Why only exterior grade?

[mwe-maxxowner]

I have been pondering something. You guys recommend exterior grade ply over cabinet grade because the cabinet grade does not use waterproof glue. At first thought this makes perfect sense. But then I stop to think about it. If your laminated glassed in stringers, deck, and transom have enough moisture introduced to them that you need to worry about the glue holding the plies failing, won't it just be rotted/weak in a matter of time anyway? Does this just ensure you get an extra season or two after it eventually does get wet?

I'm not arguing the recommendations, or trying to start a ruccous, just thinking out loud and maybe opening conversation. I'm using the recommended exterior grade in every step of my build!

 

[jigngrub]

Re: Why only exterior grade?

Why stop at cabinet grade if you're going to screw up your resto, why not the cheapest chip board... or maybe even particle board or fiber board?

 

[mwe-maxxowner]

Re: Why only exterior grade?

I'm using exterior grade as recommended. I'm just curious as to why, for the reason stated? If the wood gets wet enough under the glass that the plies can come apart, won't it be gone soon either way? I'm not arguing a point, but looking for a little education on it.

 

[matt167]

Re: Why only exterior grade?

polyester resin is not water proof, that along with screw holes that will penetrate the FG and limber holes, the wood under the deck will be subject to moisture. Exterior glue will take the elements, but the interior glue will not.

 

[Ned L]

Re: Why only exterior grade?

I understand what you are saying. The glue used in interior plywood will (and does) let go LONG before the wood begins to rot. Also, the fungus that causes wood to rot actually survives in only a fairly narrow range of moisture level, so wood can be soaked with water for extended periods of time with no harm (no rot) as long as it is then dried out sufficiently. Interior glue would never hold up to that.

 

[mwe-maxxowner]

Re: Why only exterior grade?

Thanks fellas!

 

[Woodonglass]

Re: Why only exterior grade?

I tried this once before with mixed results. I'll try one more time. The wood basically acts as a form for the glass. The glass in actuality carries most of the load but... since most of us only use two layers of glass the wood still plays an integral part in the overall stuctural integrity of the stringer. If you used enough glass and resin to build it up to 1/4" or more and made the stringer 3 - 4 inches wide, (some of the boats in the 50's and 60's did this) then the wood you use would not really matter. You could use Foam, (been done) Balsa or anything else. It's just the Form. Hollow stringers will work just fine IF they are bulky enuf. If not then the wood needs to be capable of carrying some of the load as well. If it gets wet due to water intrusion and the plys delaminate then the structural integrity of the plywood would be compromised and the stringer would fail. The Non Waterproof glue laminations would allow this failure to occur much sooner than the Waterproof glue. That's my Story and I'm stickin to it!!!!

 

[tpenfield]

Re: Why only exterior grade?

Since I am messing around with moisture in my boat, I thought that I would comment as well. . .

The fiberglass and/or resin encpsulation seems to act like a 'one-way' door for the moisture. It allows moisture in over time, and since the wood readily holds onto the moisture, it never seems to want to go back out.

Boats live in a moist environment, be it on the water or just outside or even in an unheated building. The moisture is higher than what 'interior' conditions would be. So, you could take any boat of moderate age and probaly find 15-20% moisture content in the wood with a direct measurement. The older it is the more moisture there probably is . . . The rot process takes a long time as well, since there is virtually no air exposure for the encapsulated wood.

Interior grade plywood would probably be delaminating much sooner than the rot from moisture can set in. So, it is not the sort of thing that would only be a year or 2 down the road, in terms of moisture leading to rot . . . it is more like 10-15 years down the road.

Although . . . it would be interesting to see if any folks on this forum have come accross a restore or repair that used interior grade plywood instead of exterior, and to see if it lasted just a few years or a longer period of time . . .

Regardless, since the 'time to rot' is more likely far geater than the 'time to absorb moisture' it is strongly desired to use exterior grade wood to maintain structural integrity.

 

[Yacht Dr.]

Re: Why only exterior grade?

If in doubt .. Marine Ply .. prep it right and you have the best.

Some other kind of ply could be suggested .. but its is a difference compaired to doing a 40+ stringer system to a little 20+ boat .. the system should be considered as well as the materials involved.

We dont ( I dont ) know what you have and would Never Recommend any materials/products until I do ..

Each build is different ...

YD.

 

[jigngrub]

Re: Why only exterior grade?

You want the best plywood you can reasonably afford for your wood reinforced fiberglass construction, the wood is where all the strength comes from... you'd be able to use cardboard if it didn't.

The exterior grade plywood is the bare minimum (cheapest) you should use.

 

[Woodonglass]

Re: Why only exterior grade?

Uhmm, if you do your research there JNG, you'd KNOW that There have been boat stingers made from CardBoard Formed Stringers. It all depends on the Shape of the Stringers and the Amount of Glass used. We've HAD this discussion. In the majority of the builds we see here on the forum with very few layers of glass and pour workmanship from the factory and 1/2 to 3/4" thick stringers then yeah the Wood plays an integral part in the structure. ( I think I already said that in my previous post) But IF the builder wanted to use Cardboard as a form and build 3" wide stringers with 3-4 layers of 18oz biaxial cloth then there would be no problems with the strength of the build. MFG boats were built kinda like that in the 50's and 60's using cardboard tubes cut in half as the forms for the stringers.

This is a good read from an Old iBoats thread.
http://forums.iboats.com/boat-restoration-building-hull-repair/what-stringers-really-do-290956.html

 

[jigngrub]

Re: Why only exterior grade?

Yeah, I've heard all of that blah blah woof woof before... but dollar for dollar you just can't beat the structural integrity that good ol' wood gives to fiberglass construction, and that's why we see so much of it on this forum.

 

[Georgesalmon]

Re: Why only exterior grade?

