The cheapest stable boat tender possible.

Discussion in 'Boat Design' started by Asleep Helmsman, Apr 27, 2019.

  1. Asleep Helmsman
    Joined: Jul 2008
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    Location: Republic of Texas

    Asleep Helmsman Senior Member

    I want to build an ultra cheap barge.

    Do y'all think a 4 foot by 8 foot square skift is the way to go? Gonna use it to redo my topsides.
     
  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    You cannot get much simpler than a rectangular floating box, that's for sure.
    Add a bit of rake on one end perhaps, to make it easier to go in that direction?
    The stability would be improved if you made it into a basic catamaran by removing a rectangular longitudinal block of buoyancy from the centerline (which does nothing for stability).
    Especially as you certainly won't have to worry about having enough buoyancy on a box shaped craft.
    Maybe make the hulls each 1' wide? For an 8' x 4' cat with rectangular box shaped hulls 1' wide, you would have approx. 500 lbs of buoyancy at a draft of 6" - more than enough for your weight, a passenger and the weight of the boat.
    And a cat hull form would be much easier to row or power than a brick shaped hull form.
    Or is this already getting too complicated re cheap and simple?
     
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  3. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    If all you need is a temporary work platform. Glue a few sheets of cheap "home center" foam together. Won't last long but could be long enough to get your top sides painted. Lash a pool floaty under the work side if extra buoyancy required
     
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  4. tlouth7
    Joined: Jun 2013
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    tlouth7 Junior Member

    I don't follow how taking buoyancy out of the centreline will improve stability, I would expect it to stay almost exactly the same at the cost of some extra vertical walls.

    Important things to get right will be how you secure it to the boat to minimise movement but allow adjustment from the dinghy, and having a thwart in the right place/height to work comfortably.
     
  5. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    The basic formula for stability is BM = I/V where BM is the distance between the centre of buoyancy and the metacenter, I is the second moment of area of the waterplane area, and V is the volume of displacement.
    You get very little 'I' in way of the centerline, yet you still have a lot of buoyancy. Get rid of some buoyancy in way of the centerline and the I will decrease a bit, but much less so than the corresponding decrease in volume, hence the BM will increase.
    Hence why a catamaran is very stable.
     
  6. Dejay
    Joined: Mar 2018
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    Dejay Senior Newbie

    I've tried to do the math but I'm getting some weird results. The I for a barge would be Ib=(BWL^3*LWL)/12 and for a catamaran with the same overall beam and 1/4 of the width for the amas I get Ic = LWL * BWL^3 / 48 + LWL * BWL / 4 * (3/8)^2

    So for a barge of 1m x 1m I get 0,0833 and for a catamaran I get 0.056 for the second moment of area.

    The displacement should actually be the same so the metacentric height above the center of buoyancy should be about 67% of a barge. But the center of buoyancy of a catamaran could be a little bit lower than for a barge.

    Conceptually I would think buoyancy in the center would make a boat a little bit more like a seesaw where it can pivot around the center, but you also loose buoyancy.

    So is a catamaran just better for seakeeping / better motions, or does a catamaran really have more initial stability? From what I've read the two are related, too fast acceleration in reaction to waves are what makes it uncomfortable. I figure is more like a soft suspension on water but still resists roll.
     
  7. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    A couple of other quick and dirty work platforms:

    - lash a couple of surfboards under a sheet of plywood
    - Lash a Wal-Mart oval inflatable raft under plywood
    - for awhile I've wanted to experiment with screwing a bunch of water/soda bottle caps to plywood then screwing the bottles in to provide buoyancy. Empty paint buckets would work as well
     
  8. fallguy
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    fallguy Senior Member

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  9. tlouth7
    Joined: Jun 2013
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    Location: Cambridge, UK

    tlouth7 Junior Member

    Your Ib is correct but something has gone wrong for the catamaran. Note that Ic = Ib - I(missing section) = LWL*BWL^3* 7/96 = 0.073 for your example. This is still lower than for a barge of equal LWL, BWL & Disp. This result is intuitive because you have removed some waterplane area (less bad than removing the same area from the edge though). A catamaran would be more stable than the equivalent barge if you cut the 8'x4' sheet in half and moved the halves apart. Thus you would have the same LWL and Area but greater BWL.
     
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  10. Dejay
    Joined: Mar 2018
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    Dejay Senior Newbie

    Thanks, much easier way to calculate it! :)
     
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  11. Squidly-Diddly
    Joined: Sep 2007
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    Location: SF bay

    Squidly-Diddly Senior Member

    I'd get two beater alum boats off Craigslist (about $50-100, might need sealing) then lay 3 2x8 10' planks across the sterns and secure with #9 "safety wire" with little holes drilled under the gunnels, then lay 4x10 3/4" plywood over then and secure with a few screws.
    When you are done will still have two semi-usable boats and some useful lumber.
    Maybe 5 planks, and two sheets to make a 8x10 platform on cat-hulls. Allow the platform to be shoved forward enough to clamp little motors on the transoms. People will be wanting to borrow it.
     
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  12. cluttonfred
    Joined: Mar 2014
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    cluttonfred Junior Member

    You can build a Bolger/Payson Big Tortoise (8' x 3' 2") basic hull in a weekend from two sheets of plywood and some scrap lumber. Add some styrofoam or empty plastic bottles under each end deck for emergency flotation and a little box seat for rowing. When you're done you have an actual little boat not junk.

    Here's a pic of a standard 6' 6" Tortoise, the Big Tortoise is the same but stretched to 8' and with little decks on both ends. You could just wing it but the plans are here Big Tortoise - 8' x 3'2" - H.H. Payson and Company https://www.mcssl.com/store/hhpaysoncompany/instant-boat-plans/big-tortoise. Check out Payson's books as well, the basic plans for several boats are free in the books.

    [​IMG]
     
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