Long keel leading edges

Discussion in 'Boat Design' started by tansails, Jan 22, 2006.

  1. tansails
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    tansails Junior Member

    Some full keeled sailing vessel keels are NACA foil section, others are square fronted parallel sided slabs, like the Colvin schooner shown here.

    The square front is certainly easy to build but what is the drag characteristic of such a keel ? All the text books recommend a NACA or similar section.

    By how much would performance suffer? Can anyone explain please?


    Thanks
    Michael Bailey.
     

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  2. sharpii2
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    sharpii2 Senior Member

    The NACA foil shape will cut down on your drag. But that's not the whole story.

    The NACA foil may also move your CLP (Center of Lateral Presure) forward as well. This could make your boat want to round up into the wind more.

    I design all my long keels deliberately blunt for that reason. Tapering the trailing edge would cut down on drag somewhat too. But fairing that end will do nothing to the ballance of your boat, so I would recomend it. (especially if the rudder is not attached)

    Besides, a NACA foil shape may be unacceptably wide (bouyant) in the middle as well.

    As for adding windward lift, I sincerely doubt that a foil with that low of an aspect ratio would be effective at all. With a long keel, the leeward side pushes the water aside while the windward side just agitates it. For that reason it is no where near as effective to windward as a short deep keel of the same area. But it has other vertues, hence my interest in it.

    Bob
     
  3. tansails
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    tansails Junior Member

    Thanks for the reply Bob

    Are there any guidelines as to what sort of shape a full keel with a blunt leading edge should be ? Just out of interest and for future ref.

    I presume a gentle curve form a blunt leading edge leading to a long Parallel sided section and finally a tapered aft end. What are your thoughts or anyone else as well please.

    Michael
     
  4. MikeJohns
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    MikeJohns Senior Member

    The NACA foil reduces drag particulalrly when the keel operates at an angle to the direction of travel through the water. The profile gives a laminar flow for the leading part of the keel even on a long keel design there are advantages in a reasonable foil section. The fairly brutal square LE that you show will give an immediate turbulant separation effective from the LE of the keel. However there are many long keeled boats that perform happily at hull speed that have blunt LE's to their keels, the extra resistance is only really a problem in lighter air when going to windward.

    Bob correct me if I am wrong but my understanding is that the problem of CLP shift on a long NACA keel is alleviated by means of a parallel mid body inserted in the foil section.
    Actaully it is not feasable to use a full NACA non modified foil as the ideal at say 9-12% would give far too thick a section if the foil was true to shape so designers use the lower end (9%) for a shorter keel which is then chopped and extended by means of a parallel mid section giving a NACA LE and TE.

    Also have to look at fabrication issues.

    Hope this helps
     
  5. PAR
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    PAR Yacht Designer/Builder

    There are lots of guidelines but most will not apply to your application specifically.

    I agree mostly with Bob's comments, though the leading edge wouldn't be hurt any if it had some parabolic in it. As Bob pointed out, the lee side surge would be reduced, but the flow would stay attached longer. You'd have better penetration and efficiency, but this likely wouldn't be noticeable. The full keel has remarkable nullifying qualities about it. It's unlikely NACA shapes could be added to your keel without great amounts of material being added to the sides of the deadwood. There have been studies on flat sided appendages, but again not in the aspect ratios typical of your yacht. I do think low aspect appendages generate lift, but not on the scale the taller fins do. The center of lateral plane will not move very far, on that hull, if the leading edge was to receive a parabolic rounding.

    Personally, I think some gains could be incorporated by shaping the leading and trailing edges of the boat, but they would be hardly noticeable. You may get a slight acceleration increase after a tack, slightly better pointing ability, but these things could easily be accounted for in better sail trim or boat handling. The boat you show (is this your vessel?) is a well burdened craft, capable of good motion, a secure feeling in most conditions and a fine passage maker or home afloat. The very small gains you may experience in efficiency aren't typically worth it in a yacht like this. If you need a tenth of a knot more speed to windward or a 1/4 of a second faster tacks, in order to win the local 'round the buoy cub, a racer will employ changes to their appendages like this, all the time. Your boat may be best served with a good sanding on her bottom, high quality paint, faired thru hulls and good concentration in sail set and boat handling, rather then putting the reciprocating saw and power planer to the deadwood.

