Dinghy 13 ft with bi-convex sections option

Discussion in 'Sailboats' started by Dolfiman, Apr 22, 2019.

  1. Dolfiman
    Joined: Aug 2017
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    Location: NICE (France)

    Dolfiman Senior Member

    I have investigated this option in the idea to enlarge the beam overall (to maximize the righting moment from the helmsman in hiking position) while maintaining at minima the beam at waterline (to maintain low the drag) but with offering more managable stability (especially during tack or gybe manoeuvers by rough seas). For user with less kills and/or seaching a more easy but still fun dinghy.
    The convexity can be more or less accentuated, here attached I have chosen a medium shape, for a comparison with a convex-concav one (a more common design) built at same beam overall and with exactly the same immerged hull waterlines.
    The main result is about the stability when the "payload" (i.e. the helmsman) is temporarly in the center of the dinghy : the bi-convex option can offer a greater tolerance.
    The main unknowns are about the behaviour at speed on waves :
    - the bi-convex leeward side can act as a spray rail (less wetty for the crew), can add some dynamic lift when heel is 10° or over.
    - … but at the cost of some extra drag, of some slam occurrences,
    Thanks in advance for your comments.
     

    Attached Files:

  2. Dolfiman
    Joined: Aug 2017
    Posts: 758
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    Location: NICE (France)

    Dolfiman Senior Member

    I worked again on this bi-convex option, being not satisfied by the sharp point transition between the two convexities, a probable source of extra drag and moreover not structurally good. So I introduce a complementary function able to generate a soft transition, as illustrated by the 2D presentation attached. With that tool, I can propose Bi-convex soft versions, one with the initial Bwl 0,92 m and one at Bwl 0,97 m to show also the influence of this data on the stability tolerance, the document attached gives a full comparison with the previous Bi-convex sharp and Convex-concav versions.

    The fun of such numerical tool is that it also works reversely, able to generate a bump instead of a concavity. So, I have not resisted to also investigate a Dinghy Bump version, but then of course for a very different objective : a dinghy with a lot of stability tolerance, offering an extra RM even at small heel angles, at the cost of more Bwl (1,20 m) and + 15% of wetted surface : original at least, can be interesting for a sailing school, or for a family or expedition programmes.
     

    Attached Files:

  3. Doug Lord
    Joined: May 2009
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    Location: Melbourne, Florida

    Doug Lord Flight Ready

    Dolfiman, I love the "bump" version , most especially for Emilie's application. Of the others I like soft 2.
    But, in my opinion, the "bump" is by far the best: it is modern, very good looking and impressive. Hope you post these in Emilie's thread.
     

  4. CT 249
    Joined: Dec 2004
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    Location: Sydney Australia

    CT 249 Senior Member

    To me, it looks as if rough water performance will be very problematic. Singlehanded dinghies suffer dramatically if they have much flare in the bow; the Laser may be something of an exception but good Laser sailors work furiously to ease the boat's passage through waves, and I believe that compared to contemporary boats the Laser had a fine bow angle that alleviated the issue.

    I would suspect that the disturbance to the water flow caused by the crease may increase drag more than the simpler alternative of increasing waterline beam.

    Personally I am very dubious about Open-style dinghies. They ignore the lessons of about 30-50 years of development in Moths, Int 14s, 18 Foot Skiffs, 12 Foot Skiffs, Merlins, National 12s etc. When dinghy designers created Open-style boats they did not ignore the 20 years of development in Opens that had gone before them, so why should the much longer and more active period of dinghy development be ignored? As a simple example, even the Australians largely abandoned wide stern dinghies in the 1980s, because they had more wetted surface and inferior handling qualities.

    Sorry to be negative, as I do like much of your work. My remarks may not apply in your area; some European designs perform poorly down here where the water tends to be choppier and I may therefore be putting too much emphasis on this area.
     
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