Rotten Stringers
 

I'm new here but I have some knowledge. I build boats in the 40 -70ft range and the molds. I keep reading about rotten stringers. What ever you do, do not cut them out. The stringers are installed while the boat is still in the mold. The stringers will keep the bottom design from changing. When you remove the stringers from the hull outside of the mold, the bottom will definitely flex. The bottom shape can change and the new stringers will keep that change. Many stringers are built hallow and then foam filled. The foam is for noise reduction and has no structural strength. The way I would repair it would be to grind the stringers and floor with a 24 or 36 grit paper.You need to grind 12 inches on either side of the stringer. This is the most important part. Vacuum the dust, blow it out and wipe it with acetone. The next step is 3 layers of fiberglass. You can use a 1708, 1808, a 24 or even a 36. I would use a combination of 1708 and 1808. You glass one side of the stringer and then the other side. The over lap is on the top of the stringer. The purpose of the overlap on top is that it creates an I-beam effect. The first layer is 3 inches from the stringer. The second layer is 6 inches from the stringer. The third is 9. The third layer can be one piece that goes over the whole stringer. On some boats we will put carbon fiber on toop of the stringer and down 3 inches on the sides' Be sure to grind between layers. I would not use vinyl ester resin. Its all I use but its very difficult to grind and gums up the paper quickly. Use a ortho iso blend. It will work better for you. Also use a 925 mekp. it works better also. I hope this helps someone.

So do leave the rotten wood in the boat. I am not seeing how to replace the stringers if you do not remove them.

What he is saying is the stringer isn't the real structure. The glass in the shape of the stringer is. And he has us doubling up the thickness of the glass. Not sure if I agree ir not, but I am pretty sure the stringers in my hatteras have no wood in them at all.
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I need a little more information. Keep in mind that many stringers are built hollow. Is this a performance boat and the stringers are rotted where the engines are bolted to the stringers. Or actually any other IO boat. In these areas where the engines are 800 to 1800 hp we use a very high density composite material and fiberglass over it. In other areas we use a very soft foam core material. Tell me what your situation is and I'll come up with a doable solution that you can do yourself. Do not cut them out. There are other solutions.

The interior composite of the stringer does not make any difference. The only place it makes a difference are where the engines are mounted. This is primarily due to compression, weight and torque. I'm getting ready to do a 30 ft cat project where I'm removing the deck , cockpit, everything to take it down to a bare hull. I will absolutely not remove the stringers. I know all the problems that it would create and the tremendous amount of work it would take to fix it. All of this is my opinion of what I would do. Other people will have their opinions. I build all the fiberglass parts and assemble them on 4 million dollar boats. What do they do.

Why remove a rotted stringer if it's just a form for the fiberglass laid over it which is the actual structural piece? Fiberglass doesn't rot, wouldn't it still be providing support as designed?

Because when the appropriate core is used fr the stringers and the laminate properly adhered, it makes it act like the fiberglass is much thicker than in actually is. So you get greater strength with less weight.

I have heard of people opening up the top of the stringer, leaving the glass either side of the wood intact. Then removing the rotted wood. I take it you feel this would be preferable to removing glass and stringer right down to the hull? And is there any option better than sliding new back into gap and gassing it back closed? I assume filling with resin and chopped strands would be a little heavy? Maybe something like gas tank foam? And what would one do to the engine mount section?

What about just properly supporting the boat with straight 2 x10's under the boat at the point where the stringers are located.
This might actually remove some hook of rocker in the bottom.

I have had the same argument about boats not being properly supported and than having their stringers cut out and new ones glassed in while sitting on bunks on a trailer.

the boat needs to be supported at the keel bow to stern and lightly blocked at the Chines..

ideally you block and laser level the boat off before you cut the stringers out Than inspect the bottom with a long straight edge before you do any cutting also.

i had repaired a old parasail boat That needed new stringers in years ago that had such a huge hook in it from it sitting wrong on a trailer with rotten stringers that I did the following.

I took two 12 foot 2by 12s mated with doubled up blocks of 2by4. I made up a pair of these and placed them under the boat exactly where the stringers go inside with the braces wide enough that I could run screws through washers and into the 2by 12s below thus pulling the big hook out of the bottom.but still have room to place my stringer in.

i than mixed up a few batches of thickened epoxy and bedded the new stringers down in it.

left it to dry for a day than pulled all the screws out and did the usual lamination process for installing stringers.

EasyPeasy way to make sure the bottom is straight.

i agree with the poster. I think there is a lot of old boats out there with new stringers that are hooked or twisted.

if the deck is off when your do the repair it further exaggerates the possibility of the boat being twisted as now you have a big wet noodle .

When I redo the stringers in my 19 Baron I'm just going to cut the top 1/2 inch cap off and eat the old stringer out with a electric chain saw.

Cut new stringers and butter them up with big batch of thickened epoxy with a slow hardner and push them in the slot and clamp them.

cap them with a couple layers of 1708 and vinyl ester and call it done.

no hours of grinding out good glass only to replace it.

