Concrete boat thread

Discussion in 'Fiberglass and Composite Boat Building' started by dsigned, Apr 13, 2018.

  1. dsigned
    Joined: Aug 2017
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    Location: United States

    dsigned O.R.C. Hunter

    I'm starting this as I've actually begun building the mold for my little boat, having sourced concrete ingredients that I would consider "close enough" for a proof of concept.

    The boat will be what I'm calling a "tunnel duck": which is intended to be something akin to a poor man's laser vortex. I considered doing the "$600" boat, but I wanted to start as simply as possible, and perhaps more importantly, I like the ethos of the puddle duck community.

    My current favorite from my test panels is a 2:1 vermiculite to mortar ratio by volume with chopped fiberglass. I will probably make an updated version of this that includes glass powder (replacing maybe 15% cementitious material), fiberglass mesh (instead of chopped) and pva fiber (although I would have to buy this, and it would be easily the most expensive part of the project). After being fully cured, it's very light (almost, but not quite, lighter than seawater), but still very hard and strong in the 1/4 inch thickness. I have two kinds of water reducer, which will hopefully help offset the vermiculite's water absorption.



    I'm making a female mold out of purple polystyrene foam. It's crazy easy to work with, and a nice balance of being stiff and being flexible. The plan is to seal the inside of the mold to make it water tight, and use expanding foam to make filets with a radius of 1-2" to avoid any hard corners. I will be making some cold joints, which isn't ideal, but I expect will probably work well enough for my purposes. I expect most parts of the hull to be about 1/4" thick, and am hoping for a hull that weighs about 150 lbs, which is roughly the same as a normal puddle duck from plywood.
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  2. SamSam
    Joined: Feb 2005
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    SamSam Senior Member

    Refresh my memory please, what is the concept you are trying to prove? I remember inexpensive and easy one off construction but I'm sure there was more to it.
     
  3. SamSam
    Joined: Feb 2005
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    SamSam Senior Member

    This guy (he's a member of this forum) had one or two threads on thin, super strong concrete for boats...
    mustafaumu sarac
     
  4. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Dsigned, A PDR need not not weigh 150 pounds. Few, if any, of them weigh that much unless ballasted.
     
  5. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I know of a few PRD's that are around 100 pounds (naked).
     
  6. dsigned
    Joined: Aug 2017
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    dsigned O.R.C. Hunter

    I think the biggest thing is just to demonstrate the viability of modern concrete mixes as compared to classical ferro. It will be low cost, but for a boat this size the cost of materials for almost anything is small enough to not be prohibitive. But for a large, narrow, multihull, the cost of the hulls is a significant portion of the overall cost. The idea is to target one aspect of the cost, and then look at targeting the cost of rigging, propulsion and interior fitting separately.
     
  7. dsigned
    Joined: Aug 2017
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    dsigned O.R.C. Hunter

    Yeah, I've seen some of them. Concrete mix design isn't a big secret, but it seems to be something that the boating community has been out of touch with. The big changes are pozzolans, water-reducers, and reinforcement tech although there are some other things that also can be used to achieve specific goals (different kinds of water reducers, different kinds of pozzolans, set retarders/accelerators, super-lightweight aggregate). Probably the most interesting thing I learned since first arguing about this is Helsal I. By modern standards, pre-stressed concrete is fairly mundane. But in the 70s, to make a boat of that size so thin and fast was quite the achievement, and to my knowledge, has not been replicated since. I'd love to see what could be achieved with an updated mix and post-tensioning system.
     
  8. dsigned
    Joined: Aug 2017
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    Location: United States

    dsigned O.R.C. Hunter

    Good to know. Something to shoot for.
     

  9. dsigned
    Joined: Aug 2017
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    Location: United States

    dsigned O.R.C. Hunter

    I haven't actually calculated what the hull weight will be. I will probably wind up overbuilding this hull, as I don't really want it to come apart, and even though underbuilding it might be better to figure out where my weak points are, this is a project that takes a back seat to a lot of other things.

    That said, do you have any idea how many square feet of plywood is in the actual boat (not including wasteage?). Back of the napkin (assuming 105 lbs hull weight) would be 35 sq ft. It looks like at that number, my hull will be about 50-60 lbs. At 50 sq ft., I'm at 75 lbs.
     
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