keel fairing

Discussion in 'Sailboats' started by rapscallion, Jan 12, 2007.

  1. rapscallion
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    rapscallion Senior Member

    I have a laser 28 and I would like to fair the keel. After reading some of the threads concerning this I see there are a lot of people her that know a lot more than I do about naca foils. Where would I start with this project?

    The keel is not symmetrical, so it is difficult for me figure out wich naca foil to go with. The information I have on leway angles comes from an IRC rating that when back calculated to phrf gives a score of 150. There are sailore that can easially sail the boat to a 126 rating, so I feel these numbers are suspect.
     
  2. Tim B
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    Tim B Senior Member

    I'll assume you have a single keel, not two assymetric keels.

    A single Assymetrical keel will cause you to go faster on one tack than the other. In normal sailing this isn't a good thing, though it is used extensively in speed sailing.

    Unless you really want to get involved in the very detailed design, an NACA-0010 or 0012 section is probably as good as any. The thickness will, obviously, depend on the current keel shape. Get a few profiles printed at scale (QCAD is a freee 2D CAD package if you need one). Back them with something rigid (eg. thin ply), cut them out (carefully) and offer them up to the keel. The gap between the template and keel will need to be made up somehow.

    Cheers,

    Tim B.
     
  3. Paul B

    Paul B Previous Member

    Not True. I doubt you'll find many successful racing keelboats of the Laser 28 type with either of those foils used for their keels. Rudders, maybe, keels, no.


    Rapscasllion might want to consult the class (http://www.laser28.org/) for the correct offsets and tolerances.
     
  4. Tactic
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    Tactic Junior Member

    Paul B,could you give examples of foil types you think would be of use in this case and similar cases? I am thinking of a new keel for my boat.
     
  5. Tim B
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    Tim B Senior Member

    I thought someone would say that. It depends where you want the maximum performace. Downwind with little leeway, a laminar section may well be better. Turbulent se4ctions may be better up-wind. It all depends on the rest of the yacht (and keel planform area etc.)

    Pure theory may suggest one foil is much better than another, in reality, the slightest bit of dirt or marine growth will reduce the advantage considerably.

    Keel design is not a closed book, there is a lot more work to be done. However much work is done into yacht balance , keel and rudder design, there will still be personal preferences and an awful lot of misguided advice.

    Tim B.
     
  6. rapscallion
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    rapscallion Senior Member

    I couldn't find anything in the class rules concerning the class keel shape. I believe the boat has downwind performance; I was hoping to choose a foil shape that would help with uphill performance. Any suggestions on that foil shapes to look at first?
     
  7. Crag Cay
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    Crag Cay Senior Member

    www.computerkeels.com

    These people can provide you with a keel profile template for the Laser 28. Due to the planform of your keel you will need four templates of different lengths. Total cost 250 dollars or so.

    Look under the listings for 'Universal Templates'.
     
  8. Mikey
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    Mikey Senior Member

    Be careful with trying to modify anything to gain performance in this area if you are not experienced and have quite a lot of knowledge... NACA-0010-12 is always a decent choice but will it be better than what you already have?
     
  9. Paul B

    Paul B Previous Member

    Yes, you seem to be pretty well versed in that. Your suggestion that the 0012 is "as good as any" is right up there with your earlier idea that a new I14 should be shaped like a Lark.

    If you would make just a small effort to see what is going on in the REAL world, instead of sitting on your schoolboy arse in front of a computer, you might find that competitive race boat designers figured there were better sections over 30 years ago. I'm not talking custom sections, they are standard NACA offerings.

    More recently some designers have chosen existing Eppler sections, before moving on to doing custom sections. At this level it is difficult to quantify the gains, but it is pretty easy to see the 0012 section doesn't work well for this application.

    Put a 0012 section on one boat, and one of the better foils on the rest of the fleet, and the 0012 boat will not be able to hold her lane upwind. That is a fact.
     
  10. Paul B

    Paul B Previous Member

    The class rules do refer to the keel and rudder complying with drawings, so the class must have them available.

    If they can't help you maybe you can contact Bruce Farr and Associates. They drew the plans, and should be able to provide the keel and rudder drawings for a small administrative and copy fee, plus postage.
     
  11. yokebutt
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    yokebutt Boatbuilder

    Since the Laser 28 was designed by proper naval architects, there won't be much gain in a section different from what was designed for it. Some boats (J24 notably) that have the keel (or rig) in the wrong place to begin with or had an inappropriate section chosen are another story.
     
  12. rapscallion
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    rapscallion Senior Member

    Thank you all for your input and suggestions. Contacting the class association is a good idea.
     
  13. RHough
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    RHough Retro Dude

    It's been a few years since I've done any aircraft design ... I don't remember any Eppler sections that had a drag bucket much wider than the 6 series. Has anyone looked at Selig sections?

    Here's a graph that points out the difference between a 00 series and a 6x series foil
     

    Attached Files:


  14. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    For a small keelboat on a trailer where you can polish the keel before every race it may make sense to use a 63 profile, on a dingy, wouldn't you lift the board downwind? And on larger boats you have antifouling and fouling that probably ruin the more advanced profiles?
     
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