PVC stringers?

[Billkuma]

Okay, here’s a question for you experienced folks. Can PVC “boards,” “planks,” “lumber,” available at Lowe’s or Home Depot, be used to replace stringers? It’s available in ½”, ¾”, 1” and other thicknesses, varying widths and lengths and is used for trim on new home construction and restorative work. If necessary, two pieces can be joined to double the thickness, either with adhesive or mechanically.

On the ’86 Bayliner 2150, the floor of the rear cabin, 49” x 86” +/-, was totally shot, with a couple 45” x 45”, ¾” pieces of plywood placed over the original flooring by a PO. Those pieces were resting in the middle where they met on the 54-gallon fuel tank, 0.090 gauge, 2052 aluminum, since the flooring was gone. The span between the stringers on each side of the tank is 40”, so the original ½” plywood flooring might have lasted a week after production before sagging. I will be removing the fuel tank, which was not being used, presumably due to a leak. Of four stringers, only one shows obvious damage and need for repair/replacement so far. I’m thinking about placing another stringer of PVC in the middle of where the tank was, i.e., over the keel, unless the tank is defective and I miraculously stumble across a good replacement tank. In that case, I would still need some reinforcement over the tank to deal with the 40” span between the two adjacent stringers. There is no way to negotiate a full length ¾” piece of plywood in there.

I would bed a PVC stringer in epoxy, so questions might be about epoxy degrading the PVC or vice versa, stability of the bottom of the stringer, or attachment of the floor to the top of the stringer.

Any thoughts?

 

[solar7647]

The resin isn't going to bond to PVC very well. I would also be worried if the PVC will be able to flex without failure. I would stick to a good untreated lumber. The wood soaks in the resin and creates a nice strong bond. Don't use any treated lumber for fiber glassing. The treated lumber will not soak up the resin.

 

[tpenfield]

Short answer is no . . .

I would not recommend using PVC for stringer or structural bulkhead material. As noted, the resin may not stick to it . Plus, it is heavy and not very strong for its weight. Might also be a tad expensive . . . Isn't a 1 x 6 PVC decking board about $4/foot?

If you want to avoid using wood, look into urethane foam and similar structural foams. Keep in mind that with foam, you will want to be using more fiberglass - both resin and cloth.

 

[Billkuma]

solar, good point about the resin soaking into the wood grain whereas PVC is (theoretically) nonporous. Don't know about chemical bonding.

tpenfield, yep, the material I was thinking about would be $4+/- per foot. Wasn't thinking about avoiding wood so much as wanting a good, solid substructure under the flooring as well as not wanting to go through this again. But, at my age, I don't plan on going through it again anyway...

Looks like some experimenting is in my future. Thanks to both of you for your advice.

 

[JASinIL2006]

I think PVC boards have considerably less strength than wood. I also wonder if the resin in fiberglass would chemically melt the PVC. It might be interesting to experiment with, but I certainly would build a boat around it.

 

[todhunter]

I'd stick with wood....or if you've got money, Coosa Bluewater 26.

 

[Chris1956]

Polyester resin sticks to pressure treated wood very well, as long as it is dry. I like it because it takes a long time to rot, after it gets wet, unlike soft non pressure treated wood.

Some may say that a properly built boat will not have wet wood. If so, why do we need to replace wet, rotted wood in boats after 10 years or so?

 

[KJM]

Personally I'd try it. Test some resin on a small area first to see if melts the plastic. If that seems ok, Id go for it. Those boards are made to hold people on a deck lying flat so they should be strong enough on edge with a keel underneath! Also there was no stringer there when the boat was built so the new one isn't needed for the structure of the boat. I was going to do that too but I found it was too expensive and I was getting cooza for the transom so I used it for stringers as well.

 

[Billkuma]

Personally I'd try it. Test some resin on a small area first to see if melts the plastic. If that seems ok, Id go for it. Those boards are made to hold people on a deck lying flat so they should be strong enough on edge with a keel underneath! Also there was no stringer there when the boat was built so the new one isn't needed for the structure of the boat. I was going to do that too but I found it was too expensive and I was getting cooza for the transom so I used it for stringers as well.

Yeah, I plan to test the idea. As a lab technician in a plastics lab in another life, the curiosity bug's got me. Maybe even design a matrix of the PVC boards on edge, primarily for the area vacated by the fuel tank. Here's a pic of the object of my screams and a couple of the rear "cabin". Wall on the right is bulkhead just forward of the engine, on the left is rear bulkhead of cabin proper. Outline in red is top of the fuel tank. Boxy structure in upper left is underside of first step from cockpit.

 

[tpenfield]

You should take a look at the restoration thread posted about 5 years ago (maybe more) by the member @bigdirty who had to do similar restoration work on a cruiser-style boat.

Here is a link to the thread >>> https://forums.iboats.com/threads/9...dirty-but-now-shes-clean-and-splashed.621397/

The damage probably goes a lot further than you think, so investigate more at this point and try to sketch a diagram of the structure of the boat as you go.

 

[Scott Danforth]

fake wood, AKA HDPE, AKA starboard has only 1/10 the strength of plywood, deforms with heat and nothing sticks to it.

when you use it as a deck surface, you must add more wood under it to support it.

the wood in your hull is structural. your choice is wood, or something like coosa. keep in mind Coosa is 5X the cost of wood. s 1/2" thick, 4x8 sheet is $360
your 1986 bayliner was built in the time when bayliner quality was should we say, lacking and substandard, even for the time. most likely the transom and floor started rotting in the mid 90's its surprising that your boat made it to 36 years old.

the fact that the floor rotted indicates that the stringers and transom are long gone.