    How does it sail now? Are you getting beat up by a buddy with a Tahiti ketch?
     
  6. FAST FRED
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    FAST FRED Senior Member

    If you look at flat plate aircraft lift charts they work fine , but are peaky.

    Many will outperform (higher lift) than the NACA at higher angles of attack.

    For me the real advantage of a blunt sloping keel edge is that it can be strong

    and perhaps the boat will climb up on the errant log or Sealand box, rather than smash to a stop.

    For some boats the ability to SURVIVE is more important than another .0001 Knot or .001 degree to wimdward in smooth water.

    FAST FRED
     
  7. jehardiman
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    jehardiman Senior Member

  8. MikeJohns
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    MikeJohns Senior Member

    J Hardiman

    The pictured keel doesnt even have rounded corners.

    From CFD. data and tank test results gleaned since the above thread I would add that at low velocity the foil section does give more lift to windward and the foil acts more like its higher aspect cousins. As the water velocity rises the LE can be squared off .
    Interesting that on a parallel sided keel with square LE, CFD confirmed with tank tests showed a big discontinuity where lift was lost at around 3 degrees leeway and became negative (this at lower Reynolds numbers) the lift then returns as the leeway increases, something to do with the large separation vortex forming off the front face. Obviously lots of parameters here though.

    A foil section will I think not exhibit any odd behaviour if you can be bothered getting that LE rolled in metal construction.
     
  9. jehardiman
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    jehardiman Senior Member

    I see the corners knocked off, and that's all it takes. Data I have shows that there is a big difference between a 0.05t radius and "pure" square corner with the size of the seperation vortex and loss of lift (especially in the transition region ~ Re 10^6). By the time you get to 0.1t radius there is very little difference from an ellipitical LE for c/t greater than ~15 or so.

    Anyway, as the thread I pointed to concludes, at high c/t ratios CB, CG, and/or construction concerns drive section shape for most low aspect sailing keels.
     
  10. sharpii2
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    sharpii2 Senior Member

    The problem of CLP shift to the forward end of the keel is caused by laminar flow. The blunting of the edge is done to deliberately cause turbulant flow. The problem with laminar flow is that it doesn't go on forever. It starts turning more and more turbulant as it makes its way aft. This causes the aft end of the keel to have so little lift in compareson to the forward end, that it is practically irrelevent. Unless you have all your sail up front, your boat is going to round up mercilessly under this condition. And what you save in keel resistance you may find yourself more than making up for in rudder drag caused by trying to keep the boat going straight.

    Blunting the forward edge is but one fix. Putting an aft downward slope on the bottom edge of the keel, some times called 'drag', is yet another. The idea of drag is to get the aft end of the keel into some clean flow and thereby cut down on the turbulance back there. It moves the keels CLA aft as well. If you look at a lot of 'traditional' designs you will probably notice most of them have at least some drag in their long keels. I have thought of the idea of having the drag of a long keel serving to facilitate the loading of a boat onto a trailer. The keel rollers would support all the weight and the 'bunks' would merely hold the boat upright.

    The picture posted shows a relatively fair downward curve for the front of the keel. It is probably for this reason the leading edge was left square. Much of the water confronting it will follow the bottom edge down, creating a lot less drag than one might think.

    If I had a more vertical stempost continuing down to become the leading edge of a long keel, I would give it a half round.

    It would be interesting to have some tank testing done so there would be more scientific evidence on these principles.

    Bob
     

  11. MikeJohns
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    MikeJohns Senior Member

    My sentiment exactly. Much mythology can attach itself to design issues and its always hard to know just where things stand without a nice bit of actual data. Long keel hydrodynamics are not covered in any text I have been able to lay my hands on. Marchaj refers to the success of much blunter LE's in the racing circuit with moderately full keels but goes no farther.
     
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