Using plywood that's not marine-rated isn't a wise choice if you expect your project to last. Still, if you want to go ahead anyway I suggest buying a single sheet first then cutting a small sample - 10 x 10 cm / 4 in. square - then boiling that in water for an hour, see what happens. If it holds together for the full hour I'd think you can proceed. If just 5 minutes, probably not.

What country do you live in? Any chance you can have what you really should use shipped in from another country nearby? Or order up a pre-cut kit sold by a company that uses marine-rated stuff? There's a company in the UK that sells and ships kits of CLC designs, FyneBoats.com Perhaps they can help you?

Hi esketo.

I live in Turkey and I can't get good marine grade plywood. The stuff I can get has a stamp on it saying "Su kontraplak" which is meant to mean marine grade but its quality is well short of what I could get in the UK or USA. However, it hasn't stopped me making a Skerry, a Chesapeake 17 LT, a WD 14 and sail rig and 3 Kaholo paddle boards.

I usually use a heavier grade of fibreglass especially on the bottom of the boats as I don't have 100% faith in the plywood but also because we have stony beaches around here. I also check every cut edge to look for voids and try to inject epoxy into them if they are not too bad. 

If the fibreglass is applied properly on the boat then lower quality plywood should not be something to concrn you too much. There's a lot of things that US boatmakers wax lyrical about but are not available where I live. I'd have preferred to buy kits for instance but a kit would be too expensive so I've built everything from plans. I've learned to use what is available and do as good a job as I can. I have no idea what cab-o-sil is for instance. All of my boats are held together with  epoxy and a mix of microballoons/silica. I don't use any particul;ar brand of epoxy either, MAS epoxy isn't sold here and West Systems is way overpriced. My boats are still in good order (I built the Skerry in 2015, the others in following years) and are all well used. 

I also don't have a workshop, just using available space that I can find. I do have lots of clamps of course. :-)  It would be nice to be able to get different, exotic woods here but they cost a fortune. I can get plenty of pine. I am reassured by the fact that throughout history people have made boats with what is locally available and sometimes travelling great distance in them. Spend time doing a good job and you'll be fine.

i wanted to chime in on what Yambo was saying.   if you can't find marine grade plywood, it may be challenging...but not a show-stopper.

marine grade plywood differs from regular plywood in several respects that, while more making it more useful for building kayaks and small boats, does not make it the exclusive option for a CLC project.  the three main differences are as follows:

1) the glues used for marine-grade plywood are typicallly waterproof/hghly water resistant.    while that's a nice feature, if your boat is built properly, the wood will be encapsulated in epoxy and stay dry...so you will never test this feature.   fwiw, the failure to properly waterproof even marine grade plywood is very problematic.  and as an interesting observation, none of the wood glue typically used in a cedar strip kayak is water-proof....and we happily construct cedar strip kayaks without worrying about this.   so the key thing here is to make sure any exposed wood is properly sealed with a water-proof epoxy and that when you put the boat away after using it, it is dry (e.g., don't use your boat as a bathtub).

2 the marine grade plywood does not have voids (openings) in any of the layers and the plys are typically thinner (more plys per a given thickness) than non marine grade.    again, this is an issue that is very dependent on what you are able to find, but probably the most important attribute related to this is how well it bends relaitve to marine grade plywood.    in the end, if you can make it bend to the correct shape, you are probably going to be fine.   

3 marine grade plywood typically uses different species of wood than non-marine grade and avoids certain aditives.   the most important part of this difference will be if the epoxy bonds properly to the particular plywood you find.   the challenge on this is it may not be immediately evident that you don't have a great bond.   the good news is, if it is not a catastrophic problem (e.g., you notice it immediately) it would probably happen slowly enough that you will avoid any harm becuase you will notice a blister or other evidence of delamination while it is still relatively minor.   one of the things that should be a habit is to inspect your boat with each outing to make sure it is ready to go.   this is not a big deal, just a careful look at the outside of the hull and a good peak inside the hull with a flashlight and looking under the seat and in hatches.    even boats built with marine grade plywood can develop problems that need to be addressed.

anyway, as yambo said, don't let the inability to source the exact spec of material keep you away from joining the club.   it may be a bit more experimental than the well documented performance of the material that CLC sells....but with care, you can probably find a suitable substitute.

h

   Marine plywood is great stuff and obviously the best ply to use, but I have built a number of boats with regular ply of various types and been happy with how they held up.  I figure any exterior plywood here in the US has waterproof glue in it and these days even a lot in the interior grade stuff does as well, but your country could be different.

I'd suggest looking for something that is rated for exterior use to be sure that the glue is waterproof.  Beyond that I wouldn't worry much about it if you don't have marine grade ply available.

I built a yacht tender that was stored outdoors uncovered at the marina and when in use towed behind a sailboat and used to get to and from shore, haul provisions and so on.  It saw quite a few years of hard use and was still in good shape when I sold it before a move to another state.  I think that plywood was fir.  It was fairly inexpensive stuff.

I built a sailing dinghy from som other cheap ply that was also fine for the years I owned it.

I even built a rowing dory out of cheap luan plywood.  Even that held up for at least several years of outdoor usage and was still fine for as long as I owned it.  I chose it to build on the cheap.  I only glassed the seams, but did take care to keep the paint on it in good shape.

   In the '70s I built 18ft sail boats out of exterior grade ply wood. It worked fine and they were not encapsulated in epoxy line your Wood Duck will be, just painted.  You'll be OK just get a good epoxy fiberglass coat. 

I built an Eastport Pram out of Home Depot 1/4" oak plywood because it was a proof of concept project and because it was all I could afford at the time with a new baby.  It lasted 3 years in the PNW.  My 2nd build was with marine grade okoume and it's still going strong 2 years later.

Hello everyone, 

Thanks for all the responses. I live in Uruguay and  we have a beautiful shoreside and lots of sea but we live watching our country and not our sea. That is why there's no marine grade ply here. I appreciate all the answers and will give it a try with regular ply. I will select the best ply that can be found here taking your comments in mind. 
Thanks again for all the time that you spend answering my questions, all of your words encourage me to begin the project.