I have noticed that most Airboats in the U S do not seem to have any flotation build in. Our local regulations state that you have to have 30% for inland and 60% for coastal waters, I am including 2" (50 mm) foam in my new mini, that's good for about 200% buoyancy.

This is how I installed 50 mm, 2" foam blocks in my boat and covered to form a wet deck. On my 3 m.x 1.5 m. (10 ft.x 5 ft.)
Mini it's good for 225 kg. ( 496 lb.) buoyancy.

It use to be a big thing here years ago but you don't see it much now. In my case if I sink my boat in most areas I run the motor wouldn't get wet unless of course it turned over on its side.

How much weight would this add?

The density polyurethane foam that I used come to 7.25 kg. (16 lb.)

16lbs for the whole boat?

When I complete my Airboat I will have between 4 and 5 thousand in it and a lot of work. I will run the Brazos mainly, in a half mile you may be running an inch of water and then across several holes up to 25 ft deep ,I promise you I will have flotation in my boat.
Ron
It doesnt take a lot to float a submerged hull plus sometimes a hull that is floating can save your butt.

i will be on the brazos myself, lots of rocks to play with here. flotation isnt a bad idea. it would help stiffen the floor of the boat up too. i just worry about weight since im already adding poly to the bottom.

On my big boat I had poly, but I want on this one.
I have also seen several bad experiences go down from poly tearing loose. The way I look at quick slick or those kind of coverings ,is they are basically a thin poly that I can spot patch and dont have to drill holes in my boat to apply.
Ron

The foam is mostly gas!, my boat weigh 125 kg. (275 lb.) including the motor 10 kg.(22 lb) and 5 mm. H D P E. on the bottom.

What engine is on that thing???

The only problem with the flotation is that it gets waterlogged after time. I remember in Busters build using an old hull he had to cut weight and cut into his floatations chambers. I forget the actual weight of the foam that he removed that was saturated but it was substantial on the one side.

I can't stress enough about weight on these things. A little weight here a little weight there adds up. I'm remembering seeing a guy at the landing a few years ago. It was a beautiful VW powered boat with excellent workmanship. The guy spent a hour showing me all the things he added to make it nice. A real nice control panel that folded out of the way, an electrical junction box that contained all his fuses and connections as well as other extras.

When he put it in the water he couldn't get it on plane. These things are a great one person boat but when you add a second person they have to work right up to their limits working the engine harder. Any weight you add that is not absolutely necessary takes away from the longevity and boat performance no matter what small engine package you are using. All these things are important if you want to be a happy mini airboat user.

Just a for instance, if I don't have a second person going along I leave the second seat at home. The wind that it catches when on plane makes a noticeable difference.

The engine on my boat is a Hirth f 36 (Solo 210) with a tuned exhaust and a 2.2 : 1 belt reduction, 45" x 34"
Geo Killy. wood prop, static thrust tested at 115 pounds.
Yes the foam will soak up some moisture over time so I have some 4 mm space under it and two drain plugs,
the deck plates can be screwed of for inspection or replacement.

Closed cell foam doesnt water log to bad a tilted boat trailer and plugs pulled everytime you are not using the boat pretty well stops any waterlogging .
It doesnt take much foam to float a hull that is submerged to the gunnels.
Ron

Hey Ron, I am with you! U.S. Coast Guard Compliant Marine Foam {2lb density urethane foam} will float 60 lbs. per cubic ft. It doesn't take much to save your butt not to mention the rigidity it provides. I for sure will be installing it in my boat.
Kenneth