Pivoting centerboard- Daggerboard position

Discussion in 'Hydrodynamics and Aerodynamics' started by rturbett, Jul 4, 2020.

  1. rturbett
    Joined: Aug 2005
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    Location: FINGER LAKES, NY

    rturbett Senior Member

    On our shark catamaran, we have pivoting centerboards.

    What happens of the centerboard is swung a little father forward than intended?
    Curious what effect this has on performance. Any thoughts?

    Thanks,
    Rob
     
  2. Will Gilmore
    Joined: Aug 2017
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    Location: Littleton, nh

    Will Gilmore Senior Member

    Swinging centerboard and swing keels have the advantage of slightly changing the center of lateral resistance (CLR). The partially raised board moves the CLR aft and the fully lowered board moves it forward.

    The relationship between the CLR and the CE (center of effort) are what balance the performance of your boat under sail. You can also adjust the CE in several ways. By moving the CLR forward, such as in fully extended, you tend to increase weather helm. This helps when flying bigger headsails, for example, because those tend to move the CE forward, which increases lee helm.

    Typical relationships between CLR and CE for mono-hulls, is between 5 to 15% of the LWL, CE ahead of CLR. I don't know if that's any different for multi-hulls. I would think the extra width of a leeward pontoon coupled with the more upright attitude of a multi-hull would give them more lee helm than a mono-hull. So, the CLR should ideally be more forward, if possible.

    There are other people on here more qualified to give a description of this dynamic, but that is how I see it.

    -Will (Dragonfly)
     
  3. messabout
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    Location: Lakeland Fl USA

    messabout Senior Member

    Will; this topic has been bashed around and argued in the past. If you put the CE ahead of the CLR the boat will have lee helm which is undesirable. As the sheet is eased the CE moves further forward which makes the lee helm even more prevalent. If the lee helm is large, tacking will become problematic......worst case scenario, an unplanned jibe. Sketch a vector diagram to see whether the CE needs to be before of aft of the CLR.

    With a boat, or a cat like a Shark, fore and aft trim will change the location of the CLR. The hull itself contributes, to some extent, to the resistance to leeward drift. That is to say that the hull has some lateral area and trim will change the location of the center of that lateral area. The board is almost always more influential than the hull in countering side forces caused by the sail. The immersed lateral area of a hard chined boat, like a flattie, will be somewhat more effective than a round chined boat, thus a bit more sensitive to trim.
     
  4. rturbett
    Joined: Aug 2005
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    rturbett Senior Member

    Thank you both!
    plans have trailing edge at 90 to hull, and the boat sometimes has too much weather helm, so I will stick to the plans!
    Rob
     

  5. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Rob if the boat has too much weather helm the center of lateral resistance needs to move aft a little bit. Thus, pivoting the board aft or less than 90 degrees, may relieve excessive weather helm. It is generally not advisable to adjust the elements so much that you have a completely neutral helm. Keep a slightly perceptible weather helm so that you normally use just a few degrees of rudder. With a completely neutral helm it is more difficult to "feel" the boat.

    With that said, it is possible that your sail is contributing to the problem. If the sail is blown out or is too baggy the CE often moves aft and increases weather helm. You may be able to adjust some of the sail camber and position of the camber by experimenting with mast bend, downhaul tension, outhaul tension, and vang tension. That is not all............When the boat heels, the position of the CLR may move one direction or the other. Also fore and aft trim will influence the helm feel because it moves the CLR.

    Good luck.
     
    Will Gilmore likes this.
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