Pay attention to what woody means. I have built a lot of boats without any wood in the stringers. Have used foam, rope layed snakelike and glassed over for stiffness, cardboard and hollow made on a mold to fit the hull bottom. I'm not talking little bitty boats here I'm talking boats from 30' to 165'. Really, and offshore fishing fishing in the 45 to 60 range too. And hollow glass stringers that were glued down to the hull, no glassing required or needed. New not built in the 50's and 60's. (rope was back in the 60's)

Having said that, for the home restorer I would think that exterior plywood is the way to go. But following woodnglass's suggestions if you put two more layers of that 1708 everyone is using it wouldn't matter what kind of plywood you used because when the wood rotted away who cares, the strength is in the composite layup of the stringers and the wood is just a form to hold it in place untill it cures. Some of you guys with rotted wood stringers that are in good shape otherwise could just prepare the surface to accept new glass and put a few layers over the rotted wood and the boat would be safe and strong and last another 45 years. I'm surely not as good with restoration of old boats as the yacht doctor and woodnglass and many others on this site, but I have seen and had a part in building 1000's of new boats from 10' to 165' in glass and 12' to 120' in aluminum. JMHO again

 

[Yacht Dr.]

Re: Why only exterior grade?

Yeah, I've heard all of that blah blah woof woof before... but dollar for dollar you just can't beat the structural integrity that good ol' wood gives to fiberglass construction, and that's why we see so much of it on this forum.

Yep ..

Indeed you can use cardboard to make a Form for stringers. You can also use bubble gum or thick oatmeal too .. as long as the glass is thick enough for the load its gonna take on. Expect a huge cost difference though in resin and glass to replace good ol' wood.

3-4 layers of 18 oz glass for a hollow stringer supporting a 454 block is a little thin IMHO .. be careful of what your suggesting until you know the parameters of the build/application.

Yes Max .. basically the better ply will withstand water penetration longer than other plys. The wood itself is a factor. Doug Fir VS. Cedar .. or Teak VS. Pine etc. There is no such thing as "waterproof" glue btw . Just water resistant.

YD.

 

[mwe-maxxowner]

Re: Why only exterior grade?

I agree it would be crazy for the average Joe to try to form a hollow stringer. And it sure would be foolish cost wise to replace plywood with two more layers of glass and resin. Yd, would 4 layers of 1708 be sufficient on my 18' bow rider with 4.3 on the motor mounts? Quite a bit lighter than a 454

 

[jmmacky]

Re: Why only exterior grade?

Good job playing devil's advocate Max! This thread makes for an interesting read and clears up some questions I had as well.

 

[gm280]

Re: Why only exterior grade?

I love reading all these ideas on Boat rebuilds and what SHOULD and SHOULD NOT be used. I am new but not so new that I can't figure out what each person on here will say and post about their position on THIS topic. I actually see both sides (I know there are more than just two sides) of this touchy subject. So I got another logical question for more debate. IF, like WOG suggests, it seriously doesn't matter what a person uses if they cover the item with enough 1708 and resin to make it solid and that the material used is simply a FORM until those fibers and resins cure, then why wouldn't it be a whole lot better to use 1/4" or even thinner wood 1/8” and build up the transom using multiple layers for a more stronger transom substrate? If you used lauan wood and soak the resin/clothe on letting each section literally become a solid resin soaked structure/layer and then add up those layers until you reach 1 1/2" or more thickness? Same with stringers and they would never need replaced again ever... That would follow WOG’s theory. JNJ suggests using as strong ply wood as possible for your transom/stringer builds because those items gain their strength via the wood. So a quality exterior ply wood would be the material to use per his suggestions. So I actually see both sides of these logical ideas and both make some sense per their points. So is there actual a middle ground here to cover both building ideas?

 

[mwe-maxxowner]

Re: Why only exterior grade?

Good job playing devil's advocate Max! This thread makes for an interesting read and clears up some questions I had as well.

Haha that was sort of my intent. I knew I SHOULD and AM using exterior grade, I just wanted to understand it more completely. I think it boils down to, the wood will get moist, and with exterior grade it should make it a few extra seasons past where cabinet grade will. Especially in amateur or first time restorers where the glass jobs may not be spot on. Ten more years would be OK, but I would love this boat to be passed on to grandkids kids some day.

 

[mwe-maxxowner]

Re: Why only exterior grade?

I love reading all these ideas on Boat rebuilds and what SHOULD and SHOULD NOT be used. I am new but not so new that I can't figure out what each person on here will say and post about their position on THIS topic. I actually see both sides (I know there are more than just two sides) of this touchy subject. So I got another logical question for more debate. IF, like WOG suggests, it seriously doesn't matter what a person uses if they cover the item with enough 1708 and resin to make it solid and that the material used is simply a FORM until those fibers and resins cure, then why wouldn't it be a whole lot better to use 1/4" or even thinner wood 1/8” and build up the transom using multiple layers for a more stronger transom substrate? If you used lauan wood and soak the resin/clothe on letting each section literally become a solid resin soaked structure/layer and then add up those layers until you reach 1 1/2" or more thickness? Same with stringers and they would never need replaced again ever... That would follow WOG’s theory. JNJ suggests using as strong ply wood as possible for your transom/stringer builds because those items gain their strength via the wood. So a quality exterior ply wood would be the material to use per his suggestions. So I actually see both sides of these logical ideas and both make some sense per their points. So is there actual a middle ground here to cover both building ideas?

I think these options are just too expensive and time consuming for most. But possible I'm sure and would make a longer lasting boat.

 

[Georgesalmon]

Re: Why only exterior grade?

You could make your transom from multiple layers of 1708 or 2415 or any other composite layup. It would be very heavy and expensive but would last a good long time, longer than any wood cored layup.