Dimensional vs ply for stringers

[Coors]

Re: Dimensional vs ply for stringers

Use whatever wood you want. Layer epoxy and biax, etc, over it, at least 3/8" thick.
The epoxy is is now the stringer, only worry about wood would be in engine mounts.

 

[lundnisswa]

Re: Dimensional vs ply for stringers

Just some observations.
When I took apart my severely dry rotted boat it was interesting to note that the only wood that actually stayed intact from the original manufacturer was the dimensional lumber they had used in the stringers, even though the dimensional stringers were wet they were still solid.
All of the ply wood that was used had crumbled and turned to mush, could just rip it out with little effort, much of it was vacuumed/shoveled out.
Therefore;
I chose to use dimensional lumber for the stringers, hoped to encase them well enough in epoxy and biax that it really did not matter what wood was in there.
A guy could probably use cardboard for stringers if glassed in well enough, but the key is after each use to keep the bow high, and let airflow through it to thoroughly dry after each outing.
And to never cover it when still wet!
Lund

 

[BillP]

Re: Dimensional vs ply for stringers

Bill I see you are a valued member of this forum and in no way am I meaing to disrespect you, I just happen to be in this field and would like to offer up my opinion.

Your example of plywood laying on sand is not a good example. PT plywood will fail in a forney machine much sooner once exposed to prolong moisture, that is factual lab data. The glue is very strong, as I mentioned in dry conditions the wood will fail before the glue when tested in tension. So your example of it laying on the ground, in sand no less, does not parallel to a stringer in a boat. Your ply is exposed so that it can dry out (air moving accross it) and is sitting on a surface that drains better than any other soil type (sand and silt).

I am going to assume that most boat manufacters have gone to plywood due to it's ease of use and installation. On an area like a transom there would be considerably more labor and material cost to make it from solid quality lumber than a sheet good encased in glass which seems to work just fine when done right.

Many people will go from personal experience on these types of things. If a boat mechanic haspulled out several stringers that were made from plywood and they held up good he will assume that plywood works well. This same person may have also taken apart a boat with glassed in stringers that were solid him fir that were rotten out, and will conclude that solid lumber is junk for boats. However there very possibly are other facters that were not looked at like if that glass encasing the solid lumber had a perforation in it that allowed water to enter. The dry lumber would absorb that water like a sponge and have no way to get rid of it, thus rotting out the lumber. The other stringers that he took apart may have had no such defects and remained dry and keeping the plywood in a good working condition. Thus there is always more to a senario than may appear on the surface.

A quick google search resulted with a link to a resaler for marine plywood. Most people read marine and assume wet, well check out this link and scroll down to the bottom and see what it says about the bonding glue for exterior applications.

http://www.marine-plywood.us/teak.htm


So plywood most likely will work just fine, when kept dry(encased in a quality fiberglass job), however if you have a chance that this material will get wet and you can make it from solid naturally weathering lumber (teak, ipe, bubinga, jatoba, cedar, redwood, sugar pine, cyprus). Also any lumber or plywood that will not be encompassed in glass should be painted on all sides with a quality oil based primer and paint prior to install.

Just a few more thoughts

I wouldn't say I'm a "valued member" here...but would say I have some yrs on the waterfront that I personally value collectively as useful. Regardless, I take no offense at what you or anyone else is posting. Lab testing is good and I use it often...BUT, interpreting lab data and converting to application is where I believe you are off the mark.

Yes, the pieces were able to dry out and in fact were sitting on damp ground most of their life...with numerous wet/dry cycles. Quite often in the summer months they sat in 3-6" of water for weeks at a time and the ground not dry for 2-3 months in a row. The reason I put them there was to keep the tires from bogging down in the wet soil. The same pieces are now sitting under a different boat without delamination. What does the forney machine say about that?

I'll add a little more empiracle info that should be digested by the forney machine for results...in my younger days most of the skiffs I grew up with were made of exterior ply and resorcinal glue. I've lost count but probably owned 10 or more. Some were glassed and others were just painted. Glassing only made them leak free, not longer lived. I grew up on the water so these boats were used daily and pulled on dry land at night. 2 of them (10' & 14') that I can remember lasted 15+ yrs with hard use. One rotted away from my neglect and the other I sold to a friend who kept it another 10 yrs...in the water at his dock. Rental fleets used ply boats that were kept in the water full time with antifouling paint too. Delamination was never a problem with any of them...rot was. We are talking plywood technology from the 1950s & 60s. Today's ply is light years better. In hindsight, the parts of those boats that usually rotted first were the dimensional lumber...frames, beams, etc.

Take it for whatever but if a lab test indicate ext plywood stringers will delaminate when glassed in a boat, something is severely flawed with the test. I mean like it isn't even close to being accurate.

bp

 

[109jb]

Re: Dimensional vs ply for stringers

Actually they use glulam now for the top and bottom on a TGI joist. The APA has found that the newer resins used are stronger in the engineered lumber over the dimensional. They consistently run tests that show wood failure in tension prior to adhesive failure.

I don't know alot about boat building but something that I would consider is that the glues in engineered lumber have been prone to fail with prolonged exposure to moisture. Even an A grade ply presure preated for an exterior application is not suited for what is considered prolonged exposure to weather. The ACQ pressure treating protects the lumber used in multi ply construction from rot and insect damage, but has no effect to prolong the life of the adhesive used to bond the layers of the structure together.

