Does lifting the keel going downwind makes noticeable difference?

Discussion in 'Boat Design' started by pironiero, Nov 3, 2020.

  1. pironiero
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    pironiero Junior Member

    Does lifting the keel going downwind makes noticeable difference in speed?
     
  2. jehardiman
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    jehardiman Senior Member

    Depends on the vessel and its design.
     
  3. pironiero
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    pironiero Junior Member

    Something like pogo40 or similar somewhat flat hull
     
  4. jehardiman
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    jehardiman Senior Member

    Pogo 40, probably not. DDW with a deep sharp forefoot leads to bow steer. You need the CLR aft when DDW.
    Edit to add:
    Many racing dinghies which have deep forefeet and are flat aft can raise the board if the crew moves way aft to get the forefoot up. In something like a Lark, both crew would move as far aft as possible with the board up.
     
  5. pironiero
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    pironiero Junior Member

    okay lets take hypothetical perfect hull
     
  6. Will Gilmore
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    Will Gilmore Senior Member

    Keep in mind that the force vectors change with each point of sail. DDW puts the drag resistance in direct opposition to the driving force except, there is a significant offset in hight. For any other point of sail, much of the drive force is at an angle to forward motion, leading to heel. Heeling lowers the driving force, thus reducing the lever arm between forward resistance and forward drive. The bow design will have a significant impact on the behavior of the boat under each point of sail.

    DDW sailing levers the bow down and lifts the stern. I'm not incline to think of it as CLR as much as moving the pivot point of the hull below the waterline foreward, although that is probably the same thing.

    For a swing keel or centerboard boat, raising the board has the effect of reducing drag and, if not lifting the board completely out of the water, moving it aft also. In addition, the CE when running DDW is significantly forward of the designed CE when calculating its relationship to the CLR. With the boom all the way out, the CE can actually move ahead of the mast.

    In dinghy sailing, weight distribution will have a meaningful effect; for boats like the Pogo, I doubt it. Raise the board, but not all the way.

    -Will (Dragonfly)
     
    Last edited: Nov 3, 2020
  7. pironiero
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    pironiero Junior Member

    Not a swing keel, lifting one with a bulb on the end
     
  8. jehardiman
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    jehardiman Senior Member

    There is no such thing as a "perfect" hull. Though hulls can be optimized for downwind (think pre-clipper packets), in is generally necessary to compromise in some fashion to allow windward work. For what can be done, look at an A scow.
     
  9. pironiero
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    pironiero Junior Member

    Lets TAKE A HIPOTHETICAL PERFECT HULL AND PERFECT CONDITIONS
     
  10. Will Gilmore
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    Will Gilmore Senior Member

    [​IMG]
    Does it seem that that torpedo shape might tilt downward in the water when sailing DDW? If so, bring it up closer to the hull for less drag farther away from the CE. You don't need the righting moment.

    -Will (Dragonfly)
     
  11. jehardiman
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    jehardiman Senior Member

    Ok, any hull will go faster DDW if you 1) decrease wetted surface and 2) increase waterline length while still maintaining directional stability. Yes, those two conditions are contradictory and raising the keel/board may actually increase wetted surface depending on hull design. The optimum design would be to have some net loss to the wetted surface while increasing LwL with keel/board raised. Furthermore, there is the DDW paradox (the faster you go DDW, the less driving force you have requiring even more reduction in wetted surface) and the flow paradox (boats go faster in slightly disturbed water than in DFC which increases wetted surface).
     
  12. Will Gilmore
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    Will Gilmore Senior Member

    Not that this addresses your question about sailing DDW, but if you are interested in the specific question of how to get the best preformance out of the Pogo, boats like the Pogo are designed to sail DDW by sailing a broad reach and tacking, instead. Their pollars indicate far better performance for DW-VMG, than a DDW tack.

    -Will (Dragonfly)
     
  13. philSweet
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    philSweet Senior Member

    The crappy part is that if you don't need all the RM, you probably do want all the foil area to reduce induced drag (all sail flying and low wind speed). And if you don't need all the foil area, you probably do want all the RM for best performance (sail reduced and high wind and seas). On most sport boats, you want to add sail to consume all available RM at the design heel angle.

    So in the case of the Goldilocks boat, no, you wouldn't want to raise the keel. You would have a sail plan and controls that could maintain the perfect balance of RM and sideforce so that things were always just right at any wind angle and any wind speed (good luck with that).

    If you look at how the old Volvo 60's did it, they had a bulb on a minimal strut and used daggerboards for sideforce. These were operated over a wide range of drafts. Once you have independent control of RM (via moveable ballast) and foil area (via daggerboards), then you can exploit daggerboard depth this way.

    A more traditional take on the lifting keel has all the ballast in a fixed keel and a centerboard extension. These boats were designed from the get-go as shoal draft sailboats.
     
  14. pironiero
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    pironiero Junior Member

    THANK YOU
     

  15. jehardiman
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    jehardiman Senior Member

    Ah yes, the CCA centerboarders... a good solution to a rule problem.
     
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