Hope Wood Sealer Opinions and More....

I've been beating around wood boat forums and there is a difference of opinions that leaves me stumped about wood.

OK, I'd say that most say you just have to use marine grade plywood, I understand that and it is expensive, but ya get what you pay for....

But some have said they build a small wood boat (under 30 feet) and it has lasted for years made from exterior ply, even CDX.

I understand that there are voids in the sheets of cheap ply and that not only it could be a structural pimple, it may crak, break, get rotten and so on.

It seems to me that with today's paints, sealants, epoxies and who knows what else, why CDX or underlayment can't be used and sealed up.

I'm not building an heirloom, just a work boat type.

I could enjoy sanding and filling. With care and maintenance (more paint) I think it would last as long as I would need a boat, some have said 15 years.

What's the deal?

 

Basically, you've hit it on the head.

Yeah, you can use CDX coated in epoxy and it will probably last, but you run a greater risk of rot due to the voids and openings in the wood. If it has to last, or has to look good, or has to meet class society rules (i.e. a boat built commercially for a customer) then BS 1088 marine plywood is the way to go. If it just has to work for a while and cost is a big issue, then you can use cheaper materials.

There is also a trade-off in the cost and amount of sealant you use. Epoxy's not cheap and cheap wood will soak up a lot of it, plus you have to be careful to coat every square inch, especially the ends.

 

Thank you....

Then I guess iy would be acceptable to build a hull using CDX for the first layer and then using better wood over it. A sharpie for example or Bolger's shanty use butt joints and another plywood layer over the bottom to build up the bottom, Seems that the interior ply could be cut slightly narrower allowing the sides to cover that layer and the last bottom layer would cover the edges of the sides. Then tape the outside edge and cover it with epoxy. Sand and paint, and paint and paint.

Is this right? It would also be on a frame, bulkheads, not S&G...?

 

The main problem with using underlayment or cheap ply is, what you get these days is not as good as it was just a few years ago. Even marine ply quality has deteriorated but not to the same degree. I used to use door ply for lightweight canoes and it was fine until about seven years ago then I started getting stuff with huge voids, unglued areas and splits in the face veneer. Baltic Birch seems to have retained its quality in most areas, although it's heavy. Whatever you buy, check it very carefully, even marine ply.

 

make sure the cheap plywood has water proof glue. Many grades of interior plywood do not use exterior glues. A simple soak test of a sample should answer that question if it can not be determined from grade specifications.

 

Regarding cheap plywood..
A lot of "far east" plywood is of exceptionally poor quality (marine stamp means nothing) typical problems are large voids and glue starved areas which can quite literally fall apart when cut. Also many of these type of boards use a filler between the face veneer and sub veneers which has little adhesive quality so the face peels off easily. These type of boards often delaminate and rot quickly. Sealed or not they are useless for boat construction.
I have sometimes bought Douglass fir ply that I have thought was impressively good for its price. It does have a tendency to be heavy, unstable and if constantly wet will rot but could perhaps be used if you find a good batch. Worth a look.
I don't think CDX board is atall suitable. Try getting a square of douglass and a square of CDX and seeing which is easier to put a hammer through.
Lastly "cheap" plywood sold at high prices does not improve!!!

 

The problems with construction grades of plywood, particularly the lowest of these grades, such as CDX is several fold. Veneer count is a real big problem. 3 veneers in a 1/2" sheet, means it's really weak and has quite limited cross grain strength (one veneer's worth). If this internal veneer has voids, checks, huge repairs, etc. (it will), the moment you try to bend it, it's just collapses along those areas. The quality of construction on these types of sheets is also a big problem. It's easy to find over lapping veneers, huge voids (read rot pockets), major surface defects, dozens of repairs and generally inferior grades of material internally. These sheets are intended to hang on studs, receive no dynamic loading and most importantly, are covered with a vapor barrier and additional sheathing, such as aluminum siding, stucco, etc., so real weather doesn't actual get at them. In fact, these sheets usual say right on the stamp or label, not for prolonged, unprotected exposure.

A workboat generally is a heavier duty version, of a similarly shaped pleasure craft. It's bottom is thicker, it's framing more substantial and on and on. Cutting corners on the hull shell materiel, is typically not the wisest location to save money. There are lots of places inside the boat where you can get away with lesser grades of plywood, but the hull shell isn't the best choice.

This said, if your hull is speced to have a 1/2" bottom (as an example). You could employ two layers of 1/4" CDX, glued together and you'll gain the strength and stiffness of a marine grade plywood bottom, possibly for less cost then a 1/2" sheet of marine plywood. I've speced this type of build on a few of my designs, as it permits you to go to the local Lowe's/Depot and buy $15 a sheet plywood, double it up and away you go. It means more work, cutting twice the number of panels you would normally, but it also permits you to stagger the seams so no leaks, no butt blocks, no Payson joints, no scarfs! Not a bad trade really.

In the end Wavewacker, you'll have to make the call, but if you want a workboat to tolerate the abuse, the average workboat receives, then consider your materials carefully. Would you install a cheap, likely to let you down engine? Why not? How is this any different then your choices for the hull shell?