Stringer repair

[Superg]

Hello fellow boaters. I have a mid 80s 32 ft Carver. It's a twin inboard with two stringers running between an aft and midship bulkhead. Engines attach to the stringers in that region.

No signs of structural stress, but moisture levels in the stringers and midship bulkhead are about 29 pct. Drilled some exploratory holes in the vicinity of the infamous limber holes. Soft wood. Not exactly rotten, but soft. Core sample I took revealed it is plywood. As I get farther from the limber holes, the moisture readings are pretty similar, but core samples are drier and more solid. So despite the moisture readings, there are clearly differences in the actual condition at different areas. Near the engine mounts and where stringers join the bulkhead all seems solid.


So my question....


Replacing the stringers is too big and expensive a job for a 25k boat. And seems unnecessary since after 35 years the boat has no sign of structural failure. This is something that has been happening since day one, and there are a lot of these boats still in the water leading happy lives.... But I do want to improve the integrity of the stringers. Here are options I'd like to ask those of you who are experienced experts for your feedback.


Option 1. Drill and fill method.

Drill bunch of holes in the stringers and fill with epoxy.

Pros: easy and cheap

Cons: doesn't really so the job for the poorer areas

Option 2. Wood sistering.

Sister the stringers with additional marine plywood on both sides, bolt and epoxy to the original stringers.


Option 3. Steel reinforcement.

Fabricate a Pi (greek character) shaped galvanized steel frame to fit over the area where the bulkhead and two stringers join together. Bolt and epoxy through the original bulkhead and stringers.


I'm inclined to believe 3 is the best option -- for someone like me who likes improving upon the engineering of others that is . But my experience with a matter such as this is admittedly zero, so I am requesting some more experienced perspectives from this excellent forum.


Thank you, John

[Viper]

You can forget about option #1. Epoxy won't do a thing if the wood is wet, there won't be any penetration into the wood fibers for a good bond. The rest of the options only serve to point a huge red flag for the next surveyor and a potential purchaser in the future, not to mention additional weight you don't want. You're not curing the problem which will continue to deteriorate the rest of the stringers and eventually require complete replacement.

Personally, if only sections of the inboard stringers are affected, I would cut out just the inboard sides of the affected areas and leave the other side's glass shell still intact. Remove the affect core, clean the area including the inside of the glass shell you left, let dry, reinstall new ply, and glass. This is the easiest way to do it with the engines in place. As long as you mate the new section to dry existing core, you'll get a good bond. If you rabbit the joint or make an angled joint, it'll provide more surface area for a stronger bond. This is the short version of the process but it gives you an idea of what I'm talking about.

[Cooler]

Sorry to be redundant here, but here is an experience with stringers in a 1986 Carver Mariner. The vessel was owned by a friend who previously worked for Carver. He had high moisture readings in his two bottom stringers. Has experience with fiberglassing, so decided to attempt replacement of plywood inside stringers. Cut the top of first stringer so he could dig out the "wet" wood, and then planned to fill with resin. As he started to dig out the wood, he found that wood to be soaked with resin, so he could not dig out very easily. He aborted that project and simply capped off the top of the stringer with layers of rove and fiberglass. He decided he would have weakened the stringers if he figured out how to dig out what seemed like dried resin. Continued to run boat for several years with no structural issues. This experience was attested to by the guy helping him. That guy is my mechanic who I know extremely well. The next time I spoke with a current Carver employee, we talked about that situation, as my stringers also showed high moisture readings. This employee provided the following information after consulting with structural engineers at Carver. The stringers are cored with resin bathed marine plywood. The purpose of the plywood is strictly used as a base mold. Due to the function of infra-red moisture meters, a high content reading is being generated by the wood moisture only. The meters do not read resin content. Additionally, the wood base mold does not provide the strength to the stringer. The fiberglass channel is the strength source. The reason the bilge is painted grey, is so if there is any reduction of strength, the stringers will start to spider crack at the base. The grey paint allows you to see that stress very easily. I inspect my stringer bases every time I open the hatch. There is no spider cracking anywhere. I am running 5.7 Crusaders. I am not an expert by any sense of the imagination. I only know this project is a huge project, and may result in issues that are not there now. If you want, go for it. Just thought you may want to know. Call Carver for consult if you want. Good luck. Let us know how it works out. er

[Viper]

This is a point of contention between you and I I have a problem with it so I'll throw in my two cents again since you brought it up. Not being confrontational but putting it out there so folks have both sides and can make an educated decision to repair or not.

