum plates for reinforcement.
Last, straighten out the wiring and the shifters etc.

Nothing to it, right?

Doug

 

[Greg QS]

I' m just seeing this post today. I have done a lot of this type of repair. Your info you are getting is correct. I would only add 1 thing. The stringer is notched. I would stay at the same angle and continue the notch to the back of your "rotted wood" . A little at a time until you remove it all. You are not weaking the stringer any more than the notch has. You could confine the notch forward up to the top of the stringer with no weakening effect. Then re wrap the top with biaxial fabrics. The wood will pull the moisture more laterally in the wood fibers more than down. The moisture follows the grain of the wood. . If you don't get the moisture or rot out it will come back again. This also exposed the top of the damaged area. It can also migrate down. With it exposed you can drill, or cut out, the rot going down and re pack with wood and epoxies then continue your repair.

 

[Greg QS]

pic modified

 

[sunchaser]

Dry rot is insidious. Inferior wood to begin with, not removing all the bad wood and schedule can defeat the best of repairs.

Which begs the question, was the notch cut out to remove known bad wood during the build process? If not, where did the water come from?

 

[garmstro55]

Sheesh Doug, you're have a rough go of it. It looks like I have the notch on both port and starboard. Is the the area you have identified? I wonder where the moisture was coming from....

 

[Dougcole]

Sheesh Doug, you're have a rough go of it. It looks like I have the notch on both port and starboard. Is the the area you have identified? I wonder where the moisture was coming from....

Thanks for posting this Gary, it's interesting and very helpful. Yes, that is the "notched" area, though my port side looks like they cut a little more out than they did on your boat.

Even more interesting is that Mainship did not cut the hole for the raw water pick up through your port stringer (be very glad about that, my friend) and instead ran the hose aft of the mount and through that notch under the engine. Perhaps that is the purpose of the deep notch on the port side but shallow on starboard.

I have been wondering where the water came from as well. My bilges are always dry and much deeper than the stringers. On the other hand, that hose hole was left completely unsealed under the plastic sleeve they slipped in there. No epoxy, no gel coat, no caulk or adhesive at all...just raw exposed core. I occasionally bring water into my engine room, I wash the bilge, degrease the motors etc. then spray them down carefully with the hose and suck up the water with a shop vac. I'm sure some water got in around that sleeve then.

So I likely got punished for keeping my engines and ER clean. Go figure.

Sorry to be "that guy" but you may want to pull out the screws holding the zip ties on to stringers, overdrill them, fill with thickened epoxy and redrill. Those screws were not sealed or bedded on my boat either.

Doug

 

[sealandsky]

Treat with this first - Then your epoxy and roving.

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[Andiamo2018]

I've worked on a handful of Mainship boats and they all had these kinds of problems. It isn't a brand I would want for more than a season of use.

Generally speaking you have to cut back at least 6 inches past the first part of solid wood - some say 18 inches.

The chances that you found the only bit of rot in the stringer (or the rest of the boat) is pretty slim. Considering your starting point doing the shorter fix makes sense in context of the rest of the boat.

Penetrating epoxy (Smith and Co) seems to really work (I have personally seen solid wood more than a decade after application) even though there are good sources online that say it doesn't work. BUT, it takes a long time to off gas and you are supposed to wait days at least before sealing up treated wood. In this application though you aren't too worried about bubbles in the paint or any of the other possible issues with trapped gasses.

Do the easy fix, make it look nice and it will be as good as any other Mainship out there.

 

[Dougcole]

I've worked on a handful of Mainship boats and they all had these kinds of problems. It isn't a brand I would want for more than a season of use.

Generally speaking you have to cut back at least 6 inches past the first part of solid wood - some say 18 inches.

The chances that you found the only bit of rot in the stringer (or the rest of the boat) is pretty slim. Considering your starting point doing the shorter fix makes sense in context of the rest of the boat.

Penetrating epoxy (Smith and Co) seems to really work (I have personally seen solid wood more than a decade after application) even though there are good sources online that say it doesn't work. BUT, it takes a long time to off gas and you are supposed to wait days at least before sealing up treated wood. In this application though you aren't too worried about bubbles in the paint or any of the other possible issues with trapped gasses.

Do the easy fix, make it look nice and it will be as good as any other Mainship out there.

That's pretty much what I did for the time being. I am going to open up the area around the hose pass through hole when I have time in a month or two to better see what I am dealing with. I drilled five or six test holes through the side of the stringer in various spots and found some dark wood but it was all dry.

