Please sanity-check my 5'6 dinghy design!

Discussion in 'Boat Design' started by markgoespop, Jul 31, 2017.

  1. markgoespop
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    markgoespop Junior Member

    So it is too short to be usable, although the high sides mean it may work in a pinch. I will build it anyway because I want to, but we'll probably need a more practical tender down the line.

    Adrian: Your experience with your winklebrig is interesting. And the link to Hannu's boatyard is useful.

    Mr E.: We are sort of young and sort of fit.

    JSL: The similarity to a coracle is interesting.
     
  2. PAR
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    PAR Yacht Designer/Builder

    The standard bath tub is 5'. More to the point is the ridiculousness of this dimensional proposal.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    He did say it was a "sanity check". o_O
     
  4. markgoespop
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    markgoespop Junior Member

    PAR: "ridiculous" is stronger than the other comments so far -- that might be useful to me. Can you put a finer point on how this design will fail?

    I modelled it in Carlson Hull Designer (thanks Rurudyne), and it seems to have enough buoyancy, with 7 inches of freeboard at 150kg. I am guessing it will track horribly and have a very low hull speed. It will also have a tendency to rock when rowed, but the bow and transom are quite high. Stability is likely to be poor because it will be quite light. None of that seems terminal. Am I missing something?
     
  5. PAR
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    PAR Yacht Designer/Builder

    What could be the worse that might happen?

    Simply put, designing small craft that can meet design criteria is very difficult, mostly because you simply don't have the hull volume to do anything to help the major issues that will arise. As you've noted; being out, farther from shore than you care to swim back to, in a craft with less than desirable stability, that's tracking like crap, making very little progress against tide, currents, winds and the occasional obnoxious mermaid, seems as tenuous a set of design decisions, as you could ask for. Having enough freeboard so they can eventually find your body, might come in handy though, at least for your next of kin . . .
     
  6. markgoespop
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    markgoespop Junior Member

    Nice. You make a very strong point. I will think about it.
     
  7. messabout
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    messabout Senior Member

    Mark you might do well to mock up a cardboard box, or something similar, in the dimensions that you have stated. The purpose of that is to figure out how two people can be seated in such a boat. In the process figure out where your oarsman must be in order to make the boat go. Figure out how two of you will be able to get into and out of the boat without having it pitchpole.

    I do believe that your sides are too low. If you have only seven inches of freeboard, the first rude power boater that comes along with his nasty wake will tend to deliver you to Davy Jones Locker. 7 inches of freeboard is something like that of a kayak. Plenty for the decked kayak but not enough for an open dinghy.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Why take the risk. I suspect the shrunken dimensions are to have it tucked out of the way at the stern, not creating wind-drag. Dinghies under 8' are Lilliputian.
     
  9. JSL
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    JSL Senior Member

    I am going to guess..... the stability will be like a beach ball. A very exiting adventure for all. Suggest you install a Prozac dispenser.
    As to speed, you will probably be overtaken by driftwood.
    One option you could do is arrange one area of your 'dinghy' that is parallel with a soft spot: it would give you an option lengthening by splicing in a 12" or 18" section

    "Old boaters never die.. they just get a little dinghy"
     
    Last edited: Aug 1, 2017
  10. Rurudyne
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    Rurudyne Senior Member

  11. markgoespop
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    markgoespop Junior Member

    The original plan seems like a bad idea (thanks folks). As an alternative, I found these plans for a simple 7'9 pram:

    Plan for pram dinghy https://digital.lib.ecu.edu/24078

    A nesting version would fit on the transom of the trailer sailer. Does that sound sensible?

    I will sketch these up in Hull Designer and make a model tonight.
     
  12. Ike
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    Ike Senior Member

    That is the same outfit that published the plans for the 7' 11" dinghy I built. It was designed by Edwin Monk. TheDouglas Fir Plywood Association published many plans for boats. Unfortunately they no longer exist. But the plans are still available. Good choice. I converted the plans from conventional construction to stitch and glue, which simplified the build. I would suggest doing that if you can.
     

  13. markgoespop
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    markgoespop Junior Member

    I ended up looking back at this post while pondering building another rowboat. My nesting pram dinghy came together nicely. It fits in my car, rows poorly and serves well as a tender for the trailer sailer. It doesn't seem to present that much drag when towing either -- it's so light, I wonder whether it's planing.

    Here is a pic of it being painted.

    upload_2018-5-22_1-58-28.png
     
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