Picking the perfect plywood.

[Bow Hunter Bran]

I’m going to try and make this post as clear as possible but I have a lot more questions then I have answers at this point. I have designed my own flat bottom duck boat. If your familiar with duck boats it would be considered a marsh boat. Its 11’ long and a 40” beam. It will be (unless someone or someone changes my mind) a stick and glue style. I have spent around 40 man-hours on the design and research. I am having trouble choosing materials for it. There are 3 different types of plywood I am considering each have there strengths and weaknesses.
The first choice is 3mm 3 ply bs6566 okoume for the deck (cover) and 6mm 5 ply BS6566 Meranti for the sides and bottom. This appears to be the strongest choice of the three and also the heaviest and cheapest.
The second choice is 3mm 3 ply BS6566 okoume for the deck (cover) and 5mm 5 ply BS1088 okoume for the sides and bottom. This seems to be the middle of the three strong but expensive and in the middle on weight.
The Third Choice is 3mm 3 ply BS6566 okoume for the deck (cover) and 4 mm 3 ply BS6566 okume for the sides and bottom/ This is the lightest weakest and almost the same price as the first.
Now to complicate things I need to choose the epoxy and fabric to use on the boat.
Many choices here as well 6 oz poly fabric on the bottom, side, and inside. Strongest option also the heaviest.
6 oz on the bottom and top and 4 oz inside. A good way to take some lbs off.
6 oz on the bottom and top and none inside just the epoxy coat.
Or go to 4 oz all over or 4 oz on the bottom top and none inside.
There are actually more possible configurations and I have them all written out on my legal pad here but Ill spare you anymore details.

These are the issues I’m trying to address. The boat needs to be light. I set a goal of 70lbs and it’s looking hard to hit. The boat needs to be strong enough to hunt out of. I’m not a big guy (160) but I don’t want to put the hours and hours into this and have it not fit the needs.
Here are my questions. First after stitching and gluing the boat do you do a full epoxy coat and then another coat on the fabric itself after the first on dries? The reason I ask is that on the first option by the time I put the wood together glue it do the joints with tape and seal it with an epoxy coat its 70lbs. If I then go with the thinnest fabric of 4oz on just the bottom sides and top I’m now at 83lbs. I’m wondering if you could combine the first seal coat with the fabric thus cutting down the epoxy used. It appears every coat I do will be another 14lbs.
What is stronger 6mm wood with 4oz or even 6 oz cloth on the bottom or 5 mm with both 6oz on the bottom and 4oz on the top (inside in the design actually) and take it a step further what if I were to use the 4mm with 6 oz on top and bottom of the sheet. Keep in mind the 4mm is 3 ply the others are 5.

My brain keeps stumbling over what material to make the bottom out of since it will be supporting the weight inside the boat as well. Its 40” wide so I don’t want it to flex and destroy the epoxy seal. That’s where all the questions on strengths using different fabrics above are coming from. Until I can confidently select a wood for the bottom of the boat I
Can’t move forward.
If you have read this far congratulations and thanks for taking the time to. If you can office some assistance and / or advice thanks again.

[gonzo]

I think you are overcalculating the weights. 2 1/2 sheets of lightweight plywood are about 40 lbs. The glass and resin about 10lbs. That's 50lbs. The fillets for stich and glue may add 2-3 lbs.

[Bow Hunter Bran]

Its actually 3 sheets of plywood. not 2. the first design listed uses one sheet of 3mm and 2 sheets of 6mm for a total of 56 lbs then 14lbs of constants thats the initial epoxy coat (10 lbs) and the flour and other small items.
Im starting to lean to one of the lighter wood choices and an exterior coat of 3 1/2 oz or 4 oz fiber. But Im really hoping to get the thoughts of some people that have done this before.
I do plan to test the design with cheeper materials. So far the 1/10th model seems correct.

[gonzo]

I was looking at the third choice. I estimated 2 1/2 sheets considering the cutouts.

[Bow Hunter Bran]

Originally Posted by gonzo

I was looking at the third choice. I estimated 2 1/2 sheets considering the cutouts.

Ah we were talking apples and oranges.
Yes you are correct the third choice is much lighter. They go.
#1 70 lbs
#2 54 lbs
#3 46 lbs
This is wood and constants like epoxy coat for sealing, tape, flour and so on.
There is almost no scrap on the layouts after cutting. It took alot to fit it all into the two parts.

What do you feel is a good choice for the bottom. 6mm 5mm or 4mm ( this is the 3 ply).
Im thinking regardless there will be a cloth / epoxy coat on the boat now. I also can run some strips on the bottom to help it track and give it strength. The largest part of the bottom / floor is 40" wide and 72" long and is the "box" I will be sitting in. the strips I mentioned will be running under this attached with epoxy and clothed over so that will help to support the 72" span.
I do apreaciate the feedback. I stated the layout of the "cheep" luan prototype tonight.

[Ian]

I also can run some strips on the bottom to help it track and give it strength. The largest part of the bottom / floor is 40" wide and 72" long and is the "box" I will be sitting in. the strips I mentioned will be running under this attached with epoxy and clothed over so that will help to support the 72" span.
I do apreaciate the feedback. I stated the layout of the "cheep" luan prototype tonight.

Hi, have you looked into Meranti plywood? It is probably the cheapest real marine plywood (I don't count douglas fir). It occurs to me that if you are putting strips on the bottom including a couple of transverse frames fastened through into the strips will sub-divide your bottom into panels of a much smaller span--this is much stiffer. I have done this with a light weight flat-iron skiff I built (easily roof-rackable). The frames could be very low profile and on the flat with quarter-round edges so as to be comfortable to sit on.
I believe that you would not gain any abrasion resistance worth mentioning from 4 oz glass. I have often wondered whether the six oz I have used on kayaks was much on an improvement on straight epoxy. Glass on the outside does not add much to the strength of the plywood in bottom impacts. Glass on the inside does make quite a large difference to thin plywood--it will not bow inward as easily and will flex further without rupturing. I have tested this in a kayak built of 1/4 inch luan by running a white-water river when the water is too low (not recomended). Of course, if the heaviest impact you expect is from the inside, glass on the bottom will help in that case.