While I have run lab tests for lumber compression I have not compared shear so I can't say that x would be stronger than x, but my experience with lumber tells me that after some moistier exposure that 3/4 in ACQ a grade plywood would be much weaker than an initial testing right out of production would have shown. Like I said I am not a boat builder but there are good arguements for the right types of dimensional lumber in this application.

First of all, it depends what brand of engineered joist you buy as to whether you get glulam on the caps. many of the manufacturers offer both dimensional and glulam versions. Secondly, a glulam is not the same as plywood. Glulam beams generally have most of the fibers oriented in the long direction of the beam. Plywood on the other hand is composed of veneers with alternating grain directions. This is why plywood is very good in shear loads. Diensional has all of the fibers (grain) running in the long direction of the piece. I stand by what I had said before. Dimensional is stronger in compression and tension in the long direction than plywood, and plywood is stronger in shear. Moot point though as either would be plenty for smaller boat beams.

 

[Todd Peterson]

Re: Dimensional vs ply for stringers

My project is done. I used exterior grade plywood for stringers for my 22 foot Baretta with a cuddy. I ripped 1"x1" from a 2x6 green treated and screwed to the side of the stringers flush to the top for a screwing surface for the floor.

Then I mixed poly resin and painted the 1"x"1 material. I put the green treated floor in and screwed with deck screws. I protected the green treated with poly resin before I screwed in the floor. I then gave it a second coat on the top. Nice and smooth for the carpet.

Now the engine is not working...no gas in carb...gota love boats.

 

[colobiker]

Re: Dimensional vs ply for stringers

at the great risk of throwing a monkey wrench into this thread. ..

I was at the local home center yesterday.. stumbled across some trex type plastic lumbar. expensive YES!!!. Heavy YES!! but it wont rot.. ever ever never... could make awsome stringers.. with the use of waterproof glues and fillers might be easier to deal with rather than fiberglass

any feelings??

just wondering

 

[ondarvr]

Re: Dimensional vs ply for stringers

Very heavy, no strength, high in cost, for these reasons it's not a great option.

 

[Yacht Dr.]

Re: Dimensional vs ply for stringers

Hello m8..

From what I read on there website is its basically PVC..

Not a real good choice if you want bonding characteristics ..

And again what ondar says..not real strong as a stand alone product... From what I read..

Id be worried about de-lam .. However..I will check it out with the company.

YD.

 

[ondarvr]

Re: Dimensional vs ply for stringers

Trex and the PVC products are different, Trex is polyethylene, polypropylene and other recycled plastics mixed with wood fiber. The plastics used are very difficult to bond to, were never designed for outdoor use, and have almost zero structural strength, the wood is ground up scraps of junk wood that couldn't be used for anything else.

This makes it very difficult to bond anything to it, plus the wood portion can mold and rot.

The PVC boards are better, they're a little bit stronger ( makes little difference though), can be bonded to with special resins (not anything you'll find easily, but normal resin will sort of stick to it) are designed for outdoor use and are somewhat lighter because they're foamed (the inside is full of air).

The problem is wood is lighter, cheaper, adds strength, is easy to work with and resin sticks well to it.

Since the plastics have little or no strength, the laminate needs to be designed to take the entire load, which means it will cost and weigh more (add this to the high cost of the plastic and its weight).
If you make the laminate the same thickness with wood that you did with the plastic, then you don't need to worry about it rotting and turning to pulp because you weren't relying on the strength the wood added right from the start.

 

[oops!]

Re: Dimensional vs ply for stringers

hi....i just jumpen in on the last page so if someone else said this allready forgive me...

but if you are against the plywood.....(stronger than dimensional lumber when on edge)....there are other options in the industry.....such as penski board.

however the cost is high....thus making ply the logical choice....once its wrapped in glass it will last many years.

by the way.....thanks ON-DA-RIVER for the lesson in plastics....your allways a wealth of info

cheers
oops

 

[chicknwing]

Re: Dimensional vs ply for stringers

Wow, I've been gone for a few weeks and this old (over a year) thread popped up, some great information for folks who use the search feature.

TC

 

[TitanTea]

Re: Dimensional vs ply for stringers

I used dimensional lumber in my rebuild but my boat only had 1 main stringer. The original stringer and all the dimensional lumber which was 20+ years old was still intact for the most part. All the plywood was like wet cardboard. The plywood was a soggy shredded mess. No matter what wood you use if it is not protected properly from water it will go bad. So rule number 1 ..... protect the wood from moisture.

 

[Mark42]

Re: Dimensional vs ply for stringers

Over the years I have been a member here, I read lots of posts from people trying to build a better stringer. Remember the college engineer major who wanted to make the stringers from steel (like the rafters in a warehouse)? Never heard from him again. And the all the people who used Trex or PVC pipes? Never an update from them either. Then there are the people who used OSB or MDF. Oops! Wrong kind of wood. Think they got one season before it expanded like a dry sponge hitting water.

Bottom line is wood is a great material. Strong, lighter than most other choices, easy to work with, and resin sticks well. Use pressure treated lumber and it will most likely out live you.

The only viable alternative stringers (that I know of) that are time proven to work are:

1) glass over foam core. (Effectively making a fiberglass stringer, the foam being the mold)

2) full foam structure (no stringers, entire are under deck area is solid foam.)

Someday, someone will come up with a better and cost effective material. But I won't hold my breath.