If I have to, I will cut out the top of the stringer, dig out the old wood and fill it with some sort of reinforced epoxy layup. Hopefully I don't have to do that much. We will see.

I've owned this boat for 13 years, put thousands of miles on it and done a lot of work to it, mostly diy, so I know it pretty darn well. I've found two spots of rot on it, one in the subfloor of the cabin where the fridge was leaking and now this one in the stringer. I've metered the decks and all the likely spots and found no other moisture. I've also addressed all of the known problem spots on Mainship 400s by overdrilling screw holes and potting them with epoxy. I've also been diligent with any screws or additions that I made to the boat. That process has likely helped me avoid some rot that may be showing up in other Mainships.

Mainship's general quality improved quite a bit as the years went on, their 2000's boats, though not perfect, are a lot better built than earlier generations. If you have followed my posts over the years (which it does not appear that you have done) you would see that I am pretty pragmatic about Mainship's build quality. Like most manufacturers, they did some things well and some poorly. Some boats are better than others, but they all have a wart or two. It's a 20 year old boat, it's going to need some upkeep. I just helped my buddy repair some pretty extensive rot around the window frame of his 1998 Grand Banks (he has had to replace every window in the boat), wet core happens to all makes that used wood in construction.
Saying Mainship is a one season boat is pretty uninformed in my opinion, you need to look no further for proof than my boat, which I have had for 26 "seasons" now and which will make it's 16th Bahamas trip in June.

 

[BobMc]

You can easily do vacuum bagging using a shop vac rather than buying a specific vacuum pump. AND if you have a relatively flat surface for the edges of the "bag" you can use duct tape instead of any other sort of expensive mastic. Learned that trick years ago from an old pro and used it extensively when rebuilding decks and creating laminated pieces for our trawler. Just built the laminate on the garage floor. Use plastic tubing pierced every few inches with a small drill bit for the manifold.

 

[ronobrien]

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 always thought that the CSM in the 1708 was boned with styrene and had the same issue with dissolving the binder as with mat alone.

 

[ronobrien]

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.
DougView attachment 163191View attachment 163192

I am no expert so take this accordingly. I have heard of a 2 part pour-able urethane that is used as a transom filler, which I am assuming has some structural integrity. I seems that it may easier to completely fill a void.

 

[lwarden]

You can easily do vacuum bagging using a shop vac rather than buying a specific vacuum pump. AND if you have a relatively flat surface for the edges of the "bag" you can use duct tape instead of any other sort of expensive mastic. Learned that trick years ago from an old pro and used it extensively when rebuilding decks and creating laminated pieces for our trawler. Just built the laminate on the garage floor. Use plastic tubing pierced every few inches with a small drill bit for the manifold.

For the removal of moisture you need a strong vacuum and a shop vac isn't ideal. The idea here is to "boil" out the water, so reducing the boiling point by pulling a strong vacuum is the goal. This is why heating helps since it increases the vaporization. At least this is the theory....

 

[ronobrien]

For the removal of moisture you need a strong vacuum and a shop vac isn't ideal. The idea here is to "boil" out the water, so reducing the boiling point by pulling a strong vacuum is the goal. This is why heating helps since it increases the vaporization. At least this is the theory....

If my memory of high school chemistry is right, you have to achieve 28.6" of mercury vacuum at sea level and 60F.

 

[Frosty]

I always thought that the CSM in the 1708 was boned with styrene and had the same issue with dissolving the binder as with mat alone.

I've never had a problem with what I think of as "stitchmat" -- in other words 1708, 1508, etc. where the biaxial fiberglass is stitched to the mat. I use pretty much exclusively epoxy.

That said, sometimes I also use straight biax if I don't feel I need the mat; but not because the mat has any dissolving problem - just if I don't feel I need the bulk or extra resin.

 

[BB-marine]

I've worked on a handful of Mainship boats and they all had these kinds of problems. It isn't a brand I would want for more than a season of use.

I'm surprised to hear that you found most Mainship have issues with hull stringers. I know some have issues with balsa core on the decks but not stringers. The best part in my opinion of a Mainship is the hull build and design and stringers. Mainship was not bashful when it came to using fiberglass and resin.The hulls are hand laid up with solid glass and the stringers are wood laminations with a lot of resin for sealing the wood and woven mat. If you look at the photos in the OP you will see the laminated plywood stringers with a 3/16" minimum layup of rover woven glass encasing the wood stringers. When I purchased my Pilot 34 Mainship I had questions about the stringers. I was able to locate an ex-employee of Mainship (plant manager) He was a member of the Mainship owners forum. He is a marine surveyor now. This is his response to the stringers.

