Forward swept keels

Discussion in 'Boat Design' started by idkfa, Mar 7, 2010.

  1. idkfa
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    idkfa Senior Member

    Principles of Yacht Design - Optimum relation between sweep angle and taper ratio pg 104

    To have an elliptical force distribution over the length keel and therefore lower drag, the graph shows for a foil of zero taper ratio between root and tip, it should be swept forwards 20%.

    Has anyone actually put this into practice and to what result?

    What's the paper that the graph comes from?

    Should a keel stall first at the tip or root?
     
  2. tspeer
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    tspeer Senior Member

    Before you get enthusiastic about making a swept-forward keel to get a semi-elliptical lift distribution, you might want to consider whether or not that is actually the lift distribution you want. Free surface effects significantly change both the lift distribution for a given planform shape and also what lift distribution provides minimum drag. The faster the boat goes, the stronger the free surface effects are. In the limit of infinite Froude number, the effect of the surface is reversed compared to what it is in the limit of zero Froude number.

    Instead of acting like a rigid wall, resulting in halving the induced drag, the induced drag near the surface is doubled at high speed. In between, the effect will be highly dependent on the influence of the hull on both the keel and the surface. As a result, the ideal keel lift distribution is less loaded at the surface, rather than having its maximum load at the surface.

    The real condition for minimizing the induced drag of the keel is not its lift distribution but its sidewash distribution. The keel should impart a uniform lateral velocity to the water all along its span. This is true even in the presence of the hull and surface effects. Calculating the planform shape that will give you a uniform sidewash with a given hull at a given Froude number is not easy. But it isn't going to result in an elliptical or semi-elliptical lift distribution.
     
  3. idkfa
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    idkfa Senior Member

    Thanks Tspeer, any ideas on stalling characteristics? And is there a preference in which end stalls first?
     
  4. Brent Swain
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    Brent Swain Member

    Hit a ball of kelp with a foreward sloping keel, or a fishnet, or floating line, and you will definitely stall , dead in your tracks . What works in a tank or theory ,and what works at sea can be two very different things.
     
  5. messabout
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    messabout Senior Member

    Brent Swain you are a sensible and practical man. Any kind of catching device under the boat is a no no.
     
  6. idkfa
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    idkfa Senior Member

    What about this device?
     

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  7. Joe Petrich
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    Joe Petrich Designer

    That's what they make kelp cutters for. Many racing boats have them on their bulb keels.
     
  8. tspeer
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    tspeer Senior Member

    That depends on what you want to happen when it stalls! If the root stalls, the bending moment and heeling moment on the keel will go up as the load is transferred to the tip. So I'd think stalling the tip would be preferable.

    Better yet is not to stall at all by providing adequate area.
     
  9. Boston

    Boston Previous Member

    kelp edging is ok but best to not get hung up on the stuff in the first place
     
  10. Joe Petrich
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    Joe Petrich Designer

    If I were racing and had the choice of a faster keel that caught kelp occasionally and a slower keel that didn't, I'd choose the faster, occasional kelp catcher.

    Nice fish tanks by the way.
     
  11. idkfa
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    idkfa Senior Member

    I was thinking more along the lines of boat handling. Guess the worst scenario is in a seaway. Forward motion is lower than normal and we have a less effective span, with the flow tending towards up and down than the normal lateral flow? Maybe one end is more likely to stall than the other? Think tacking has a lot more to do with momentum, if the speed is kept up then it is a non-issue, so much easier with a heavy boat.

    Hmmm, what’s the additional stiffness requirement on a keel, beyond carry the bulb at 90deg, due to lift? Lift is opposition to gravity?

    My aspect ratio 7+, with a fair % of Disp in the bulb, so already a design challenge.
     

  12. BTG YACHT DSGN
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    BTG YACHT DSGN -sailing is believing-

    Well, IMHO, I think that this 90 degrees load case is the maximum reasonable assumption for transverse stiffness. You should be carefull when choosing the keel foil thickness to meet this transverse requirement, of course.

    Regarding longitudinal stiffness: there are certain requirements for bolt sizes in the ,,25 % area", to deal with possible impact force(refer to the ,,Principles of Yac.." for more details) . But are there any scantlings on this issue- I'm not sure... For sure, however, keel has a lot more stiffness if examined longitudinally ;)
     
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