From winged keel to bulb keel

Discussion in 'Sailboats' started by Mad for sail, Nov 15, 2011.

  1. michael pierzga
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    michael pierzga Senior Member

    Which is more effective. The endplate of a flat bottom keel or the endplate of a Scheel keel ?
     
  2. Paul B

    Paul B Previous Member

    Adding the inches ahead of the fin will ensure you catch weed, lobster pots, and anything else on the keel and it will stay there.

    You are doing a lot of work and Mikko is doing a lot of analysis, but the problem is you are using a poor section for the bulb. You should consider something much different.
     
  3. Mad for sail
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    Mad for sail Junior Member

    Ok Paul, I am open for suggestions.
    You don´t like the bulb I designed or you don´t like to usea bulb?
    What you suggest to add to the fin?
     
  4. Paul B

    Paul B Previous Member

    I am saying if you are going to have a bulb you should be looking at a different section than the one you are using.

    The section you will want will be significantly lower drag than the one shown in your model. Note this does not mean it will be thinner than the one you are currently using.
     
  5. Joakim
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    Joakim Senior Member

    Maybe not, but what would be better and would it also work as well without seaway? This type of keel is used in many successful designs since about mid 90's (X-362 Sport, Dehler 33, IMX 38/40 etc.) to new designs (J/111, First 35, X 35/34, several Arcona models). Even many of the T bulbs have flat bottom.

    I think the flat bottom has several advantages over a round bulb. For the same draft it has lower CG, deeper lifting surface and better end plate function.

    It was funny to read comments about the keel of J/111 when first pictures where shown. Many people ranted about its keel being old fashioned and that a T keel would have so much been better. No that J/111 has shown its performance both up and downwind nobody says anything about its keel.
     
  6. sorenfdk
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    sorenfdk Yacht Designer

    ...but it most certainly means that the leading end should be much finer ("thinner").
    Try searching these pages for bulbs - I'm sure you'll find something usefull! And if not, try a NACA 67-020 (or maybe thinner).
     
  7. Paul B

    Paul B Previous Member

    Not a bad suggestion.
     
  8. Mad for sail
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    Mad for sail Junior Member

    Ok but how do I design it? Is it a half bulb attached to the bottom of the fin?
     
  9. Joakim
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    Joakim Senior Member

    I'm not a keel designer and I have never designed one, but I it would be very close to following:

    1. Each horizontal cross section is a NACA profile. The bulb is made by changing the chord length and/or the thickness of the profile.

    2. The vertical cross section of the bulb is almost triangular (sides a bit rounded) or a cut triangular (= vertical near the bottom of bulb). This will depend on the needed volume. Avoid too thick NACA profiles. A heavier bulb is deeper not much thicker. Typical seems to be 13%, but some heavier ones are 16-18%.

    3. The leading edge of the keel is as it did not have a bulb. Some have a bit thicker leading edge at the bulb.

    4. The leading corner of the bulb/keel is rounded and the sharp edge of the bottom starts from 1/4-1/3 of the bottom chord. (J/111 keel is less rounded)

    5. The trailing edge is sharp also on the top of the bulb. The trailing edge can be a straight line leading to a single sharp point at the end of the bulb (e.g. J/111) or it can be curved leading to an almost vertical trailing edge at the bottom of the bulb (e.g. several X-Yachts).
     
  10. Mad for sail
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    Mad for sail Junior Member

    OK, how do you like my new L-Keel?
    It´s 40% deeper than the wing one and I guess some 200Kg lighter.
    Suggestions before I dive into my worksatation?!
     

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  11. Joakim
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    Joakim Senior Member

    Looks good, but I have the following questions or suggestions:

    1. Is 148 mm the maximum thickness? If yes, it is much smaller than typical 13% of the chord (2424*0.13 mm = 315 mm)

    2. Compare the CG and longitudal center of lateral effort to the original keel with wings and try to keep them equal, if the boat floated and behaved OK. You can move the bulb by modifying the angle of the leading edge of the new part, if necessary

    3. The bulb is very long for the size of the boat. Around 1.5 m is more typical. Your keel fin has much longer chord, thus the keel will have a shorter "L" with a "normal" size bulb.
     
  12. Mad for sail
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    Mad for sail Junior Member

    I will reduce the cord to about 1900 to give it some simetry to the top.
    The maximun thick of 250 will give me about 1000 kg which is the desired weight.
    Trimming the bulb profile to grant same CG won't be that easy ...
     
  13. Joakim
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    Joakim Senior Member

    You don't need to have exactly the same CG. Just remember that moving it, will change the trim of the boat. If you move 1000 kg 0.4 m towards stern, stern will sink about 20 mm and bow rise about 30 mm (calculated for another similar size boat). You certainly have CG closer than that.
     
  14. Mad for sail
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    Mad for sail Junior Member

    Checking the current keel dimensions I concluded that the fin profile is basically 63xx family with thick % chord from 6,5% to 8%.
    I will be building the bulb using 63xx profiles from top to botton increasing from 8% to 13% as discussed before.
    In order to reach all those parameters yet reaching to 1ton I need the chord lengtth in the bottom must be at least 1900.
    It will be interesting to see how it behaves in the CFD simulation.
     

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  15. Joakim
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    Joakim Senior Member

    Do you have enough volume? The keel is now quite much lighter than the wing keel.

    If you need more, you could use the bottom section as is for the lowest 15-20 cm and then taper to the keel faster. Or you could use a profile thicker than 13%.
     
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