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Thread: How do flooded boats float, foam , air spaces ?

  1. #1

    How do flooded boats float, foam , air spaces ?

    I know a little about buoyancy and displacement , but...What does the foam in a boat do to make the boat float after it has been swamped? And that's the only time it does right ?
    A boat will only float at the surface of the water when full of water, At least that's the plan.
    Is it that the foam takes up space and not let the water completely fill the boat?
    In my mind this is the only expiation like a sealed compartment full of air, kinda.
    Other wise foam would only ADD weight to a boat.

  2. #2
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    Re: How do flooded boats float, foam , air spaces ?

    You're right, the foam does add weight to the boat. It also keeps water out of those spaces, in a general sense. The foam captures air in thousands of small cells, thus eliminating the ability for the water to push the air out of a space, if there's so much as a pinhole in that wall of that space.
    Imagine if you will, a large ship, with a double hull. Enclosed in that hull is millions of small plastic balls. If there's a swamping, or even a hull breach, those balls will stay in the hull, keeping the water from completely filling those compartments between the hull. Foam acts the same way, but on a smaller scale.

    Roger

  3. #3

    Re: How do flooded boats float, foam , air spaces ?

    I wander how a person estimates how much foam you need to make your boat float.
    .

  4. #4
    Member Bart's Avatar
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    Re: How do flooded boats float, foam , air spaces ?

    there are a couple of boat makers that use this tech. one is the Boston Whaler. they use closed cell foam, which doesn't absorb water.
    you may want to have the foam in a sealed containment of some sort, though. so water can't seep between the foam and the liner or hull. worked on a few fishing boats back in the day. the owner thought it would be a great idea to spray foam the hull. the idea worked good for a few years, but after the boat material expanding and contracting. along with the twist and flex, water and the fish slurry would seep in the cracks. talk about a rotten smell.
    but on a positive note, i googled "foam in jet ski hulls" and it opened up quite a few links, and different methods that many have used. i havent had time to look much more into it. but the link below talks about some pros and cons, so there is more than a few that have when with this idea. good luck
    as far as knowing how much foam you would need, google "foam buoyancy calculator" there are quite a few links on how you and find a good estimate on your needs.
    http://www.x-h2o.com/index.php?threa...-first.107635/
    it is kind of fun. with you asking some of these questions, because it makes me stop and take a second look at how and what some manufactures have done and where the industry may go with some of these ideas.
    what area's are you thinking of placing foam on your build?
    keep up the good work.

  5. #5
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    Re: How do flooded boats float, foam , air spaces ?

    Quote Originally Posted by Coyotes-R-Us View Post
    I wander how a person estimates how much foam you need to make your boat float.
    .
    Water weighs about 62 pounds per cubic foot or about 7.5 gallons per cubic foot..... one gallon is 231 cubic inches...
    Weigh your boat and do the math for the amount of flotation required to keep it from sinking to the bottom...

  6. #6

    Re: How do flooded boats float, foam , air spaces ?

    Quote Originally Posted by Bart View Post
    what area's are you thinking of placing foam on your build?
    keep up the good work.
    In between the cats, about 24"x 6" by 11 feet.
    And a 2 foot by 2 foot by 3 feet n each cat almost in the back
    directly in line with the motor/drive.

  7. #7

    Re: How do flooded boats float, foam , air spaces ?

    Quote Originally Posted by kent View Post
    Water weighs about 62 pounds per cubic foot or about 7.5 gallons per cubic foot..... one gallon is 231 cubic inches...
    Weigh your boat and do the math for the amount of flotation required to keep it from sinking to the bottom...
    With that and what I plan to use foam=39744 qb" of foam=172 gal water=1421 pounds ????
    With about half that would hit the water first before the cats filled with water . HUMM
    What do you think ? plenty?

  8. #8
    Member Bart's Avatar
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    Re: How do flooded boats float, foam , air spaces ?

    here are a couple of links, that might help out
    from looking at the specs on the second link they have this, measurement to go by. a 10" x 20" x 9' will float 665lbs. or one cubic foot of EPS will float 60lbs of weight. I remember working down at the city docks one summer, replacing the floating docks. there was a couple of the foam logs that we had to place under a couple of the docks. one of them shot out about 30 feet. before we could get it secured to the float.
    both of the links even encapsulate the foam in a liner similar to you plastic on a gas tank. and does custom sizes. something like that would work between the cats.
    looks like you are having to much fun.
    http://www.geofoamamerica.com/products/flotation-foam-logs/
    https://univfoam.com/products

  9. #9

    Re: How do flooded boats float, foam , air spaces ?

    I'm going to use "Pink" foam boards cut to fight , rapped in nylon water proof fabric like a pillow case closed with Velcro.

  10. #10
    Super Moderator Corky's Avatar
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    Re: How do flooded boats float, foam , air spaces ?

    Here's the other part of the equation you haven't thought about...Buoyancy center of gravity...I know this the hard way...I have foam under each seat in my small red mini jon..{foam under the front "step",middle seat and rear seat under the engine}...If it fills up with water the bow will float but the weight of the engine,prop and cage will weigh the rear down and it sets on the bottom...Now, the foam is almost enough to float the rear and if you wade out and lightly pull up on the cage it will come right off the bottom...

    Here's the catch : The engine and cage has such a high center of gravity the foam tries to roll the hull !!!..Much like trying to hold a beach ball under water...With the hull completely full of water there's nothing to stop the boat from simply rolling on it's side and just keep going until it's completely upside down...Which ever side you stand to steady the boat it's guaranteed to try and roll out the other side...I always have to pull the boat to shore as far as it will come{to keep the bow steady} and start bailing with the gunnel right at the water line...It's a pain for sure and probably the reason you don't see the foam used much in an airboat..
    You on the other hand would have an advantage because the pontoons would make for a nice,wide footprint under the engine,cage,and prop..But foam placement is going to be tricky you can't just add it where it's convenient it almost needs to be designed with flotation in mind to make it work properly...

    Aluminum .120 14' X 76" hull
    Teledyne 4a084-4 engine
    Circle S 1.69:1 reduction
    67" Whirlwind "Razor X" prop



  11. #11

    Re: How do flooded boats float, foam , air spaces ?

    HUMM , Interesting . Let me think on that.
    I'll try to get a diagram and put it up.

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