Do Keels need to be aerofoils?

Discussion in 'Boat Design' started by stonedpirate, Sep 3, 2011.

  1. stonedpirate
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    stonedpirate Senior Member

    Hi,

    Just wondering if you could use a flat board with a rounded leading edge as a keel or does it have to be shaped as an aerofoil to work?

    Thanks
     
  2. Doug Lord
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    Doug Lord Flight Ready

    ======================
    Generally, the better they're shaped the better they work-but you can get by with less than perfect. A boat I grew up racing-the Windmill had a tall narrow daggerboard that was basically flat with the first and last 2"(approx-don't remember exactly) trimmed down. The leading edge to a parabolic shape ending in a 3/32nd in. rad.(approx) and the trailing edge sharp. Worked very well but could surely be improved upon-but not and still be a Windmill...
     
  3. PAR
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    PAR Yacht Designer/Builder

    "Do Keels need to be aerofoils?"

    Nope . . .

    It would help if you define your question as the subject is so broad . . .
     
  4. tunnels

    tunnels Previous Member

    Do you see aeroplanes with square wings ??
    do you see boats box shaped ??
    do you see cars that are box shaped ?
    SHOULD KEELS BE SQUARE !!:confused:

    WOW !!
     
  5. stonedpirate
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    stonedpirate Senior Member

    Thanks guys.

    After much reading, i have to disagree with tunnels.

    A square keel automatically becomes an aerofoil because of the angle at which it hits the water plus with the combined effect of the rudder angle creates the long and short routes for the water to generate lift.

    Probably not explained very well but the angle of the keel and the rudder create the wing.

    Have a wing shaped keel is optimal but not essential.

    Cheers
     
  6. PAR
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    PAR Yacht Designer/Builder

    Generally, the more aspect ratio you have, the more important foil shapes become. A long shoal appendage will gain very little from foil sections, just easing the leading edge corners will be enough. The only exception to this would be very light air sailing, where a foil section, even if very shallow will have less resistance and better "penetration" then the square faced appendage. On the other hand if you have a flat plate centerboard boat and an identical boat with a foil sectioned centerboard, it'll out preform the flat plate on all points of sail, except a dead run with both boards fully retracted, at which point they'll be the same.

    Again without knowing what you're attempting, the discussion will weave into all sorts of unnecessary avenues.
     
  7. stonedpirate
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    stonedpirate Senior Member

    Its a full keel on a 10 foot heavy displacement hull.

    The keel is 3/4 length of the hull.
     
  8. tunnels

    tunnels Previous Member

    OK PROVE YOUR THEORY !!
    why is every one on the planet wrong!
    make a square keel and go racing !!
    i sort of understand your reasoning , dont get me wrong
    there a mountain of foils each one has its uses !! :eek:
     
  9. stonedpirate
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    stonedpirate Senior Member

    who said anything about racing?

    its a 10 foot tub with a sail and a top speed of 4 knots.
     
  10. tunnels

    tunnels Previous Member

    Thats the perfect place to try a square keel and a foil shaped keel !!
    USE A GPS TO CHECK YOUR SPEED !! :?:
    Racing is where inventive and creative people dare to venture and prove there ideas are better than others because they go faster . if we didnt have racing we would still be walking in bare feet dragging a cub and eating raw meat :p.
     
  11. daiquiri
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    daiquiri Engineering and Design

    Stonedpirate,

    I agree with the following indications you have been given so far:
    1) Everything else being equal, a foil-shaped lifting device will be more efficient than a flat-plate, at least in the Reynolds number range of practical interest for boating.

    2) Due to a very-low aspect ratio (AR) of your keel, the side-force it generates will be mostly due to a huge vortex swirling around the bottom edge of the keel (called tip-vortex, or leading-edge vortex, as the two are practically indistinguishable for very-low AR keels) and will depend in much lesser extent on the foil shape. So, in your case, you will imho notice a very little (if any) difference between a flat-plate keel and a foil-shaped keel. However, the leading-edge shape is important for operations at low angles of attack, so try to keep it beveled or rounded.​

    In the attachment below you can see a colorful depiction (I had some spare time today :) ) of the said LE vortex (delta semi-wing in the pic - vaguely resembling a very-low AR keel).

    Cheers!
     

    Attached Files:

  12. FAST FRED
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    FAST FRED Senior Member

    Do you see aeroplanes with square wings ??

    Not the question , If you look at the L/D graphs for airfoils , you will see a flat surface has great lift , just the angle of attack is limited compared to more complex shapes.

    FF
     
  13. PAR
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    PAR Yacht Designer/Builder

    How much of the keel, compared to the hull is in the water? By this I mean, do you have a clean separation and vertical, lateral area or is it a built down hull with no real distinction between appendage and hull. If there is a distinct separation between hull and appendage and you have more then half the total draft of the vessel in the appendage portion, you could benefit from some shaping.

    An example would be a hull that has 2' of belly with 3' of appendage under it, for a total of 5' in draft. This would show some gains with shaping. On the other hand if the hull has a total draft of 4' and the belly is 2.5' and the appendage is 1.5', then just round over the leading edge corners and call it a day, unless your sailing is often in very light air, in which case, put a healthy round over or ellipse shape on the leading edge.
     
  14. troy2000
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    troy2000 Senior Member

    Getting a little snotty there, aren't we? When did they pass a law that says you have to be a speed maniac or go home? I'd venture to say that racing was hardly on the minds of the people who originally developed fishing boats, ferries, cargo vessels, warships, yacht tenders or other practical applications.

    The simple truth is that for most practical or recreational purposes, the difference between a flat plate and an aerodynamic (hydrodynamic?) foil isn't worth staying up nights worrying about.

    I don't hand out much negative rep, but I hit you for this post. I see no point in you dumping on this guy, simply because he isn't building a race boat (and I bumped the wrong button, or it would have been signed; I don't give anonymous rep).
     

  15. Leo Lazauskas
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    Leo Lazauskas Senior Member

    In general yes, but it depends on the application.
    On some rowing shells and canoes, it is desirable to generate high lift and low drag at low angles of attack, but for the fin to stall suddenly at higher AoA to prevent "over-correction". That can be achieved by using profiles that are not quite airfoil-like.
     
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