Dimensional vs ply for stringers

[chicknwing]

The title of the thread says it all! What are your suggestions? Original stringers were ply, previous replacement was with dimensional. Pro and cons to each?

TC

 

[Squid Billy]

Re: Dimensional vs ply for stringers

Plywood is by far stronger due to all the different grains being cross glued together making it multi directional. Thats all there is to the subject, IMHO.

 

[tashasdaddy]

Re: Dimensional vs ply for stringers

totally agree.

 

[Mark42]

Re: Dimensional vs ply for stringers

I would use plywood also. I should add that I would also use pressure treated plywood. Just make sure its had time to dry out well before use. Pressure treatment leaves a lot of moisture behind that needs to evaporate before encapsulating the stringers with epoxy or polyester.

 

[Bondo Again]

Re: Dimensional vs ply for stringers

Ayuh,...... I Agree with Mark....

Dimentional lumber really doesn't have any place on a Boat.....

 

[chicknwing]

Re: Dimensional vs ply for stringers

Thanks for the replies. I did some more reading after I posted this and found the same answers you all have suggested. I got DRY 3/4 inch exterior grade ply yesterday. Thanks for the responses.

TC

 

[Robj]

Re: Dimensional vs ply for stringers

Really either will work. But I think ply is better and that is what I used in my restoration.

Have a great day,

Rob.

 

[Squid Billy]

Re: Dimensional vs ply for stringers

Dimensional lumber really doesn't have any place on a Boat.....

Well I wouldn't say that, ... it can be used as trim.

 

[Ezrider_92356]

Re: Dimensional vs ply for stringers

well i think it has more to do with the size of the boat, my boat was bilut with dimensional lumber for stringers and i decided to re-do it with dimensional. for one dementional lumber will have a little give to it so it might give a slightly better ride. another is the stringers lasted 50 years and were still pretty solid, the only thing that actually came out in one piece. all the ply that i took out broke apart when pulled on.

 

[BillP]

Re: Dimensional vs ply for stringers

I used dimensional for the center stringer and 3/4" ply for the side stringers in my Cobia. All pressure treated wood. I had the dimensional piece already or would have used all ply instead. The only thing about ply is it's smaller footprint might make stress points on the hull (I saw 1/2" ply stringers & bulkheads being used in a 24' Pursuit and they (the factory) put strips of foam under the ply to keep it from happening...and they officially call their glassed over ply stringers "composite stringers"). Ply need longer and fatter screws to grab the end grain. I used #14 screws in the ply end grain and #10s in the dimensional to screw the floor down.

bp

 

[tashasdaddy]

Re: Dimensional vs ply for stringers

food for thought, when they build structural engineered wood beams, what do they use for the core strength. plywood, with a 2x4 on top and bottom.

 

[109jb]

Re: Dimensional vs ply for stringers

You can't say plywood is stronger than dimensional because they each have their place. Plywood's greatest strength is in shear which is why it is used for things like sheeting on walls and roofs of houses. Dimensional lumber is much stronger in compression or tension loads than a piece of same dimension plywood. This is exactly why those engineered house floor beams have dimensional lumber on top and bottom. They are using each for their greatest strength. The top cap in compression, the bottom cap in tension, and the web of the beam in shear.

As for the stringers on a typical small boat, either will work fine in my opinion. I'll be using plywood when I do mine after this boating season is over.

 

[Mark42]

Re: Dimensional vs ply for stringers

food for thought, when they build structural engineered wood beams, what do they use for the core strength. plywood, with a 2x4 on top and bottom.

Tash,

Great info. I am currently in the process of bonding a 2x4 to the top and another to the bottom of my boat. Should be really strong, no?

 

[boatboy999]

Re: Dimensional vs ply for stringers

Putting 2x4's on top and bottom is a lot of weight... I used 1/2 inch marine ply for the stringers and capped the top with a 2 inch wide strip of the same gauge marine ply (to give a big bond area for the floor sheeting), all fiberglassed together. I bonded the bottom edge of the stringer to the hull on both sides with an epoxy saturated sawdust filler (to get a large wetted profile on the hull) and then overlayed the fillet with a heavy glassfiber cover. The fillet means the the glass mat is not having to go round really tight curves, which makes for much easier work). The bond area to the hull is enormous compared to the original Sea Ray manufacture and the completed project is absolutely solid. the construction I used gives the strength of I beam construction like the floor joists mentioned elsewhere (that's why I beams are used in steel construction).. without large weight penalties which solid timber tends to give. Heavy gauge doesn't necessarily bring great strength gains.

