Squeezing more RM out of keel with controllable flap?

Discussion in 'Boat Design' started by pironiero, Jul 30, 2021.

  1. Squidly-Diddly
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    Squidly-Diddly Senior Member

    I'm talking about a keel with airplane wing style tab for RM that looks just like an airplane wing except its pointing down, and the tab to produce the RM is circled in red and just like on a airplane where the ailerons produce roll, the tabs on keel would be at the tip for leverage.
    When extra RM is not required it would be as streamlined as a normal fin non-bulb keel (but I guess it could have a bulb on the end...just like some airplanes got fuel tanks on tips of wings.)
     

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  2. pironiero
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    pironiero Senior Member

    But why do you need a whole wing in front of it? Why don't you make the wing itself adjustable? It seems kind of illogical to me, fish fin is moving whole, not just the tip.
     
  3. Squidly-Diddly
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    Squidly-Diddly Senior Member

    I'm not a big time sailboater but I'm guessing it would be hard to stay on course and that you really want 80%+ of the keel fixed for directional stability, and that since water is denser than air just a small tab will provide lots of extra RM.

    Also what about drag where the keel meets the hull if the keel is canted and sharp corners happen and you don't have flexible fish skin? Maybe if the hull was flat at the keel and its a big flush circle plate and the keel smoothy joins the plate. But now thats a major new radical design.

    I'm talking about taking a run of the mill removable keel unballasted sailboat and cutting out a tall thin notch and adding a tab with a rod with control hand lever on top. You have to set the tab to neutral to insert into dagger sheath.
     
  4. tlouth7
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    tlouth7 Senior Member

    That is complex in the context of the most highly loaded part of a boat's structure.

    Fundamentally though, any sort of righting moment addition on the keel suffers from the same problem of being to windward of the centreline, such that to add righting moment the force must act downwards. In contrast a foil to leeward provides force upward, thereby reducing the effective displacement of the boat.
     
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  5. pironiero
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    pironiero Senior Member

    i havent thought about it this way, unfortunate, really.
    but what if in reaching and running conditions we use this foil to push hull out of the water while compensating tilt with water ballast? And in beam and close reach we use it to gain RM and simultaneously max waterline length?
    im talking about fixed keel boat in this case
     
  6. CT249
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    CT249 Senior Member

    No, not at all. The RM on a sportsboat keel comes from ballast weight, which is there at all speeds.
     
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  7. CT249
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    CT249 Senior Member

    How do you impose worthwhile force with "two cords"???

    As a comparison, one may just as well add RM by just adding wings. You could make the wings 20m wide on a 6m sportboat, and you could therefore develop huge RM and a huuuuuge rig.

    In real life, of course, things don't work like that. In high performance boats you often don't really need RM when everything is going well. You need RM when something has gone wrong, such as a slow gybe or a sudden luff or gust. At such times, when speed is low, dynamic lift is also low just when you need maximum RM.

    Many years ago, at one stage 18 Foot Skiffs went for a wingspan of 30ft when they did a championship in open water. As soon as they returned to normal water, they had to cut the wings down to 23' because RM in steady-state conditions is largely irrelevant in normal sailing.
     
  8. pironiero
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    pironiero Senior Member

    i meant that at slow speeds high aspect keel cant keep the boat from sliding sideways, isnt it?
     
  9. tlouth7
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    tlouth7 Senior Member

    Remember a keel is doing two jobs - the foil/strut is providing lift to windward and so limiting leeway, and the weight/bulb is providing righting moment.

    You can't increase righting moment by rotating the foil or adding a tab along the back without also increasing leeway. In fact, keels with tabs along the back are used, but specifically to decrease leeway so the precise opposite of what is being proposed by some here!
     
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  10. DCockey
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    DCockey Senior Member

    I recall a patented system from ten years ago or so which had foils which extended horizontally from the leeward side of a sailboat to provide lift and thus righting moment. I haven't seen mention of it recently.
     
  11. DCockey
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    DCockey Senior Member

    tlouth7 is correct. Sailboats heel because of the vertical offset between the aerodynamic force on the sails and hydrodynamic force on the hull/keel/centerboard/rudder. If the boat is sailing in a straight line the lateral components of the aerodynamic and hydrodynamic forces will be equal in magnitude and in opposite directions. Adding tabs or rotating a keel or other vertical foil will only change the angle of leeway. It will not change the magnitude of the side force.

    To use hydrodynamic force to change righting moment the force must be in a vertical direction and horizontally offset from the CG of the boat. This is the exact same mechanism by which any boat develops a righting moment.
     
  12. tlouth7
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    tlouth7 Senior Member

    DSS or Dynamic Stability System. It's still being fitted to a fair few boats, but the grand prix monohull racers have moved onto bigger leeward foils which meaningfully lift the hull out of the water.
     
  13. pironiero
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    pironiero Senior Member

    also seen custom built wooden boa with this DSS
     
  14. pironiero
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    pironiero Senior Member

    but the idea is to keep flap horizontal to water surface and pitching it to gain more RM
     

  15. container
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    container Junior Member

    Its actually the opposite, IMOCA and the Volvo 65s have their keel pins inclined (front of the pin is higher than the back) anywhere from 2° to 10°, it varies on the IMOCAs and i cant recall exactly what angle the volvos use. This means the AoA increases as the keel cants, lifting the bow and reducing displacement. However it also significantly reduces righting moment at speed so you need a wider hull to somewhat offset this. The compromise must be worth it as they all use it.
     
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