Is this design crazy?

Discussion in 'Boat Design' started by gathem, Oct 10, 2014.

  1. gathem
    Joined: Oct 2014
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    gathem Junior Member

    I want to buy a bunch of 16' ft long X 4' X 4' blocks of EPS.
    Leave the hulls bottom square (higher drag coefficient, but more buoyancy), use a hot wire cutter to shape the stern & bows to a point.

    Paint the hulls in black latex paint, then put on several layers of fiberglass mat with polyester resin. When in doubt over engineer, so I was thinking 1/4" thick fiberglass. I view the as EPS as 1.) a form; not a structure, 2.) as a flotation in the event the hull was holed, it wouldn't sink.

    Then weld/hot dip galvenize/attach a steel deck to each of the two hulls forming a 32' catamaran.

    I do not want any through hulls, or any access to the inside of the hulls. Solid foam core, with a fiberglass shell... just like a surfboard.

    I can attach an outboard motor to the deck. I may have a fiberglass box bolted to the deck as well, to house any components required by the coastgaurd.

    I am thinking of something 32' LOA * 16' beam, with 1' draft, and a motor that is only capable of pushing it along at 4 knts.

    Most of the design decisions were based on the requirements:
    * Easy for me to single handedly build
    * Something I can afford to build.

    Please... tell me if this is crazy, and if so... how I should go about making it a little less crazy.
  2. Saqa
    Joined: Oct 2013
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    Saqa Senior Member

    I have done a 8' canoe by making polystyrene former and glassing over that. Works well. What will happen with what you describe is eventually the skin will break loose from the plug. What needs to be done is cut slots into the foam so you can fit some timber or ply stringers and frames and glass over that. It is best to use epoxy on glass over raw finish polystyrene. Works fine for small craft but I dont know about something 32'
  3. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Your latex paint and EPS foam are chemically attacked by polyester resin, making this boat out of epoxy is going to be super expensive, aside from the engineering shortcomings.
  4. gathem
    Joined: Oct 2014
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    gathem Junior Member

    Thanks Saqa and Mr E.

    Mr E: I am aware polyester resin eats foam. However, I have had success before, with using a thick coat of sprayed on Latex paint, to act as a barrier. Once the first coat of polyester dries, I was home free. I could even use a sprayed on first coat of epoxy, and then apply polyester for all subsequent glassing. Epoxy is expensive, which is why I wanted to go this route.

    The foam plug is not there to be structural. It is simply to provide a shape/make it unsinkable should it ever become holed. I can add marine plywood stringers in between blocks of foam. In fact, I can reinforce it in any way necessary to make the hull alone be the structure.

    Can you enlighten me on some of the "engineering shortcomings" so I might adjust the design?
  5. Trent hink
    Joined: Sep 2013
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    Trent hink Junior Member

    If the eps foam form is entirely covered with glass then you may need to cut vent holes to compensate for the expansion and contraction of air inside the foam.

    Then again if the eps is really only serving as a form then you might as well dissolve it with gasoline or acetone and get rid of the excess weight, not to mention the fact that lightweight eps will absorb water.
  6. gathem
    Joined: Oct 2014
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    gathem Junior Member

    I thought long and hard about disolving the EPS, but I have two main hesitations:
    1.) thats a ton of foam, and if I disolve it, its basically a ton of napalm which is dangerous, bad for the environment, and hard to safely dispose of.
    2.) Most cats need the hull space for living area/storage. I don't. I just want the hulls to be unsinkable. There are lots of submerged logs, and rocks which keep me awake at night. I will add a vent plug to the hulls.
  7. upchurchmr
    Joined: Feb 2011
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    upchurchmr Senior Member

    Fiberglass mat has almost no strength.
    Better use cloth.
    You are right there will be a ton of foam.
    Why not just use 2" thick slabs to form the shape (hollow) and avoid the cost and weight of all that foam.
    You need to get somebody to calculate if the hulls will snap in two from the weight of the deck, etc.

    This will be a very expensive experiment without some engineering.

    IMO, yes this is crazy.
  8. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    A 1/4" laminate isn't sufficient on a 32' long 16' wide, 12" draft vessel. Lets assume it's flat bottom and double ended, it's be about 18,500 pounds at 12" of immersion. The 2:1 length/beam ratio is absurd and will be grossly inefficient to say the least. If you make this a cat, you'll lose some displacement, but this is just way out of your league in terms of the scantlings, let alone the hydrodynamics involved.

    Simply put, don't quit your day job, and after you launch, stay within swimming distance of shore, as you'll likely be wet on the way back in. This technique has been done previously, but you do need a clue about what going on. Judging by your previous questions in the other thread, you seem to be looking for some simple formulas and equations to solve a host of issues, without grasping the physics, engineering and dynamics involved. It would be nice if we could plug in a few parameters into an equation and have all the questions answered, but unfortunately it doesn't work like that. It's a bit like telling a doctor you have a fever, then asking what's wrong with me.
  9. Petros
    Joined: Oct 2007
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    Petros Senior Member

    it seems to me building plywood hulls stitch and glue, and glassing them over in the conventional way would cost less and weigh less.

    buy plans from a proven design, it will save you a lot of work and get you something that will actually work well for your intened use.

    Or even better, for the cost of that EPS, and fiberglass cloth and resin, you can likely buy a ready to use used boat. There are lots of older pontoon boats that meet your description that can be had cheap, do some minor repairs, paint, etc. and you will be on your way in a week or two rather than many years from now (if ever).
  10. messabout
    Joined: Jan 2006
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    messabout Senior Member

    Petros has delivered the voice of wisdom, and experience Gathem.

  11. bpw
    Joined: May 2012
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    bpw Senior Member

    If you want a cheap boat go buy a used one, it will be much cheaper than a home built one.
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