The owner of the $4 million dollar boat probably trades it. :)

I agree with tommy. Support abd the keel and corners of the transom. Leave the hull and the top together. What I did when I did this was cut one side of the stringers fiber glass and the top off. This will leave you one side to position the new stringer. You grind it and adhere the new stringer to it with thickend epoxy. Then wrap the exoosed side up and over the old fiberglass. The nice part of doing it this way is you have a template for where it goes and the height at which it needs to be. So when it goes back together your floor is in the correct location. But i believe you need to get the boat off the trailer and support it properly before cutting anything

Gotcha, thanks for the explanation!

you can't leave rotten/wet wood inside of the fiberglass it will start to delaminates, boat that are made with sandwiched wood in between fiberglass they need that wood, can you imagine what would happen if you take out all 3 ply woods from the transom it would be empty flexing shell.

How many boats hulls or decks are built with sandwich construction of plywood. Only cold molded boats that are epoxy saturated are built this way. The rest may have plywood as a core material in the deck not in the hull. The previous discussion was about stringers not transoms. Solid fiberglass laminates will not delaminate because of rotted wood inside a stringer. As previously stated, if the stringers are where the engines on IO boats are located the core material needs to be replaced. But you still do not cut them out completely. Remove the top of the stringers and remove the rotted core. There are numerous materials that can be used. After, glass over the stringers.

In the transom if the wood is rotted what are you to do? If you leave the rotted wood and you have outboards it will break off. The outer and inner laminates will not delaminate just because the core is rotted. What would cause the delamination. Statements are easily made if they offer no resolution to correct the problem. To fix a rotted transom without removing the outer or inner laminates, do the following. First fill all the holes inside and outside the ransom. Not completely only to seal the holes. Cut as little as possible the laminate on the top of the transom. Maybe a 1/4 to 3/8ths. Once the rotted ply is exposed take a chain saw and remove all the rotted wood. It will eat up a chain so you will probably need 2. Once you have removed the rotted wood clean it all out. Now you will replace the wood with a pourable liquid ceramic that you buy at the fiberglass supply store. It comes in 5 gallon pails and you will need all of it. You catalyze it with mekp. It pours right in and dries hard as a rock. This is what it was designed for. I've done several this way and have never had an issue. Of course this is only my opinion. But beware of opinions that have no experience or solutions.

I forgot to mention to glass over the top of the transom and finish it with gel coat or paint.

Can you explain to me how you dig down a 3.5-4' deep transom to get all the wood out with a 20" chain saw? and without either removing the deck or cutting a bog hole in it that will need to be glassed back together, faired gelled and most likely repainted? I could see your method working on junior's 18' boston whaler , but not on a 38 foot scarab.

I speaking about outboard boats. You can get a chain saw lover than 20 inches. If you want to avoid painting and fairing on a Scarab repair it from the inside. Pull the engines, drives, everything from the engine compartment. Remove the glass and then the core. Don't use plywood again. There's all different types of transom material. Rebed the first later of core into the transom with core bond. It must be hot coated first. Still relief holes to get all the air out . Add multiple layers to get to the desired thickness. Reglass and reassemble. A lot of work but not that hard to do.

The fact is that wood didn't got rotten it self or with insect, it got rotten because water got in and it can't get out, so water captured inside of the stringers, floors and transom cause delamination and this is proven of happening thru out the years, ive seen in person what captured water can do to a fiberglass.
Another fact is that most of the old boats that are built with wood and need that wood for the strength especially on the transom side.
Ive seen in person well known boat builders cut the stringers out with one side of the transom get all rot out and fix glass that has started to delaminates, hull moves nowhere as long side, bottom and transom outer shell is connected as one.
And of course I'm talking about derigged boat, you can't have heavy outboards hanging of back of your boat while you cutting the stringers in that case transom might bent.

My boat 31' with outboards and I had to replace transom, floors and 2 whole stringers, there's no way we could do what you saying with transom unless we chop off whole top pieces on the back of the boat.
this is how's been done thru out the decades, so there's no reason to reinvent hot water, we are not trying to get to the moon here.
and yes my boat has completely wood core, all around, and please stop saying that water will not delaminate the fiberglass and do some at least reading before makeing such a statement.

If you are talking about the delamibation between the core, ie plywood between the laminates I totally agree. The fiberglass layer them selves delaminating is caused by poor preparation between layers. In areas that are non structural you do not need to remove the plywood. According to the naval engineers if you have more laminates on top than the sides you create a I Beam effect. There are many boats out there that are built with hollow stringers. If you're repairing the boat why go through all the extra work when it's not necessary. We build a 61 with 1800hp man's that only has high density core in the engine room. The rest is so light it crumple if you hit it with your foot. It's the glass giving it the strength not the core. These boats are designed and engineered by Naval Architects. They are the experts. They do all the load tests. Just because it's always been done a certain way over the years, does not mean it's the best way. Technology has changed. I do it according to what the architects specify.