differnce in plywood
 

whats the diffrence in the ply you by at home depot and marine ply and if covering with fiberglss does it make a diffrence.

In simple terms, about 100 bucks and 25+ years of service life.

Composition-wise the difference will be in the quality of the wood (species and heartwood vs sapwood), the number of layers, the voids in the wood and the glues used.

My experiences have taught me when you buy plywood at Home Depot, you get what you pay for. A professionally oriented lumber yard will have much better exterior plys than what you'll find at H-D. They may cost $50 instead of $30 for ABX or ACX but they will typically be much better. If you can't budget for marine ply look for a pro lumber yard and get the better exterior ply. I have used some real CDX that was better (read durable when exposed to the elements) than HD's typical ACX. Some of Home Depot's stuff barely qualifies as sheathing...:mad:

Covering the lesser plywood in fiberglass will not cure its structural shortcomings in marine applications, although for some things it may suffice.

Ditto what McGillicuddy said.
Marine fir plywood is similar to a good quality A/C Fir plywood,but in most cases has more Plys, and there are no voids in any of them.Regular plywoods used in the construction industry will have voids in the plys that can allow water intrusion.
You will find that the Marine plwood will also be a more rigid and stronger plywood than the regular plywood.
Personally I'd spend the extra few bucks for the marine product.The last time I bought Marine plywood was about 3 years ago,and at that time I paid $78.00 for a 4x8 sheet of 3/4".
If you have any specialty plywood houses in your area they will sometimes sell it to you directly.If not any good independent lumber yard can order it for you.

Here's a picture of two pieces of 3/4" marine fir glued together.The dark line(the black dashes) in the center is so that you can see the epoxy glue joint where I glued them together for my transom.
You can also see there are no voids,and you can see that there are more plys than regular plywood.

http://i2.photobucket.com/albums/y7/easy2/IMG_1030.jpg

"Some of Home Depot's stuff barely qualifies as sheathing..."

Gilly......you are so right!!!

thanks guys i know what i gotta do to make it right .

if you're smart,you're gonna move away from the whole plywood core thing...

wood cores,these require the use of epoxy - to insure the core doens't become wet,from the pourous polyester resins...

fact: wood rots...


composites - you should look into them...

Make sure you don't use fir ply form (Used for concrete forms). The ply form at most home centers is labeled B - B Structural 1. Its good quality plywood but is treated with release oil.

How is Polyester resin any more or less pourous than Epoxy resin? Neither is "pourous" as I see it. These SeaCrafts are built primarily out of Polyester resins and are Very water tight. Please explain. Comments?

In a related thought.....

I'm planning on redoing the transom on "Katrina" in the spring and i am seriously debating the use of plywood vs. composites. So i'm very curious to see where this thread goes and interested in knowledge opinions. So far, I'm leaning to plywood due to:

1) Availablity
2) Cost (about 1/2 the cost of Coosa)
3) Reasonable lifespan (20+ years if done correctly)
4) Workability (with standard woodworking tools)
5) Holds screws better, thus basically reduces the need to thru-bolt.

I do plan to use Epoxy resin, but hear that Vinylester resin is a close second and Polyester have been successfully used too.

In a word, osmosis. Polyester resin layups will absorb water. Vinylester is better, epoxy is best. They rate in the same order on secondary bond.

Jim, if I was going to redo a transom, I would probably go with a Coosa core and vinylester or epoxy, probably epoxy because I can get it at a good price, and I'm familiar with its working characteristics. I think of it this way; if I used plywood and polyester, I'd be too old to fix it when it goes bad. If I use Coosa and epoxy, I'll be dead before it goes bad.

Dave

Plywood is fine for a transom. There are several areas where plywood is superior to the foam cores (compressive strength, ability to hold a screw, and ability to absorb resin and bond to laminate/skin). Penske/coosa is great for transoms, but plywood is proven to last for 30+ years with poly resin (even with poor installations).