Need to clear something up first; the repair I mentioned above is not done through the top of the stringer, I have no idea how one would even think that would be doable in a proper manner. The technique is to remove the inboard side glass shell only as I mentioned above. This gives you full access to the core and a means by which you can properly install and glass in the new core, something that is impossible to successfully accomplish properly from the top of the stringer with both side shells still in place. Don't get me wrong, it's doable that way but not properly doable if you're using recommended fiberglassing techniques. Note that I said "glass in" and not simply pour resin in from the top over the core!

While a boat manufacturer might claim their core is saturated and completely impregnated with resin, my personal experience and that of every glass tech I know dictates otherwise even when a vacuum process is used. If they were completely saturated, they'd never crumble but given time we all know they do!

I'll say again that there is no way I would trust a cored stringer to be structurally supportive once the core has deteriorated, I've seen the results of that, so I don't quite understand what Carver is saying. The logic of saying the core was just a mold doesn't make sense to me either if that's the case. In an industry where pennies in production can be the difference between life and death for an OEM, if the core doesn't matter, why impregnate with resin and why use expensive marine ply when there would be much cheaper materials to use for molding purposes only? Their own production materials tells me the core serves more than just being a mold. The thickness of the outer glass skin alone wouldn't be sufficient to support heavy loads and flexing stresses IMO, hence the vast difference in glass wall thickness of the Top Hat system of stringers that are in fact designed to support heavy loads and stress with no core. Why are hollow Top Hat stringers so much thicker then if thinner walls would suffice? It just doesn't add up to me. If the core was just for the molding purposes and doesn't matter if it crumbles, then the system doesn't work because I've seen what happens when the core fails. If nothing else, you certainly couldn't hope to secure anything to the thickness of glass alone that makes up a cored stringer. Once your core failed, you better not take the boat out for a ride or truck it anywhere, assemblies and machinery would come loose from fasteners popping out of glass less than a quarter inch thick, which is exactly what happens when the core fails! So many things steering me away from that theory about the core.

I agree about interpreting your readings properly and other factors that need to be considered, that's why you take core samples for getting a true picture of what's going on. If the core is just wet that's one thing, it's still usable and structurally sound but eventually it will rot, no question about it. How long that takes and how stability is affected leading up to a failure depends on several factors and conditions. The effects of wet cores starts with glass delamination then progresses from there. While not a structural problem at first, even outer glass skin delamination can lead to problems that accelerate cored stringer failures. Tests have shown that the core condition and integrity curve will be a very slow and steady slope at first for years but will then reach a point where things start deteriorating exponentially and that curve changes drastically. I've seen this happen several times in the field. Repair it now or don't? I have a saying that goes with that; "Pay me now, or pay me MORE later"

[bud37]

Viper can I ask a question that might apply to the ops repair........now if you were to cut out 3 or 4 feet in the method you suggest would the new wood be bonded to the old stringer only and held off the hull or would you epoxy the new section to the hull and the old stringer section ?

Something I often wondered as it seems there may be some confusion how to finish it right.

[Viper]

The old bad stringer areas are removed. In simple terms, you're bonding the new stringer to the section of existing stringer that is still good, and yes the bottom is bedded/bonded onto the hull.

I should mention as I've done before, a properly equipped shop can dry the core out if it's wet but still sound. Using specialized equipment to apply a vacuum to it will boil off the moisture and remove it. You'd be surprised how well this can work. Of course it's all for not if you don't take measures to prevent water from getting in there again to begin with. Because there will likely be delamination as a result of the wet core (eventually), once dry to a targeted spec, the same system can be used to then vacuum bag the assembly and introduce resin throughout which will fill in gaps and clamp the surfaces together if you will while everything cures. This is the way a lot of boats are built these days, much less rolling out glass mat compared to before.

