Noobie question - building a wooden flat bottom pontoon

Hello, first time posting here. I'm currently building my first boat, a 12'x6.5' wooden flat bottom pontoon.
I have currently built two pontoons (12'x2'x16") out of marine ply (1/2" on the bottom and 1/4" on the sides. The insides have been framed out with treated 2x4's that have been ripped in half with lots of good quality construction adhesive and screwed. The front 16" of the pontoon slopes up at a 25 degree angle but they are essentially rectangular boxes. I have not put the tops on them yet and put them in my parents pool for a while and they already float and don't really leak, just a little moisture (no pooling) after about 30 minutes in the pool.
My plan since this is my first boat and I'll never be able to see into the pontoons is to completely fill the pontoons with 2" EPS foam.. that way even if I end up with a huge hole in the boat it won't sink and it should add a little bit more rigidity since all the foam will be cut the fit perfectly.

My questions are, before I put the foam in and put the top on, how should I finish both the interior and exterior? I was told by someone a gallon of roofing tar applying liberally will give the interior a strong seal. Should I fiberglass the exterior? What steps would you do from filling countersunk screw holes to sealing and painting? I've never worked with fiberglass but have worked a bit with upholstery and am fairly decent with woodwork and urethane so feel like I could do a decent job.

I don't plan for this to be my forever boat but would like to build something solid. Basically practice for a couple years from now when I'm more experienced and know what I really want and can sink more money into it.

 

Welcome to the forum.

If you haven't used stainless or more "noble" fasteners, then they're going to corrode. You can put putty over their heads and a 'glass skin on the hull, but unless you embalm the inside and outside, in which case there's no guarantee, these fasteners are going to rust. They will not melt and die with the mentioning of moisture, but in a few years, you'll see rust stains, where the fastener heads live.

You don't need 'glass, though it can help seal up the seams. I'd recommend 6 ounce 'glass tape down each seam, covering as many screw heads as you can. It'll help seal things up and make the seams more water tight. Then just paint 'er up with good quality house paint and call it a done deal.

You can use foam if you want and many do, but used milk jugs (dozens) are probably cheaper. Just glue the lids on, stuff them in the 'toons and put the deck on. Packing peanuts work too. Just pour them in, until flush with the top of the 'toons and put the top on.

Keep it clean, dry and chase down dings, divots and repairs when necessary and it'll last pretty well. On your next build, you'll want to look into proper materials and building techniques, but for now go have fun.

 

I used outdoor decking screws.

I was thinking of using some square restaurant oil containers with screw on lids in the pontoons so I use less foam. They fit perfectly with some 1" foam on the sides.

 

I realize the design is amateur but what is wrong with the materials? Not questioning you just curious.

 

Decking screws are typically coated mild steel, which will rust (guaranteed) in time, so we avoid them (understandably). PT lumber and plywood is usually about the worst grade of material available and though suitable on the side of a tool shed, not so much on a boat you'd like to keep. For your uses it'll be fine, just not very strong for its weight and PT coatings prevent coatings (epoxy, paint, etc.) from getting a good grip on the surface.

The oil containers will do fine. You're just looking to fill up the available volume. Go for it.

 

The exterior is marine ply, the only treated wood is the 2x2" frame.
I can still pull the screws out and replace them since nothing is patched yet. For future knowledge what should I have framed the interior out of and what type of screws should I use?
I'm using this boat as a practice for building something similar that is larger a couple years down the line. Next time I would put more into the design but I saw this as a good practice for putting on a fiberglass or epoxy coating and learning some basic boat building before doing it for real with a more complicated design.

Sorry for my complete lack of knowledge and thanks for responding!

 

Marine plywood should be sealed, at least along the seams and ideally completely with epoxy and fabric. The fabric is for abrasion protection and to help waterproof the seams. If it's a learning experence, just tape the seams. The real issue is the PT lumber inside the boat. The chemicals are very corrosive to fasteners. This is why the deck screws are coated, just to protect them from the PT chemicals. Again, they work for several years, but expect some rust after a few. The usual fastener choice is stainless.

 

Your boat design sounds a lot like the boat I built. The hulls or pontoons on mine are 32" wide by 6" high with a 7" tunnel between them it's 6' wide by 12' long at the water line. I've had the boat on the Mississippi for the last 5 summers and I'm really happy with the way she has performed. The hulls are completely filled with closed cell sheet Styrofoam and there are four water tight compartments in the bows. I used triple coated deck screws for all joints that are not exposed and sealed with epoxy and stainless for any exposed fasteners. Take a look at my thread (A boat a motor and a trailer) in boat building.

 

Just read the entire thing. Looks great! How did you treat the hull?

 

Hi ALM

I used 6" wide fiberglass tape on all seams and corners of the hulls and 6 0unce fiberglass cloth on the outside of the boat. I used 4 gallons of epoxy resin to put three coats on the entire boat I didn't use any cloth on the top and inside. Thanks to Par for steering me in the right direction here as I was originally told to use polyester resin and that stuff has fumes bad enough that there's no way you could use it in a closed building in the Winter and apparently you have to put it on real thick to make it water tight. I got the first epoxy from Epoxy USA and when I added another layer of 1/2" marine plywood to the bottom of the boat I used 2 gallons from Raka. I always use 2 to 1 mix non blushing marine epoxy and I always use a fresh mixing cup for each batch. You can buy 1 quart mixing cups by the dozen on line from any automotive paint supplier pretty cheap. I don't trust pumps for mixing epoxy and I don't believe in paying the high prices for resin from West systems or other suppliers. Always check the shipping costs because this can add a lot to the cost of the Epoxy. I applied 4 coats of high gloss polyurethane to the inside of hulls before installing the foam sheets. I would recommend just using fiberglass tape and epoxy on all the corners and seams and not using cloth on the rest of the boat and just coating the rest of it with three coats of epoxy it will be much faster and easier and this will still make it water proof. The main thing is that all the plywood must be really dry so it is best if you can work in a heated building in the Winter and this is one of the advantages of building a small boat. I used Elmer's carpenters waterproof wood glue max for all tight joints that are screwed together applied with a disposable brush. The advantage is that any unused glue can be poured back into the container and it doesn't have to be mixed and supposedly it's stronger than the wood but I would only use it on tight joints. Good luck with your project and if you can post some pictures.