Keel Bulb attachment angle

Discussion in 'Sailboats' started by Ron S., May 3, 2023.

  1. Ron S.
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    Ron S. New Member

    I have been trying to educate myself about keel design, but there is a very basic thing that I can't seem to find the answer to. Any help would be appreciated. Use the line drawing as an example. If I draw a line from the tip of the bulb to the tail of the bulb, should that line be parallel to the waterline? Intuitively that would cause the least drag. But I also have heard that the bulb should be angled slightly upward. I know there are many complexities to sailboat design - any general guidelines or common practices I should follow?
     

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  2. jehardiman
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    jehardiman Senior Member

    FWIW, you should never use a bulb unless the rules or the operational depth force you to have one. Based on your PDF, I'd make the foil wider without a bulb unless your GM needed it.
     
  3. Ron S.
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    Ron S. New Member

    The class rules do require the bulb, so if I have to live with a bulb, any input on optimizing the angle compared to the waterline?
     
  4. jehardiman
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    jehardiman Senior Member

    Not the waterline, but the flow lines. FWIW, that keel sucks..big time....for what you/they want it for.
     
  5. gggGuest
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    gggGuest ...

    Its a one design with closed class rules. Are you not required to comply with the orientation of the keel in the kit as supplied?
     
  6. alan craig
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    alan craig Senior Member

    Ron S. hasn't stated but this is a R/C model boat - Odom = one design one metre. Keel and bulb therefore look completely normal.
     
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  7. Ron S.
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    Ron S. New Member

    I used the line drawing of an ODOM out of convenience. It is a one design class with established class rules. I am trying to define class rules for a larger radio sailing model. My question of bulb angle came up while trying to define the rules for the keel on the larger model. I can find a lot of great information and discussions on fin and bulb shape and theory, but have not found anything that addresses my question. I realize every boat design has its own unique design requirements and that the topic is complex. Perhaps you could simply recommend a book or other resource to help me understand better. If I knew then what I know now, I would have studied naval architecture in college - it is far more fascinating to me than what I chose. The question is meant to be very general in scope, not for a specific boat. Thank you for anything you can share.
     
  8. baeckmo
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    baeckmo Hydrodynamics

    You have to think of the main purposes of the combo vertical foil plus bulb. Together they prevent the vessel drifting to leeward, and the bulb weight is supposed to reduce its heeling. Now a cylindrical bulb will generate a lift when moving throught the fluid with an angle of attack. If that lift is adding to the foil lift it may be worth a slight increase in resistance; meaning that there will probably be a "sweet spot optimum" when the bulb is not aligned with the flow.

    You have a similar situation with an aircraft fuselage/hull designed to provide some lift that comes from a slight aoa; "the hull drag is unavoidable, let it do some work instead of just dragging along....".
     
  9. messabout
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    messabout Senior Member

    The surmise is that when the boat is powered by the sail, the bow will be forced downward to some extent. If that is the case, which it probably is, the trick is to angle the bulb in such a way that it assumes zero angle of attack when the bow is depressed.

    It is also a reality that the bow depression angle will vary with different wind strengths and with the point of sailing.....beating, reaching, running, and even with the trim of the sails.
     
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  10. Ron S.
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    Ron S. New Member

    Thank you! I just needed confirmation from people who have studied these things. The point about lift does seem to bear out even in the scale of the models that we race. It is particularly noticeable in strong winds going downwind.
     
  11. SuperPiper
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    SuperPiper Men With Little Boats . .

    If this was the last question on a trigonometry exam and I was completely out of time, I'd GUESS that the upward bias of the bulb was somehow related to (or perhaps equal to) the amount of leeway. OR, if it was a multiple choice exam, I'd pick answer 'B'.
     
  12. peter radclyffe
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    peter radclyffe Senior Member


  13. DCockey
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    DCockey Senior Member

    When sailing to windward the boat will be heeled and making leeway. In that situation the orientation of the bulb relative to the zero heel waterline is not directly relevant.
     
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