Maple and burch plywood are pretty heavey and 1/4 inch is to thick. Both those plywoods are going to delaminate if they get wet . If you glass inside and outside your looking at an extremly heavey Kaholo. 60 pounds plus. If you are in Milwaukee your right on the lakes which mean there are boat builders around. Get on the internet and start looking for boat builders, specialty lumber companies, plywood suppliers, Marine supplies. Call them on the phone and ask about 1/8 marine plywood, maranti plywood. you will find it . 

 Years back on an impulse purchase I brought home 10 sheets of a mystery hardwood plywood which resembles maple, maybe birch, possibly layers of oak or hickory. It had no knots but flawed due to a wavy finish which sands out nicely if the wrists can hold a sander long enough and an off beat thickness of 5 mm. The lumber yard here had bought it cheap for ice shacks. I cut the combing parts out of it for my CLC triple, no voids. I left the 9 remaing sheets against the basement wall and to sit through a couple of floods, damp floor and sweating water pipes dripping on them for the last 6 years. Surprise, I thought they were total garbage. No delamination, no rot, trace of mold and some nasty stains I wish were not there. I believe I read somewhere all plywood is glued with exterior glue? Only way we would have delamination is if the wood was repeatedly saturated and dried or other breakdown like rot such as what you get with exposed fir or spruce, which the fiberglass would prevent. 

   Just in addition to my last post on using off beat or crap plywood for boats :-)

I read somewhere to put samples of the plywood in a dishwaher runnig regular cycles, hot water and dry time. Can't remember how may cycles or how long specifics but something we may be able to google up more information on. If you can get hold a piece of approved marine plywood for a compairison in the dishwaher may be a fair test. 

Russ

Do your homework before using cheap plywood for a boat. I used 5.2mm Underlayment for a "Cheap Canoe". It is supposed to go underneath kitchen and bathroom floors to help level them out. It was not supposed to have no voids and use waterproof glue.   I "saved" about $100 bucks on three sheets verses Okoume. It started delaminating on its maiden voyage. It is only 2 years old and already has 5 patches on it. I quit trying to save the wood. I cut it out and replace it with mat and cloth. I have it rigged for a small sail and was thinking about taking it on our next weeklong trip, but now am not sure it would last that long, without needing repairs. Also, be carefull with the grain direction, the cheap stuff has paper thin outside plys for looks. The real plys are running the wrong way. If you need a second opinion google Mike Monies, he built a red SCAMP. It has now been sold to someone that is handy. Good Luck. JRC

   Kettle Moraine Hardwoods, Hartford and Calidonia WI.

  Check our Alpine Plywood in Milwaukee.  800-236-6324.  Okume is the best and lightest marine played but there are others that are a little heavier and marine grade.  JRC from the above post was correct in saying to be very cautious of some good looking plywood with paper thin outer layers.  You will have time and money invested so use good plywood that is the correct thickness, 3mm.  Google "Okume , (name of city that is a reasonable drive from your home)". 

This feedback is exactly what I needed to hear.  I don't feel like risking (or repairing) anything once the project is done.  Okoume it is.  I had called Alpine and a few other places in the past, but never thought to give Kettle Moraine Hardwoods a try (even though I've been there many times for other projects)  They're fairly local too.  Obviously a higher cost than buying some inexpensive birch sheets, but the piece of mind is worth it.  Thanks all!

Matt