Using cheaper construction plywood instead of Marine plywood

On a regular basis the question shows up on these forums regarding saving money by using construction grade plywood instead of the more expensive Marine Grade Plywood.
Today I cut straight line through an 18 mm sheet of construction grade and this was the result. I think this illustrates it. If you don't get it then don't build boats...go buy one from a reputable builder. The savings is no where near worth the risk of having this kind of failure hidden away in your boat.
I tried to word the title to this thread so that it can easily be found through a search. I would encourage other experienced builders that can contribute to this topic to do so. Sometimes it feels like we end up explaining some things repeatedly.

 

that piece of plywood does not meet even the lowest construction grade, it is junk and should not be used on your house either.

This is not a good example, you will find defects and out of tolerance issues with any manufactured item, be it marine plywood or alloy steel, if you look hard enough.

I have bought AC plywood and found no voids and a perfect bond, I have also found voids in marine grade plywood. It is meaningless, you have to carefully inspect every piece of wood, and every other component, before you install it. Marine grade is much better and the defects are fewer and far between, but that does not releive the builder of inspecting each piece.

 

that piece of plywood does not meet even the lowest construction grade, it is junk and should not be used on your house either.

That is rather obvious isn't it?

This is not a good example, you will find defects and out of tolerance issues with any manufactured item, be it marine plywood or alloy steel, if you look hard enough.

It actually is a good example. It was stamped as acceptable for use in construction of houses and as a whole piece showed no sign of its internal problems.

I have bought AC plywood and found no voids and a perfect bond, I have also found voids in marine grade plywood. It is meaningless, you have to carefully inspect every piece of wood, and every other component, before you install it. Marine grade is much better and the defects are fewer and far between, but that does not releive the builder of inspecting each piece.

No one said anything about being relieved of prudence in your use of materials. As to being meaningless, I find it a little curious that you can so offhandedly dismiss all the testing and regulation behind the "Marine Grade" stamp. As I pointed out, many amateurs come here for advice on choosing materials, this is just an example of why going to Home Depot to buy your material carries risk. I still would advise anyone considering saving money by using construction grade materials to go the small increase in cost to get marine grade

 

Agreed Mr Greenwood and your example is getting to be all too common with much plywood coming from unknown sources. Petros was a bit strong in his remarks but the advice to inspect all wood, plywood or lumber, is valid anyway.

 

"...small increase in cost to get marine grade."

Try up to 5 times as much per sheet when you include shipping to the middle of corn country. I paid over $100 per sheet of Joubert 6mm and I had an 8 hour round trip to go get it from the nearest place that has it. To build a knock about skiff I'll stick to the construction stuff that I pick through.

 

Once again, take that increase in cost and calculate what it represents in the overall cost of building a boat. Then take that resulting value and weigh its value in terms of importance in the structure of a boat.
Chump change is what it adds up to. Particularly if we are talking hull, deck or bulkhead material. And especially if we are talking about someone who is new to the process and may not be particularly good at selecting materials or optimizing their strengths.

And for fair comparison you have to compare species to species. Comparing top grade construction fir to top grade marine fir would be a more fair comparison. That is not to say that getting marine grade out to where you live is cheap!

 

Im suffering thru a bad plywood decision now. Couple years back I needed to build a set of dockside boarding steps from ply and epoxy fillets.
18mm ply...the marine stuff was expensive, so I cut corners and used 18mm common exterior grade.
Only one corner is filleted on these steps. The fillet held strong..so strong that the ply has split apart, just like the ply in your photo.

The whole structure is now a wobbly piece of junk.

I wont make that mistake again
I wont make that mistake again
I wont make that mistake again
I wont make that mistake again.........

 

100 is too much. Shop elsewhere.

Marine Meranti BS6566 is available for 45 per sheet.

 

In Iowa, including shipping?

 

Like I said... 3 times that with shipping. Other than where I got my last stuff... I am 500+ miles from the nearest place that has marine grade plywood.

 

In traditional boat building, the planking stock is usually some of the finest found on the yacht, which to experienced builders is obvious. Why this logic seems to be falling from favor, is beyond me.

 

Correct me if I'm wrong, but I think the only difference between marine and construction ply is the glue used in the manufacturing process. Faults as shown in post #1 can happen to both, a serious source will apologize, take it back and replace it free of charge, also if it is "only" construction grade.

Such large faults can often be seen already before a sheet is cut. In fact, the store where I can buy plywood doesn't have a single flawless sheet in stock, but I found an alternative. It is impregnated under high pressure, one side almost black and shiny, the other side has a fine line or dot pattern.
The primary application seems to be reusable concrete molds, it is about 30% more expensive but survives outdoor use without any paint.

 

The number and quality of the plys is the difference between marine and exterior construction grade.
For structural application...boats that will operate at speed with slamming loads.... structural marine ply is the way to go.
If you are a wood aficionado, you may have the eye and have access to higher quality non marine ply.

I dont...what is available locally is poor quality with thin face veneers and obvious flaws in the interior plys. .

 

There are several differences between construction and marine grades, if you look at other than APA stock.

 

Just throwing this into the pot;

What if someone found a place to buy the individual ply's , and glued them together yourself with maybe marine epoxy, and put weights on top to compress while it cured?