[tomschauer]

From experience, I totally agree with Viper. If there is wood in the stringers, it is structural. If not they could have used Styrofoam and saved big $. Its not just carver, most manufactures used and some still use wood stringers, even in high dollar sport fishers.
Its not a bad thing, wood used properly is very strong and more importantly more flexible than solid fiberglass. Which is a big plus for boats that are constantly pounded off shore.
Some older Carolina custom built sport fishers had wood stringers that were bare wood and not encapsulated in glass. Many of these boats are still going strong today, as wood can get wet and not rot, so long as it can dry out. Encapsulation, keeps the water in and accelerates the degradation of the wood.

[Viper]

Totally agree Tom. Really all you need is a means to secure a wooden stringer to the vessel/hull. This is usually done with glass tabs. Nowhere in training, through literature or instructors, was I ever able to find a logical reason that made sense for encapsulating a wood stringer in glass. There are benefits to encapsulation, perhaps the most important is that it stabilizes the wood from some of the effects of ambient air such as expansion and contraction from ever changing humidity levels, and serves to stabilize the hull. In reality though, encapsulation only works if it is complete and the shell's integrity is maintained. If you leave areas of exposed wood such as limber holes, it's technically not fully encapsulated. And simply painting the exposed wood in the limber holes with resin doesn't cut it but it's a quick fix during production. Couple that with poor equipment installation techniques throughout the vessel's life, and it's easy to see how water gets in. That's evident not only in stringers but the cores of decks, swim platforms, and hulls. Once moisture gets into the assembly, it's likely to be there for a long time in the absence of air.

I've also seen older builds utilizing solid bare timbers for stringers. The condition of the ones I've seen would lead one to believe they've just been installed even though the vessels are 50 or so years old. Antique wooden boats that I've seen be restored are usually under the knife because of exterior planking and decking. Problematic ribs and stringers while they exist are much less prevalent, even though many of them usually get wet. Where they fail is usually mating surfaces that trap moisture for extended periods.

Glad I don't have to worry about stringers. I think one of the best things Carver ever designed was their Top Hap hollow stringer system. Can't beat it IMO, one less thing to worry about.

[Superg]

Thank you all for such in-depth coverage of this topic and helpful responses.

I am certainly inclined by Viper's logic that a wood core by definition should be an active core. That said, I have also heard what Cooler has heard. As this is a big job indeed, I will try to get a response from Carver and will post it here. I am also going to have a surveyor take a look and get his opinion since he will be able to see it and knock it with a hammer first hand.

Because the damned limber holes are right next to the mid ship bulkhead, the main problem with replacing the stringer section is that I will have to recreate that joint between the stringer and the bulkhead. Also, of course, as mentioned here, once you start cutting into a stringer you never know what you may find and may cause more problems than leaving it alone. The boat does work and perform now, after all.

Regarding option 1 in my original post, I understand it may be illustrative of trying to mitigate a problem; however, there isn't a boat out there of this age to my knowledge that doesn't have the same issue. So that's why I thought Option 1 would fix it once and for all and illustrate this as an improvement to a new owner. I would certainly feel better about taking that approach rather than trying to splice in new stringer sections which probably won't function as solidly as a single original piece of timber. But this is also why I am grateful for opposing viewpoints. So thank you.

A fourth option might be an even easier solution, and that is to add fiberglass to the stringers to in fact ensure that the fiberglass is the main structural component. This essentially takes the load off the original core. Heck, it would be easy to add an additional 1/4 Inch of glass over the problem area of the stringer and tab it to the hull and bulkhead. There would still be the problem of existing moisture in the core though, which would spread over time and create new potential areas of weakness. So I think regardless, that would need to be addressed.

[bud37]

There is another angle to consider.......will you keep this boat for a while after the repair....if so what value would you put on it. It is possible you could put all sorts into this and get the same amount out. You could just use the boat up as it is then move it along when you want to move on or up..

I mean even if you find you need to repair this now, in another 5 years what is the value especially if you use up the engines in that time.....just another point of view to consider.....

I guess you will see what the surveyor has to say and that is a good move on your part....that moisture % you mentioned doesn't seem all that bad for now, but again that can be very tricky to measure accurately in the bilge.

Fibreglass Boats are funny things IMO, they are designed to flex so any repairs need to be done correctly or other issues will crop up.....good luck man and let us know what you decide.