Bulb Keel Shape, Volvo Open 70

Discussion in 'Boat Design' started by ICE, Feb 24, 2009.

  1. ICE
    Joined: Feb 2009
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    ICE Junior Member

    We're sailing a 28' boat, called a brøker (see picture here)...
    We're planning to add a bulb to the keel - with a weight of 80kg... (the keel is atm only 385kg, and should have been 510kg)

    I've been unable to find the bulb design used on the Volvo Open 70... The bulb seems squeezed, so that it's wider then tall - unlike a normal "drop-shape keel"....

    Could anyone tell me if the V70 bulb shape would be better then a "standard bulb" for our boat...

    Any information regarding the V70 bulb are welcome!

    more picture of the boat can be found on www.teamice.dk under "galleri"

    I hope someone can help us out...
    Thanks!
     
  2. Tim B
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    Tim B Senior Member

    Hmm. Interesting project. Given that it's a class boat, has anyone else done this? and how did you come to have a boat short of 130Kg of keel???


    Anyway. You have a few things to worry about here, before you start on the keel shape.
    Firstly, do you actually need a bulb? could you add the ballast with faired cheek plates on the existing keel?
    Secondly, where do you need the ballast to get the best handling and performace?
    Thirdly, if you need part of the bulb forward of the bottom of the keel, what about fowling?
    Fourthly, how does the structure hold up to having the extra weight on, with standard safety factors?

    When you've answered those questions, then start worrying about the shape. If you want anything like a definitive answer on the keel, then you'll need to get someone with some CFD knowledge to run a series of bulbs. That can be a time-consuming process.

    Hope this helps, I'm not trying to discourage you, but just going through the standard engineering methodology.

    Cheers,

    Tim B.
     
  3. alberto88
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    alberto88 Junior Member

    I think you are much smarter to rebuild the entire keel and do similar for sportboat, with very heavy bulb, on a lightweight keel structures, made carbon or similar.

    Look up Thompson boat have a similar size sort of keel to this; shape of bulb seem to be moderate importance but some variance; just look howe sport boat are do and then do similar for that.
     
  4. SViau
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    SViau Naval engineer / Designer

    Don't forget these bulbs are made of tungsten (Open 60"), which allows to give them a shape that lead could probably not accept for strength reasons.
     
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  5. ICE
    Joined: Feb 2009
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    ICE Junior Member

    Thank you for your answers...

    Tim B:
    The boats weight is normally 1600kg, with a keel weight of 500kg. Our boat is only 1200kg with a keel on 385kg.

    The boat is extreme fast, and very demanding to sail – it sails 12knots in only 12knots wind with spinnaker.

    The boat is very “touchy” and if one man goes down to trim the gib, the boat WILL react. We are only 4 men, and want to keep it that way – unless it’s very windy. We simply seek to add as little weight as possible, but with the most “counter affect”…

    The 80kg bulb, is the maximum amount of weight we can add before the penalty is to big… (Keel Weight will move from center to low or “bulb weight”)… But by only adding 80kg – we will only get a few seconds in penalty. (according to Danish rules).

    There will be no problem with the structure to hold the extra kilos.

    Alberto88:
    We do not want to change the keel from a middle-center weight keel to a bulb-keel… The penalty is to big…

    SViau:
    Thank you for pointing this out...
    We have 150kg of lead we’re planning to melt for the new bulb… We removed these kilos from the center of the boat early last year…
     
  6. fastwave
    Joined: Jan 2007
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    fastwave Junior Member

    Just take a NACA65 profile and use a body of revolution. That way you can cat the lead in a pipe and then take it to a lathe with a 2D profile (very cheap). Use a Length/diameter of more than 5

    If you want to squash it then go ahead but then you need a mould etcetc
     
  7. ICE
    Joined: Feb 2009
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    ICE Junior Member

    I'm very curios about what will happen, if we squash the bulb. Is it only "possible" to do if you have canting keels, or will it also be a optimization to do on a "standard keel"...

    Do you have some good references on the NACA65 profile?

    Thanks...
     
  8. fastwave
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    fastwave Junior Member

  9. fng
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    fng Junior Member

    thats quite a small bulb, aproximately 7 litres of lead or about 840mm long for a round 65-018 , or 1000mm long for a 65-015.
     
  10. ICE
    Joined: Feb 2009
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    ICE Junior Member

    keel design

    I'll try and post my keel design v1 tomorrow...

    - With the 80kg keel, the keel will still be a standard-keel... Not even a low-weight og a bulb-keel - this is why we try to keep it around 80kg

    If we squeeze the keel, we migh have to remove 3kg - not that it matters much in the end... The 80kg is still like have an extra man for instant counter weight everytime
     
  11. ICE
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    ICE Junior Member

  12. fastwave
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    fastwave Junior Member

    Looks like it could be a bit longer and more slender. L/D of 5ish if for an axisymmetric body submerged away from the free surface and in isolation. IN reality due to the free surface and lifting fin the optimum lies above 5.

    So if you wanted you could push it a bit more in that direction.

    I presume LCG change etc is all accounted for to have an ok trim on the boat. 80kg is not much anyway. just an extra person sitting on the side
     
  13. strather
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    strather Junior Member

    actually what you did looks pretty good! had u considered to make the trailing edge of the bulb a beaver tail like?? (with a quite wide ending it might increase the effective aspect ratio little bit more) Can u let us know of the overall results in the performance / stability??
    cheers!
     
  14. capt vimes
    Joined: Apr 2009
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    capt vimes Senior Member

    if you take a standard planform of a keel and add a bulb to it... don't you decrease the effective draft by doing this?
    if so - this might lead to a smaller lateral-plan and therfore lesser sideforces from the keel resulting in more leeway - isn't it?

    i am no NA but this is something i came accross in larsons + elliasons 'principles of yacht design'.....
     

  15. strather
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    strather Junior Member

    @ capt vimes: In fact what you say is correct. but it very much depends on the shape of the bulb itself along with other parameters as well. A round bulb might lead to decreasing the effective aspect ratio because it may actually make the flow from the high pressure side to the suction side easier thus encouraging the spanwise component of the flow, leading to increases in induced drag. However, and this is exactly why I asked the guys if they considered a beaver tail squashed bulb shape, the latter tends to keep the flow more perpendicular to the span ... decreasing the induced drag ...
    But still, judging from the appearance of their work I innocently presume that they had significant benefits in terms of stability whereas the increase in induced drag (IF any) and the undoubtable increase in frictional resistance would not cause a remarkable draw in performance. I am curious though about the results if they actually managed to notice something objectively....
     
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