"The stringers are designed to need no core. It’s just a form excepted in the way for the engine bolts where it takes the compression load. Mainship did not use pressure treated plywood Silverton did. I almost guarantee the stringers have picked up some moisture the fact that some of these boats are approaching 50 years old with no stringer failures I’m aware of should give you some comfort. I think core samples would be a waste of time and money. Just one mans opinion. I worked there 1986-2007"
To further the our discussion he stated "So if memory serves the bottom is 3 layers of 24-15 fabmatt over all doubled thru the keel and if memory serves 3 additional layers in the keel so 6 + 3 layers in a hard to reach area…That is a lot of glass to roll out perfectly"

"Again the longevity of the boats out there should comfort you, these imperfect boats made with minimum wage labor have held up."

Andiamo2018 we all have opinions and I respect yours but also disagree. The Mainship Brand that has not been built for over 16 years. It is the most popular make doing the Great Loop. Some of them are known to have been around the circle 5 and 6 times and still going. AGLCA Stats for Mainship ranked #1in 2021 Mainship (13) 2022, Mainship (20), 2023 Mainship (32), 2024 Mainship (31), and there are a bunch of them out there this year 2025 too! I don't think your opinion holds up with the general public. The Mainship maybe not the best boat ever built but the most popular one for coastal cruising the great Loop. I personally have owned mine since 2021 and have well over 8000 miles under the keel and plan on possibly another 6000 miles in the next two years. The boat has performed flawlessly for us for 4 years. and the privies two owners of the boat for 19 years. That kinda throws your 1 season out the window! I do appreciate and respect your opinion I hope you do mine too!
Brian

 

[Dougcole]

I'm surprised to hear that you found most Mainship have issues with hull stringers. I know some have issues with balsa core on the decks but not stringers. The best part in my opinion of a Mainship is the hull build and design and stringers. Mainship was not bashful when it came to using fiberglass and resin.The hulls are hand laid up with solid glass and the stringers are wood laminations with a lot of resin for sealing the wood and woven mat. If you look at the photos in the OP you will see the laminated plywood stringers with a 3/16" minimum layup of rover woven glass encasing the wood stringers. When I purchased my Pilot 34 Mainship I had questions about the stringers. I was able to locate an ex-employee of Mainship (plant manager) He was a member of the Mainship owners forum. He is a marine surveyor now. This is his response to the stringers.

"The stringers are designed to need no core. It’s just a form excepted in the way for the engine bolts where it takes the compression load. Mainship did not use pressure treated plywood Silverton did. I almost guarantee the stringers have picked up some moisture the fact that some of these boats are approaching 50 years old with no stringer failures I’m aware of should give you some comfort. I think core samples would be a waste of time and money. Just one mans opinion. I worked there 1986-2007"
To further the our discussion he stated "So if memory serves the bottom is 3 layers of 24-15 fabmatt over all doubled thru the keel and if memory serves 3 additional layers in the keel so 6 + 3 layers in a hard to reach area…That is a lot of glass to roll out perfectly"

"Again the longevity of the boats out there should comfort you, these imperfect boats made with minimum wage labor have held up."

Andiamo2018 we all have opinions and I respect yours but also disagree. The Mainship Brand that has not been built for over 16 years. It is the most popular make doing the Great Loop. Some of them are known to have been around the circle 5 and 6 times and still going. AGLCA Stats for Mainship ranked #1in 2021 Mainship (13) 2022, Mainship (20), 2023 Mainship (32), 2024 Mainship (31), and there are a bunch of them out there this year 2025 too! I don't think your opinion holds up with the general public. The Mainship maybe not the best boat ever built but the most popular one for coastal cruising the great Loop. I personally have owned mine since 2021 and have well over 8000 miles under the keel and plan on possibly another 6000 miles in the next two years. The boat has performed flawlessly for us for 4 years. and the privies two owners of the boat for 19 years. That kinda throws your 1 season out the window! I do appreciate and respect your opinion I hope you do mine too!
Brian

This is fantastic information, thanks for posting it.

The concept of the stringers deriving the majority of their support from their thick fiberglass skin is an interesting one, and is something I have heard before. It makes a ton of sense in my case, as there are no visible signs of stress on my stringer (cracks etc). In theory, it would also explain, I guess, why Mainship wasn't worried about sealing the edges of the hose hole through my stringer....they didn't care about the core as it is not expected to add strength.

That said, I'm still going to dry that spot out as best as I can and see what comes of it.

 

[BB-marine]

That said, I'm still going to dry that spot out as best as I can and see what comes of it.

