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