: Is it always a bad idea to use non-marine grade ply, or do some of you think
: it will work out ok?

No, it is not ALWAYS a bad idea. Yes, it COULD work out OK, but.....

http://kudzucraft.kudzupatch.com/temp/101_0920.JPG
http://kudzucraft.kudzupatch.com/temp/101_0921.JPG

Check out these two photos. There were no obvious voids here. Cheap exterior plywood I cut out to use as a forms from a new cockpit coaming. As I said, it looked fine on the outside.

This is an extreme example, but all plywood is not the same. Marine grade is better quality. I have built a couple of boats using cheap plywood and it worked. BUT it created extra work because the voids were not visible till I cut the sheets open. I needed d something to fill those voids and that creates extra work.

Years ago I built a DK-13 tortured plywood boat and after I got the hull together the plys started to delaminate before my eyes. My boat was exploding in slow motion! I managed to save it but it took a lot of extra work.

Here is my way of deciding. How long do I want/expect the boat to last? If it is a prototype then I use cheap (not marine) woods since I don't expect it to last long. If it something I will be happy with if it lasts 2-3 years I use cheap wood.

If it something I am proud of and want to last longer, I use good wood. The real cost in the boat is not the materials. It is the hours and hours of labor. It is a false savings (to me) to save $100 or $200 in material when I am going to put a hundred or more hours labor into building the boat. Hopefully spend hundreds of hours more paddling it.

All that said, sure you can do it and it may turn out great. I am not saying don't. Just wanted you to be aware of the downsides. Many boats are built from cheap plywood. Problem is you can't judge a sheet of plywood by it's face.