Homebuilt sailing dinghy for children

Discussion in 'Sailboats' started by Ara, Sep 19, 2016.

  1. sharpii2
    Joined: May 2004
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    Location: Michigan, USA

    sharpii2 Senior Member

    One possible solution would be to skip the idea of a bendy spar rig and go with a flat cut sail instead.

    I have had considerable experience with such on the Super Snark I once owned.

    Its flat cut, Boomed Lateen, sailed feathered into the wind so well I was able to sail it in 30 kts of wind and make upwind progress (quite slow with lots of bailing).

    Just make the sail about 25% larger to somewhat make up for lost efficiency.

    Another solution is to look up the spar sizes of an Optimist dinghy, as it has roughly the same loads and crew weights as your boat is likely to have. Even thought the rig on your boat is different, the section size of the mast and Boom can give you a real clue of what sizes you're likely to need.
     
  2. Ara
    Joined: Jan 2016
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    Location: Europe

    Ara Junior Member

    Free!Ship-File and Coordinates

    Here I try to upload the .fbm file as an attachment.

    The coordinates of bottom, bow, sides and transom were obtained from Free!Ship from the menu Tools > Develop Plates and then the "Txt"-button for "Export plates to a TXT-File". I did not manage to get the bulkheads in this way, so I used the menu File > Export > DXF 2D Polylines and then sorted the .dxf file in Excel to get rid of the unnecessary entries. (The next time I would rather try to get access to a large format plotter to print these shapes directly from the .dxf than transferring all the coordinates to the plywood with pencil and ruler.)

    With kind regards

    Ara
     

    Attached Files:

  3. gggGuest
    Joined: Feb 2005
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    Location: UK

    gggGuest ...

    As you're obviously comfortable with exotics this needn't be too onerous. You can work out approx section sizes and wall thicknesses from various manufacturers web sites. Then simply target your spar on the too bendy side, and add carbon unidirectionals if it turns out too soft.
    Too bendy isn't that fatal if the truth be told. I have a mast that's really too soft on my current boat and the result is that the top mast bends off and depowers the rig in just about any wind before I get to the point of really knee and body wrecking sitting out. This doesn't strike me as a fatal flaw in a youngsters recreational boat.
     
  4. JRD
    Joined: May 2010
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    Location: New Zealand

    JRD Senior Member

    As noted above a sailboard mast can work well, cut off the base rather than the tip to keep flexibility. You can make fixtures from the carbon offcuts you will have. If you plan to have a bolt rope sail you can buy plastic sail track and attach it with a flexible glue.

    3/4 height stays are usually a good start and as offered by others you can change the stiffness by adding or sanding off carbon unis. Experimenting with attachment height of the forestay is also a good way to alter the bend characteristics.

    The sail size for the Firebug dinghy is probably a good starting point for you.

    Your hull shape reminds me of an old style moth, nice work
     
  5. tdem
    Joined: Oct 2013
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    Location: NZ

    tdem Senior Member

    A square top main will give better gust response for a given stiffness mast?
     
  6. Ara
    Joined: Jan 2016
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    Location: Europe

    Ara Junior Member

    New spars

    During winter new spars were built to replace the weak bamboo sticks. Starting material was tiling battens from the hardware store. Two halves were prepared by milling grooves using an end mill (see picture of boom parts). The inside was coated with epoxy, and the two halves were glued together using epoxy. Many rubber bands cut from old bicycle tubes were used to press the pieces together while the epoxy was curing. The corners were then rounded using first a planer and then sandpaper.

    The hollow wooden boom had the same weight as the previous bamboo boom (approx. 380 g), but it was clearly stiffer: when the bamboo was supported at both ends (distance 166 cm) and weighted in the middle with 272 g, it flexed about 1.0 mm. The spruce spar under the same load flexed only about 0.4 mm.

    The boom was then wrapped into two layers of carbon (200 g/m2, +- 45 degrees), using video tape to squeeze out excess epoxy (see pictures). While this technique worked great in the past to build round tail booms for radio controlled glider airplanes, it was a bit of a disappointment in this project. Although the corners of the initially rectangular boom were rounded to give it an oval shape, the pressure of the video tape was insufficient on the less curved faces of the oval. The result were wrinkles in the carbon cloth and an excess of epoxy remaining in the laminate. The bending under the standard load went down from 0.4 mm of the bare boom to 0.3 mm with coating.

    The mast was built in a similar way. It is hollow, with a diameter of about 36 mm, and coated with a layer of carbon/kevlar tubing. The mast finally has a weight of 1050 g.

    The attached pictures show also the parrots on hull and sail. With these the requirements specifications from post #1 are for the most part fulfilled (at least until testing on the water shows the next points to be improved).

    With kind regards

    Ara
     

    Attached Files:


  7. Ara
    Joined: Jan 2016
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    Location: Europe

    Ara Junior Member

    After nearly one season of sailing the whole family is satisfied with the little dinghy. The most important modification during the season was the addition of a second sail. With about 3.8 m2 the original square-top sail is suitable for older children or an adult sailing with a child. A simple triangular sail of about 1.9 m2 is large enough for the 5 year old son with a weight of 18 kg (see pictures).

    With kind regards

    Ara
     

    Attached Files:

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