Thanks guys, this has been quite educational.

I'll be doing a new engine hatch myself this winter. I WILL spend the money for marine plywood. A) Marine plywood uses waterproof glue unlike outdoor plywood which uses water resistant glue. B) Marine plywood generally has more ply's than pressure treated making it dimensionally more stable. C) Marine plywood is laminated from "premium" veneers so that there are no voids and, once again, premium venners are dimensionally more stable. D) As was mentioned, encapsulation is a good idea, but WILL NOT eliminate moisture from entering the wood and when moisture enters, it may never dry out promoting dry rot. Non encapsulated marine plywood will, based upon my experience, last 5 to 7 years with reasonable care.

So, do yourself a favor, call your nearest specialty (they sell hardwoods typically) lumber yard and get Marine Plywood.

BTW - No barn yard or domesticated animals were harmed in the creation of this response, however, if this response is directly responsible for the cutting of very pretty fir trees, mahogany, teak, or other rain forest inhabitants that result in the creation of wooden boats, go hug a pine tree in someone elses backyard.

Marine grade plywood is the only way to go. Forget the Thompson's Water Sealer. It is crap. As for the wood, you could get away with half inch and fabricate ribs using the same material. The resin and cloth route will give added strength and weight, which is why I would go with half inch plywood. I have done this job and it worked out well for me. As for the cost, it is not a lot of money. I know people that have paid twice as much building 5 Liter Hydros. Just my $80.00- $79.98.......

Mike If the big stores do not have the wood you need call Jefferson County Lumber in Imperial and they will be able to help you. Forrest

Are you any good at fiberglassing? If so, I would suggest using 1" Balsa Core off a quickie mold. That is if the hatch is fairly flat and it won't take much too make a mold for it. I made these off a flat mold I made with pre-finished panels from the HOMER...

Topside...

These hatches were about 10 feet long. So you could only imagine how heavy they would have been if I didn't use Balsa...

Sharkey's right I did the same withair lite foam core for the race boat save the weight if you can!!! It will last longer just because it is lighter!!

Sharkey, nice work!

The wood product you want to use is called boat panel. It is treated plywood designed for boat building and carries a 40 year guarantee.