Brainstorming help needed: Inflatable + polystyrene + polyurethane

Discussion in 'Boat Design' started by engineer137, Jun 4, 2018.

  1. engineer137
    Joined: Jun 2018
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    Location: Salt Lake City

    engineer137 Junior Member

    Wow! This was the most helpful post so far. Thank you very much.
  2. Skyak
    Joined: Jul 2012
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    Skyak Senior Member

    If you want to learn about about relevant hull performance search "Displacement mode hull".

    I notice you were impressed by the better connection between the rental kayak and your body -that's a critical characteristic for initial stability of a kayak.

    There are plastic foams used to build boats -made in sheets and shaped (subtractive) to use as core under glass reinforced plastic.
  3. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    The odds are, you insert any expanding foam into a flexible container, it will go out of shape by the time it sets up. And no way of rectifying the situation. Not worth it.
  4. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    I'm glad you're still interested in building your own, but your supply of aluminum would not make a man portable boat.

    As an experiment, cut a piece of 1/4 inch ply to line your inflatable with. Should save some effort by not having to drag a saggy bottom around.

    If you compare the bottom to side intersection between the inflatables and rigid, the rigids are way less round. These corners add stability and true tracking. Much less energy is used to stay upright and much less course corrections.

    There are many websites offering canoe/kayak plans. Glen-l is the first to come to mind. Following a proven design saves much frustration. I repair fiberglass boats. Labor is by far my greatest expense. Materials only account for 5% of my billables. The difference in cost for proper high quality materials that will provide decades of service, vs cheap funkydoodles which die after a season or two, is trivial. The paddle board without the hard shell would not survive one day around my family.

    There are plenty of members here who would gladly assist your effort to build a proper watercraft. However, we don't want to be associated with dangerous or haphazard constructs.

    Want to start a pool as to how many of the beads actually make it into inflatable! My bet is on less than 1%

  5. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    As an engineer you should have no problem calculating the weight of a kayak made of thick aluminum. You will need three or four people to carry it. Aluminum canoes are made of 0.040 (18 gauge) and weigh about 60 lb for a 13 footer.
  6. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell "Whatever..."

    First off, as has been said, make sure the pressure in the kayaks is up to spec, it makes all the difference.

    My brain storming ideas:
    Sell the inflatables for cash.
    Buy used kayaks that meet your needs.
    Build two skin-on-frame sit-on-tops for $75 each.
    Rent when you need them.

    But, whatever you do, don't do all the work you described above.
    You'd be making a very large mistake.
  7. engineer137
    Joined: Jun 2018
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    Location: Salt Lake City

    engineer137 Junior Member

    I really appreciate the wonderful feedback I got very everyone. I have decided not to modify the inflatables at this time. I want to do some small scale experiments first. I can get a 16oz bottle, weight it, fill it full off Styrofoam, weight it again, add a little water, shake it up, pour out the excess, then spray polyurethane foam inside the bottle. After cutting off the bottle, I would weight it again, then test how strong it is. I can submerge it underwater for a few weeks, and see how much water it retains. Stuff like that.

    I am still going to build something for the family reunion next month, but after all I've learned recently, I am leaning towards a Sawfish style paddleboard. Something with solid foam core coated with PMF.
  8. trip the light fandango
    Joined: Apr 2018
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    Location: Rhyll Phillip Island Victoria Australia

    trip the light fandango Senior Member

    I'm glad you 're thinking of something other than your foam idea,there is so much of that stuff floating around in the ocean we're kind of obliged to keep it for a while and filling a cheap old inflatable with it sounds like instant garbage after one go. The cheapest best kayak/canoe I have read on this forum was made of dacron painted with a waterproof paint /glue. A light wooden frame{could be many things] ., it packs away small and is light. This approach could work well with your use of materials that are cheap or thrown away, you can make a quality water craft this way that actually works very well. Start looking into it and you will see what I mean, I saw a rubberised canvas version in the 70s called a Klepper[?] ,it is easier now with modern materials,..make something nice,..good luck. And yes I suppose it could be made in aluminium sheet with styrofoam filled plastic bags strapped into each end, but it wont be as good unless you are very good with working ally sheet., cheers
  9. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    You can use xps for the paddleboard. Glue it up say 3" thick and form and fill any depressions the next day. It sand easy or you can rough form it with a sure form. 8' long will be a bit short, but probably adequate. If you want it longer; turn the xps on edges and build it longer with staggered joins. If you use 2" xps; you could rip it at just under 3" and get 16 rips which is plenty for a nice board.

    Use 6 oz glass and epoxy in a wrap to keep the foam from breaking.


    Takes a week. Cost is under 100 unless you buy fancy paint.

  10. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    A cored paddle board is an awesome 1st project. XPS and EPOXY and 6oz glass cloth is a durable solution. XPS isn't strong enough fot boats large enough to need an engine over ton horses. Plenty strength for paddle. XPS is not compatible with polyester or polyvinyl resins (desolves into stupid ugly goo).

    I hope your not going to use the web tutorial I just viewed on how to make sawfish kayak. It provided zero advice on hydrodynamic shaping. The lack of fun factor on the inflatable was due to its inability to maintain its marginal hull form. Sheathed XPS will maintain its carved shape. How will it achieve a higher fun factor if it is also shaped poorly? I strongly urge you to seek advice on hull furm dynamics before carving. Why spend many dozens of hours carving if the end result is unsatisfactory performance.

    The real alarm bells went off when I realized PMF stood for poor man's fiberglass.

    I am out of time for now. So I'll continue soon
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