Here's some home cheapo Arauco plywood acx I used in my bracket. Did my 23 transom with the same wood . . . come see me in 30 years ;) $30 sheet.

http://www.wellcraftv20.com/gallery/...195_Medium.jpg
http://www.wellcraftv20.com/gallery/...Medium_001.jpg

I'd rather use the crappiest plywood made with a thin epoxy than the best marine ply with poly. I absolutely understand why others do what they do, but thats about how I feel about it. Epoxy and dry clean plywood might as well be a piece of plastic for decades, even with just 1 thin layer of light glass.

epoxy is completley waterproof - it will adhere,no water

polyester resins are pourous - these will allow small amount of water to be absorbed,and passed through - the reason a boat's bottom is epoxy barrier coated - to assure no water is absorbed by the hull...


when bonding a glass - fiberglass to wood - polyester resins,they provide a surface bond - due to the fact,they will not fully "soak" in the wood

epoxy,with it's long kick time,and the fact it's waterproof,it provides a stronger,much more reliable bond,when glassing to wood...

epoxy has a much longer kick time,versus polyester - longe cure,stronger bond...

All i can say,is I'm completely satisfied with my restoration that was done entirely with marine ply,and ployester resin,(except for the glue up for the plywood faces).I'm fully expecting this resto to last well beyond the time I'm six feet under.
I paid very close attention to detail when I did the glass work,and I'm not the least bit worried that I will have any kind of failure with the glass or resin.
I have a close friend in the glass and resin business,who sells to several members on this site,so I could have got a good deal on any resin,be it epoxy,polyester,vinylester,dion,etc,but Honestly I didn't see the need to spend more than what's necessary.
There are thousands upon thousands of boats, from commercial lobster boats,to yachts and recreational boats layed up with polyester resin that are still functioning,and structurally strong years and years after they were built.

Rarely is it the resin system that fails,it's usually because of sloppy workmanship,or careless maintenance that causes issues.Wrapping a piece of wood of any kind with epoxy resin and cloth won't help a bit if you do a sloppy job.The wood's still going to rot if water gets to it.
I don't think that there are loads of boats on the water that rotted because of water absorbtion from the hull bottom up.No offense to anyone,but I think that although poly resins can absorb some moisture,it's blown way way out of proportion.

No doubt that epoxy resin is more waterproof,and is stronger with a better and stronger secondary bond,and is more flexible.There are other advantages as well,it has no odor,so it's nice for repairs in closed spaces,it allows for more layup time with the different hardeners.It is a stronger resin system.
I'm just not sure it's all that necessary,for general resto work,it's much more expensive.It won't dissolve the binders in mat so it cant wet it out properly.It will although wet out 1708 stitch mat as there are no binders in it.
Either way most high quality resins,(not boatyard resin),applied properly will do a good job.
I just don't think you need to be worried about polyester resin being a bad choice,because it really isn't.It's been used succesfully in loads of rebuilds.

You have to weigh out what works better for you.Epoxy is stronger but generally much more expensive.Vinylester is more water resistant than polyester, but in my opinion is to close to the price of epoxy,so I guess if money isn't an issue than go with epoxy,but absolutely don't be afraid of using polyester.They all will do a good job.

X2 . . . well said.

In a nutshell; there's more than one right way to do the job. Lots of wrong ways too, so ya gotta make sure you know what yer doin'.

Dave

X3 My thoughts exactly. My boat lasted the first 30+ years of life with Poly, and in another 30 Ill rebuild it again. Ill probably swear at the last 'hack' who rebuilt it the first time! :D

Ha . . . that's the truth. My favorite mistake so far was forgetting to add mekp to the first 2 layers of my first bracket tub and then trying to get it out of the mold. I learned so much working with Strick.

http://www.wellcraftv20.com/gallery/...7/DSCN5875.jpg

Ahhh the good ole days....now your a bracket making machine! Caught 15 bass yesterday with Lisa...... they are starting to move in !!

strick