I agree. I would dry it out and seal it. I believe the wood does add some structural strength too.Theses quotes were from someone that worked at the plant and managed the plant. You don't want rot. The point of my post. Mainship may have not done everything perfect in the build when they were building boats but the hulls were built well and are still holding up well.
Brian

 

[Frosty]

The concept of the stringers deriving the majority of their support from their thick fiberglass skin is an interesting one, and is something I have heard before.

Just to extrapolate slightly. You can find more on this in the WEST "Fiberglass Maintenance and Boat Repair" manual which is (at least used to be) free for download. Not that this is the only place to find such information, but I mention the WEST manual because it's easy to come by and very useful. Cookbook approach to many fiberglass repair jobs.

Anyway, they refer to them as "Active core" stringers or "Inactive core" stringers. Essentially were they designed with the center either empty or with something non-structural that just serves as a mold for the glass skin that does the real work? Or were they designed with that "core" as a structural part of the stringer.

They go on to talk about Active Core materials (eg, not plywood since half the layers run counter to where you want the strength).

I imagine some builds might hover somewhere in the middle of the two concepts.

Okay, back to the flow of the thread.

 

[Andiamo2018]

I'm surprised to hear that you found most Mainship have issues with hull stringers. I know some have issues with balsa core on the decks but not stringers. The best part in my opinion of a Mainship is the hull build and design and stringers. Mainship was not bashful when it came to using fiberglass and resin.The hulls are hand laid up with solid glass and the stringers are wood laminations with a lot of resin for sealing the wood and woven mat. If you look at the photos in the OP you will see the laminated plywood stringers with a 3/16" minimum layup of rover woven glass encasing the wood stringers. When I purchased my Pilot 34 Mainship I had questions about the stringers. I was able to locate an ex-employee of Mainship (plant manager) He was a member of the Mainship owners forum. He is a marine surveyor now. This is his response to the stringers.

"The stringers are designed to need no core. It’s just a form excepted in the way for the engine bolts where it takes the compression load. Mainship did not use pressure treated plywood Silverton did. I almost guarantee the stringers have picked up some moisture the fact that some of these boats are approaching 50 years old with no stringer failures I’m aware of should give you some comfort. I think core samples would be a waste of time and money. Just one mans opinion. I worked there 1986-2007"
To further the our discussion he stated "So if memory serves the bottom is 3 layers of 24-15 fabmatt over all doubled thru the keel and if memory serves 3 additional layers in the keel so 6 + 3 layers in a hard to reach area…That is a lot of glass to roll out perfectly"

"Again the longevity of the boats out there should comfort you, these imperfect boats made with minimum wage labor have held up."

Andiamo2018 we all have opinions and I respect yours but also disagree. The Mainship Brand that has not been built for over 16 years. It is the most popular make doing the Great Loop. Some of them are known to have been around the circle 5 and 6 times and still going. AGLCA Stats for Mainship ranked #1in 2021 Mainship (13) 2022, Mainship (20), 2023 Mainship (32), 2024 Mainship (31), and there are a bunch of them out there this year 2025 too! I don't think your opinion holds up with the general public. The Mainship maybe not the best boat ever built but the most popular one for coastal cruising the great Loop. I personally have owned mine since 2021 and have well over 8000 miles under the keel and plan on possibly another 6000 miles in the next two years. The boat has performed flawlessly for us for 4 years. and the privies two owners of the boat for 19 years. That kinda throws your 1 season out the window! I do appreciate and respect your opinion I hope you do mine too!
Brian

As I said, I've worked on a handful - maybe 5 - where I had to do something with a structural item or stringer. Everyone of those times I was surprised at how little there was to hold things together. I don't know the years or even the models but the boats did look mostly nice from the outside.

In one instance the client wanted a removeable step installed and he had his own design for it which seemed fine to me. Just a cleat mounted on the side of the stringer and I can't remember what we did on the other end. It needed to be strong because it was where he would stand to lift anything into or out of the engine bay. The stringer looked fine but when I went to drill holes the drill practically fell through the stringer. What was there was damp and the stringer was hollow. I had to glass in blocks of wood to both sides of the stringer and then through bolt them together. I asked a respected project manager at a good boatyard about it and he shrugged and said 'Mainships are lightly built'. He had worked on dozens if not hundreds which was way more than the handful I worked on personally.

But I still like all the Volvo engines I've had or worked on. They are incredibly common (west coast and PNW) and parts have been expensive but they all ran well and lasted. Even so many people clearly don't like them at all. So there isn't much weight from the numbers of them out there or the general opinion.