I would simply build it with wood, do it right per the stickies and it will be around at least another 30 years.

at the top of the page are the stickies. open this one https://forums.iboats.com/threads/how-tos-and-other-great-information.283508/
look at links 14, 15, 18, 2, 3, 4a, and 4b. read them in their entirety. look at every picture and video.

 

[mickyryan]

someone told me recently that parker boats uses pvc board and there is a boat maker in Virginia who uses pvc. for all of their stringers will it work most likely but as much as i love pvc for certain things i would go with well sealed wood .

 

[briangcc]

If you're dead set against using wood, you might want to look into Seacast instead. It'll be pricey BUT it's designed for this application - pour in transom, possibly stringers. There have been a couple builds with the stuff - check out Archbuilder's threads for some very high end work using Seacast.

I would echo everyone else on not using PVC boards as structural components in a boat. It's not designed/spec'd for that so why take the chance with friends and family aboard your boat?

 

[briangcc]

someone told me recently that parker boats uses pvc board and there is a boat maker in Virginia who uses pvc. for all of their stringers will it work most likely but as much as i love pvc for certain things i would go with well sealed wood .

Possibly BUT according to the Dual Console manual and the warranty for new owners, it covers structural components supplied by Inland Plywood. Dunno?

Here's where I was looking: https://www.parkerboats.com/catalogs

 

[JASinIL2006]

I think in this thread people are equating PVC boards with composite plastic wood deck boards (e.g., Trex) that are often made of recycled plastic bags, sawdust and other stuff. They are not the same products.

PVC is very different from Trex, in that PVC isn't as dense/heavy, and it can be machined/tooled with normal woodworking tools. It also takes most types of paint well, which is why it is often used in trim work on houses (interior and exterior) as well as decking.

While PVC is certainly more rigid than Trex, I don't think PVC is nearly as strong as wood, and when it gets cold, PVC becomes very brittle. PVC also is much less environmentally friendly than most other building materials, both in terms of the chemical processes required to make it, as well as the fact that when it's useful life is over, it will occupy landfill space for a very long time. (PVC can be recycled, but if it's coated in fiberglass, that seems unlikely.)

I'd be interested to hear how it holds up to resin, since many solvents can melt PVC.

 

[Billkuma]

Thanks, tpenfield, Scott, micky, brian for your input and directions to other resources. Been following them, watching, and learning.

KJM, experiments are on. I used PVC clear cement to bond two pieces of 1/2" PVC sheet. After all, that cement has been used in PVC plumbing (chemical bonding) for decades. I then bonded two pieces with epoxy. This morning, I can't separate the PVC manually in either case. Unfortunately, I don't have access to the instruments to test tensile strength or shear strength any longer. But both methods provided secure bonds between PVC pieces and epoxy bonding to fiberglass should not be in question after decades of that use. I also bonded PVC with PVC cement to a scrap piece of fiberglass coated with gelcoat. As expected, it stuck but separated by forcefully peeling the gelcoated piece away from the PVC, speaking to its tensile strength, force 90° to the mated surfaces. Even in this case, shear strength was strong.

Thought about the effect of heat on warping an underlying matrix of PVC boards on the edges. The softening temperature of PVC is (nominally) 82°C/180°F and, if the inside of that cabin gets there, I'm outta here! I'll go back to planes - it's a lot cooler up there. Besides, the heaviest items I plan to store in that cabin are pillows, cushions, and PFDs, maybe a 9yo young'n temporarily on her iPad. BTW, I'm not addressing full-length stringers from stern to (or near) the bow. It's a limited application of short stringers, 48 - 50", under the floor of the aft cabin. Full-length stringers are a completely different story for PVC application and I haven't gotten there yet. In fact, haven't come across a problem there - yet. I may look at this option in the Four Winns if/when I get to it.

Don't have an aversion to using wood - worked with wood most of my life in aircraft, new home construction, repair, renovation, etc. Just looking at alternatives that don't rot, easy to work with, and sturdy and safe enough for the application. I'll keep poking, literally and figuratively.

 

[KJM]

Interesting, I'd say it will work. You are only supporting an area that had no support before so whats the worst that can happen? If the floor in the cabin sags then chaulk it up to a learning experience. No big loss.

 

[Billkuma]

Exactly. Contrary to my worst fears, the transom, cockpit floor (lower and upper), and outdrive show no signs of problems. Have been using the full swim platform for access/egress and no signs of failure. As always, I'll make sure when I get to it. Now, I'm sorta, kinda excited about the PVC possibilities for the full-length stringers on the Four Winns.

 

[kcassells]

So at the end of the day you are back to the form of the glass around the pvc. Not the structural use. I would think then the glass would need to be added on as heavier. It's kinda like hollow stringers filled with foam.
I think I would still use proper wood and the typical installation.

 

[mickyryan]

REMEMBER WITH PVC CEMENT YOU ARENT ACTUALLY GLUEING YOU ARE MELTING THE TWO PIECES INTO ONE sorry about caps but pvc glues is a misnomer as you dont want it "glued" you want to melt them into one