 

[Todd Peterson]

Re: Dimensional vs ply for stringers

Great info. I have to replace the beam where my mercruiser 470 motor mount bolts to. The marina said it was hollow under the fiberglass. I do not see or hear the hollow sound. They wanted to pull the engine to get at the beam. I could cut it out with the engine in place. They wanted $1,200.00 to pull the engine and replace the beam. I will pick it up on Monday and do it my self. I will use plywood for the beam and beef up the stringers on each side with plywood and fiberglass.
How many layers of fiberglass and rosin do I need?

 

[2rnd]

Re: Dimensional vs ply for stringers

You can't say plywood is stronger than dimensional because they each have their place. Plywood's greatest strength is in shear which is why it is used for things like sheeting on walls and roofs of houses. Dimensional lumber is much stronger in compression or tension loads than a piece of same dimension plywood. This is exactly why those engineered house floor beams have dimensional lumber on top and bottom. They are using each for their greatest strength. The top cap in compression, the bottom cap in tension, and the web of the beam in shear.

As for the stringers on a typical small boat, either will work fine in my opinion. I'll be using plywood when I do mine after this boating season is over.

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.

 

[Todd Peterson]

Re: Dimensional vs ply for stringers

I pulled out my beam today that the motor mount was lag bolted too tonight. It was bolted into a 2x6. It was so soggy. I could not tell the species of wood. I will put back plywood stacked on edge on the bottom of the hull and then put 2x6 material to give the lag bolts something to bit into and then fiberglass the heck out of all of it. I will replace the rotten stringers with plywood. I will use 3/4 inch plywood that will be sandwiched together and glued and fibreglasses. Then foam.

It should not move.

 

[reelfishin]

Re: Dimensional vs ply for stringers

A lot would have to do with the application or dimension needed. If the stringers had to hold screws or were fastened to other than with resin, I'd have to say whole lumber would be the ticket, for overall stress resistance and rigidity, laminated plywood would be best.
I just pulled the deck out of one boat that had no wood, never did, just hollow round fiberglass tubes made with heavy woven glass mat.
It was super rigid and will never rot.
I also had one boat built with cedar or redwood stringers, which are very rot resistant as well.

My concern would be the glue holding up in the plywood, like mentioned above, even PT isn't meant to be submerged, so in the event that the wood is exposed to water, it could be prone to failure. I think dimensional lumber might have an advantage in very wet environments. I cut up one boat, an unknown flat bottom which used half round cardboard tubes over which they laid glass mat. I don't suppose that they intended the cardboard tubes to add any strength but they did survive at least 30 years encased in glass and resin.
I've also seen stringers formed with foam cores, fiberglass batting and honeycomb paper. None were large boats but all held up well. I cut apart an early Duo 15' a few months ago, it had no stringers but the entire hull was double layered and between each layer was a foam core. The boat had a network of dimensional lumber forming the deck support, none of which was actually encased in glass. There also was no foam below the deck.

One big advantage to using plywood though would be the ability to cut and form extreme curved stringers while retaining full strength. Cutting dimensional lumber to match the curve of a hull will no doubt weaken it as it would require cutting across the grain. Plywood also doesn't tend to warp or need to be seasoned before use.

 

[BillP]

Re: Dimensional vs ply for stringers

You're slicing froghairs. Ext and pt ply has no problems with delaminating... especially when glassed in for stringers. Pt has little delamination issues when set outside in on the ground for several yrs here in Florida. I have a few pieces of pt used under my trailer tires on raw hammock land that stayed over 10yrs without doing anything but warping. The only reason I moved them was so a house could be built on the property.

 

[2rnd]

Re: Dimensional vs ply for stringers

You're slicing froghairs. Ext and pt ply has no problems with delaminating... especially when glassed in for stringers. Pt has little delamination issues when set outside in on the ground for several yrs here in Florida. I have a few pieces of pt used under my trailer tires on raw hammock land that stayed over 10yrs without doing anything but warping. The only reason I moved them was so a house could be built on the property.

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