Leeboard Orientation

Discussion in 'Hydrodynamics and Aerodynamics' started by Justaguy, Nov 27, 2015.

  1. Justaguy
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    Justaguy Junior Member

    Hi,

    This is my first post, so please "be gentle" ;-) I'm trying to get up to speed on many aspects of boat design and building in preparation for a DIY project -- generic sailboat with leeboards. For purposes of understanding this concept, I don't want to clutter the issue with my preferences.

    At the moment, I'm trying to understand what seems like a contradiction in leeboard design. Assuming the leeboard is a simple foil, the information that I've been able to find recommends that, during installation, the leeboard should be oriented with its convex side toward the hull. To my understanding, that is counter-intuitive.

    If a leeboard is a foil, instead of just a flat plank, it will produce lift in some direction (as dictated by standard foil dynamics). If the convex side of the leeboard faces the hull (facing windward), instead of the lifting force of the leeboard helping to counter the heeling moment, it would seem to exacerbate it. In other words, the leeboard would contribute to the same longitudinal rotational force caused by the sail, worsening heeling.

    So, I think the opposite -- the leeboard should be installed with the convex side facing outboard / away from the hull, so that its lifting force can help to counteract the heeling moment.

    My only guess about my likely misconception is that the lift generated by a leeboard may be so minuscule so as to not really matter which way the board is oriented, except that in the prescribed way, it will at least hold itself tight against the hull during use.

    I tried to mock up a drawing (thanks to the original artist) to show what I think would happen. Hopefully it is attached. Please take a look and tell me what I'm not understanding properly.

    Thanks for your time.
     

    Attached Files:

  2. TeddyDiver
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    TeddyDiver Gollywobbler

    It's not heeling moment you should think about. Instead it's the amount of leeway you try to minimize.
    BR Teddy
     
  3. Joakim
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    Joakim Senior Member

    First of all "just a flat plank" produces lift just fine very similar to symmetrical foils, but with more resistance and different stall. An asymmetrical foil produces lift with zero angle of attack, but that is more a question of how angle of attack is defined. An asymmetrical foil can have even less resistance than a symmetrical, when lift is produced.

    The hydrodynamical purpose of a keel or a leeboard is to produce lift as a counterforce to side force from sails. Without that a sailboat would just go downwind. As Teddy said the purpose is to minimize leeway.

    As an unavoidable side effect heeling moment is also increased.

    You could also use a symmetrical foil or even a flat plank installed at an angle to get the same effect as having the convex side to windward.
     
  4. jehardiman
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    jehardiman Senior Member

    Another way to think of it is that the arm from the CE of the leeboard to the CG is ~ 1/10th to 1/20th the length of the arm to the CE of the sail. For all practical purposes, the leeboard contributes an insignificant amount to heel. Having the board lift 'out' rather than 'in' just reduces the ability to go to windward.
     
  5. Paul Scott
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    Paul Scott Senior Member

    Anyone who has railed up on a D2 or longboard might not agree that the heeling effect is minimal-

    My main problem is that the leeboard is right by the hull, and if the lee rail is down, you've got a big bump there slowing you down. Maybe a windward board is better in some ways- the side attachment lump is out of the water more often than not, you can see what it is doing, angle (adjustable?) it to pull the windward side down? The big downsides are that it gets pulled out of the water when the rig heels the boat (most of the time:p), heeling to leeward there's not as much board in the water, and it pulls away from the hull at the low attachment point.
     
  6. TeddyDiver
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    TeddyDiver Gollywobbler

    The amount of 'bump' depends of the geometry of your leeboard. Any keel, rudder and leeboard are more effective (less drag) when the top end is meeting the hull below the waterline ie acting as an endplate for it..
    The heeling moment is not affected by the keel or leeboard. It's becouse of the heeling forces induced by the rig and sails. If there were no keel at all it would only mean more leeway until the drag of the hull created becouse of this transversial movement would counter the side forces created from other forces.
     
  7. Joakim
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    Joakim Senior Member

    When you calculate the heeling moment you need to take the vertical distance of center of efforts of the hydrodynamical and the aerodynamical side force. In equillibrium these forces are equal and to get the moment you multiply with the vertical distance.

    Keel or a leeboard most likely lowers the hydrodynamical center of effort and thus increases the heeling moment.
     
  8. latestarter
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    latestarter Senior Member

    I am interested in canoe sailing and attach a sketch I drew for a discussion on another forum, to illustrate Joakim's point.
     

    Attached Files:

  9. daiquiri
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    daiquiri Engineering and Design

    As usual, it comes to the well-known fact that nothing comes for free.
    The increased heeling moment is the price you have to pay to Mr. Leeboard for the service of decreasing the sideways drift of your boat, which is his main job. ;)
     
  10. TeddyDiver
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    TeddyDiver Gollywobbler

    Keel for sure some, but a leeboard might even rise the hydrodynamic CE in some cases. There's no definite answer becouse everything changes. Without a keel when leeway increases also the angle of attack increases and so do the aerodynamical heeling forces. But it depends how the equlibirium is achieved so IMHO it's impossible to have anything conclusive.
     
  11. TeddyDiver
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    TeddyDiver Gollywobbler

    In other words, without a keel of any kind you can't point as high in the wind as with, so beam on the wind is close to the best scenario in most cases. How then having a keel or not would affect the aerodynamical heeling force?
     
  12. DouglasEagleson
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    DouglasEagleson Junior Member

    This is sort of off topic. It is about ballast in leeboards not shape. But it concerns me. I think there might but be a fallacy or misconception about ballast in lee boards.

    The leeboard righting angles are confusing. Ballast efficiency?

    I don't know how to calculate for lee boards. It seems like the down side board ballast(if you want it) just swings uselessly until it reaches the boat center of mass. That is alot of angle. Just placing ballast on the deck sort of fits the scenario.

    Any thoughts?

    Thanks doug
    eaglesondouglas@gmail.com
     
  13. Justaguy
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    Justaguy Junior Member

    Covered all bases

    All,

    Thanks to all for an excellent collective reply. Special mention to Joakim and jehardiman for covering the essential technical aspects, and to daiquiri for summing up the key points useful to a beginner like me.

    Justaguy
     
  14. Paul Scott
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    Paul Scott Senior Member

    Um, if I look at the cartoon correctly) doesn't the sailor's weight (if it stays in the same place) have more leverage on the heeling moment of a leeboard than a centerboard, or a windward board?

    Are any hydrofoils not surface piercing?
     

  15. Doug Lord
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    Doug Lord Flight Ready

    -----------------------------------------------------------
    Well, yes and no. Surface piercing foils like Hydroptere uses are much different than fully submerged foils like the Moth uses. Fully submerged foils require an altitude control system=wand(s). "Surface Piercing Foils" like Hydroptere uses(for main foils) regulate altitude by speed-the faster she goes the less foil area required.
    That's the cut and dried stuff. The complicated stuff is UptiP foils that "ideally" are fully submerged foils using leeway coupling to control altitude(backed up by manual adjustment of the foil rake which changes the angle of incidence of the foil). In practice , a number of designers use the uptip portion of the foil as a surface piercing foil to provide backup.
    Both surface piercing foilers and fully submerged foilers use a trailing rudder T-foil to control pitch.
    Even on fully submerged foils the strut(daggerboard) that supports them does pierce the surface but is not ,generally, called a surface piercing foil.
     
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