Stringer Replacement

[SS MAYFLOAT]

Planning to replace a bad stringer in my Bayliner (imagine that). What would be the better wood to use. I was told that if I took marine plywood and laminate it to the dimensions that I need that this would be stronger than the original. I have the capability to do this, but should I do both sides to keep the continuity between the sides? The other side is in great shape. The one side is rotten due to a long time leaking problem in a holding tank that was never maintained. It was stored about 10 years outside and the holding tank was never emptied, ot if it was it was filled by rain. What about pressure treated lumber, will the fiberglass adhere to it as well as a good quality wood? The holding tank after the stringer has been replaced will be lined with new glass and will be sealed and painted with a good epoxy swimming pool paint. After about 4 coats of this, it will be about 6 ot 8 mils thick. Or is there a special holding tank coating that I should use? Thanks to all that make this website the best!

 

[joe parker]

Re: Stringer Replacement

PT lumber is pine; not a good choice for boat stringers. it cups, bows, warps, and you will never find a piece, without a big ole knot every couple of feet.Look into using douglas fir, or white oak; both are real stable species. even better, would be engineered lumber; called a micro-lam at the lumber yard. it will be the laminated plywood,that you spoke of, but in longer lengths. There are other species of dimensional lumber that are suitable; what part of the country are you in?

 

[joe parker]

Re: Stringer Replacement

duh? I just saw that you're from Ohio

 

[MIKE F]

Re: Stringer Replacement

How about trying to get some of that composite material many boat builders are now using and staying away from wood and it's problems?

 

[12Footer]

Re: Stringer Replacement

The material MikeF is talking about(starboard and Starlite) is great. They also make a foam insert matrial.The cost is higher, but the length of time it lasts for make it a good alternative.

 

[MIKE F]

Re: Stringer Replacement

12footer<br /> I was thinking of the foam type material not the starboard type stuff <br /> Do they use starboard for stringers?

 

[Valv]

Re: Stringer Replacement

I have to replace my stringers also and I was wondering if material like Plastic Lumber or others like MikeF mentioned will work. I don't see why not. They'll last longer than the owner of the boat (in my case not long ).

 

[suzukidave]

Re: Stringer Replacement

If you don't go for the plastic stuff, many boat builders up here have used kiln dried fir with multiple coats of wood preservative applied before they are glassed in (pick out tight grain pieces).

 

[mworthan]

Re: Stringer Replacement

I've just replaced the stringers , floor and transom in my 17.5 Bomber and thought long and hard about this. Remember that an exterior ply or marine grade ply (fir for example) uses a waterproof glue (Resorcinol I believe)to prevent delamination. The wood itself will rot. So using the time tested materials of boatbuilders for decades I opted for white oak, the shipbuilders favorite frame wood. I cut out the old stringers and braces, used them for templates and fashioned new ones out of oak(remember white not red). I bedded them in thickened epoxy and ran two layers of 6" 9oz. biax tape down each side to tie them to the hull. There has been some worry expressed about the compatability of epoxy to oak due the tannins in the oak, but I've never had a problem. <br /> I am as big a fan as any of space age materials, and use composites quite a bit. However, you have to remember that most boats incorporating them exclusively have been designed around there physical properties. A composite foam stringer for example relies heavily on the fabric used to transmit most of the load. It also relies on a super strong two stage epoxy and probably vacuum bagging to reach optimal resin content. You see foam core material cannot handle stress and load itself. The compression and tension loads must be transmitted to the skin and this skin must be able to handle it. It can be done but must be done properly.<br /> As far as using an engineered plastic wood, like the decking stuff sold at "Big Orange" that may prove to be a great option, but I would want to test adhesive compatibity first, and would like to see some data on strength. I have a feeling that a long board made with short fibers and resins may buckle under load like a piece of particle board, but it is intriguing.

 

[johndezman]

Re: Stringer Replacement

from what i've read about the subject, the strength comes from the glass encapsulation of whatever coring is used. the coring just occupies space and the strength comes from the lamination, just as in steel I beams the strength is from the top and bottom flanges and not the web-it's there to hold the flanges apart.<br /><br />in other words, if the fiberglas laminate is done properly, the glas shell can exist after the wood has rotted away and there would be minimal impact on strength. its when they cut corners that you have problems

 

[mworthan]

Re: Stringer Replacement

The way I understand the principles of monocoque construction, you're exactly right, but there is the problem of shear in the core itself. As you stated the core acts as the web in the I-beam analogy, but the web must remain intact across its length. Inherently a rotted out encapsulated board (stringer, 4 sides) would be stronger than a rotted out hull section that is only two sided, but now your I-Beam has turned into a piece of tubing that places the stress on the walls in line with the direction of force instead of the walls that face direction of force. Maybe there's little difference in a practical application, just have to make sure that the lamination in those two walls can handle that all by itself without rupturing (that's why I emphasized having to use a high performance fabric and resin that goes beyond simple encapsulation). I understand the rotted away core scenario, but in a honeycomb core with most of its material removed, it still transmits load via a series if walls connecting the two skins. The honeycomb works because the connection between the skins are continuous. The quality of the core material itself can interfere with this skin to skin transfer of energy. I've seen this when using polyurethane or polyethelene foam and the connection between the skin and foam stayed together but the foam had sheared away from itself right below this glue line. Now one skin is moving independently of the other and the web of the I-Beam has been effectively split down its length. This is the only reason why I (and this is just one man's humble opinion, I don't consider myself an expert) feel safer using nature's own high performance core material: wood. The shear strength of a wood core is incredible, it comes in rot resistant varieties and can be soaked on CPES or epoxy for maximum rot resistance. The quality of the core is so good that regular E-glass and standard resins can be used to basically encapsulate the piece and provide enough protection under load to prevent fracture. Plus I like the smell while I'm working it. Good luck.

 

[SS MAYFLOAT]

Re: Stringer Replacement

Well, It sure seems like alot of options. I understand about the integrity in most of the options available. This boat is a heavy one weighing in about 7800 pounds. She has 4 stringers with two on each side. The inner stringers are comprised of fiberglass over a foam core. The encapsulating glass around this core is about half inch thick. These seem to be just as good a condition as if they were new. But the two outside stringers are of the typical Bayline design. They only have the glass up the vertical sides of the stringer. With it being open on top of course lets all kinds of nasty stuff into the grain of the wood. I think I will see about using a composite core type and redo both sides. If anything it just might help me loose a little bit of weight. Thanks Skippers and Captains for your input, it is mighty helpful.