Please sanity-check my 5'6 dinghy design!

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

  1. markgoespop
    Joined: Jul 2017
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    Location: UK

    markgoespop Junior Member

    I would like to sanity-check my first design with you folks. I will be building a dinghy to go on my tiny 18ft trailer sailer in the next 3 weeks, come what may.

    I have the following design constraints:
    - cheap as possible
    - *really* easy to build
    - carries 2 (we're ~150kg)
    - won't flood in rough-ish costal water
    - max length 5'6
    - max width 3'6

    My current design is a flat bottomed pram. It is fore/aft symmetrical. It has extreme rocker: 7" deep at the centre, so that the waterline at 150kg sits at the join between the bottom and the bow and stern transoms. The sides are 14" deep, and at 150kg there is 7" of freeboard at the lowest point in the centre. The bow and stern reach 7" higher. There will be a 7" deep skeg on the back half of the hull.

    I will build it from cheap 6mm ply, following the construction method of the 'Elegant Punt', i.e. external rails, glue and screw.

    Does this sound sensible enough, given the design constraints? Can you see any refinements?

    Here is a paper model to help you visualise:

    [​IMG]
     
  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    A small inflatable would seems more suitable.
     
  3. markgoespop
    Joined: Jul 2017
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    markgoespop Junior Member

    Thanks Mr. E. I am not keen on inflatables: they are wider than the spec when inflated, and not ready to go if deflated. Plus I would like to make a boat.

    Is the design problematic somehow?
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Where are you going to put it ? In the trailer-sailer ? If you tow it, it will act as a brake much beyond 3 knots.
     
  5. Rurudyne
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    Rurudyne Senior Member

    It's a pram.

    As for the design, if you intend to build a plywood boat you might be profited by using Calrson Boat Designer. It is free and can be downloaded from the links here: Free or Low Cost Yacht Design Software : the Boat Design and Boatbuilding Directory https://www.boatdesign.net/Directory/Software/Free_or_Low_Cost/

    Good news is the download comes with lots of sample hulls to compare to. It will also do so basic calculations.
     
  6. markgoespop
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    markgoespop Junior Member

    I am hoping to attach high on the transom, on its side with the open top facing forward.
     
  7. markgoespop
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    Location: UK

    markgoespop Junior Member

    Thanks Rurudyne, I will take a look.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    It is very tiny, and any mention of "rough-ish" water, would make me nervous. But, with caution, might be OK, if only because it has a little bit of beam to help.
     
  9. Ike
    Joined: Apr 2006
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    Ike Senior Member

    For me around 8 feet is about as small as it gets. My dinghy is 7' 11", weighs 55 lbs. can hold two without danger of capsize or sinking and is easily transported. It was easily built of plywood using stitch and glue construction. The whole boat (not counting labor) cost about $300.00
     
  10. Tiny Turnip
    Joined: Mar 2008
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    Tiny Turnip Senior Member

    Mark

    I'm no naval architect, but my gut feeling is your boat is too small to be safe for a tender for two. I have owned a 16' winklebrig for 10 years, and struggled with the same problem. I ended up using a small inflatable as a tender for the times I could not moor to a bank or pontoon, or anchor shallow enough to wade ashore. Have a look at Hannu's boatyard; he specialises in small, very simple boats, but even he baulks at 5 foot boats for anything other than beach toys. He's built them down to 4 foot. For a boat that size, I would say some buoyancy is vital - bags or built in.

    with very small boats, (according to Hannu), much of the difficulty can also be in the tight bending radii needed. perhaps 4mm ply for some elements.

    Some of the geodeisic aerolite boats may be of interest; super light and very beautiful, though of limited robustness, and a rather more technical build.

    Might your desire to build a boat be a separate project from the need for a tender?

    Might an inflatable SUP be a solution for the tender?

    Adrian
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    A couple of people who are young and fit and nimble might be able to cope with it, but otherwise not, I'd say.
     
  12. JSL
    Joined: Nov 2012
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    JSL Senior Member

    this is almost like a very small coracle. Make sure you wear your PFD.
    I had a friend with a 17' daysailer. He carried a folding boat- worked very well. If no luck at marine stores, check out hunting stores etc.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    That seems like a good suggestion, the folding boat. 5' 6 is a bath-tub !
     
  14. Rurudyne
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    Rurudyne Senior Member

    Butcher, Baker and Candlestick Maker?
     

  15. Ike
    Joined: Apr 2006
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    Ike Senior Member

    even bath tubs are longer than that,
     
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