Dr. Gerritsma Righting Moment

Discussion in 'Sailboats' started by SuperPiper, Dec 28, 2010.

  1. SuperPiper
    Joined: Jan 2003
    Posts: 362
    Likes: 5, Points: 18, Legacy Rep: 58
    Location: North Of Lake Ontario

    SuperPiper Men With Little Boats . .

    I tried to reply to a thread started by Yades but it is too old:

    http://www.boatdesign.net/forums/sailboats/mast-compression-loads-21474.html

    So I am starting a new thread for the same subject.

    Yades showed an estimate of Righting Moment proposed by Dr. Gerritsma in 1993. I'm having trouble using the formula. I start to get plausible results if I substitute Sin(phi) for phi and Sin^2(phi) for phi^2 in the length coefficient. I've tried googling Dr. Gerritsma but I can't find a reference to this formula. Can someone help me?
     

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  2. SuperPiper
    Joined: Jan 2003
    Posts: 362
    Likes: 5, Points: 18, Legacy Rep: 58
    Location: North Of Lake Ontario

    SuperPiper Men With Little Boats . .

    I was able to access the thread, but when I tried to reply, a "more than 507 days old" message shut me down.

    I'm very interested in the Gerritsma formula because of its simplicity. However, I'm wondering if there is a typo error because I don't recognize the results.

    Has anybody tried it? Can you lead me to the original Gerritsma document?
     
  3. SuperPiper
    Joined: Jan 2003
    Posts: 362
    Likes: 5, Points: 18, Legacy Rep: 58
    Location: North Of Lake Ontario

    SuperPiper Men With Little Boats . .

    Not much traffic on this thread. Did I mention that I need to know the righting moment so that I can hydrofoil on the boat's canting keel with a kite sail?

    The Gerritsma formula uses basic boat dimensions to estimate righting moment. From that, GZ, AGZ and AVS can be approximated and I'm on my way to computing a STIX for this boat. Is anyone aware of a similar formula?

    I just want to be able to say something like: "Based on the Gerritsma approximation, my boat has a STIX of 19.3."
     
  4. sorenfdk
    Joined: Feb 2002
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    Location: Denmark

    sorenfdk Yacht Designer

    I have some of Dr. Gerritsmas papers somewhere. I'll have a look and see if I can find something.
    In the meantime, there's something else about this formula: The RM seems to be dependant on the speed. This makes me think that it should only be used in certain, specific connections.
     

  5. SuperPiper
    Joined: Jan 2003
    Posts: 362
    Likes: 5, Points: 18, Legacy Rep: 58
    Location: North Of Lake Ontario

    SuperPiper Men With Little Boats . .

    Sorenfdk, you are absolutely correct!

    I finally found a reference to this formula in a van Oossanen article: http://www.swzonline.nl/swz-archief... stability requirements for sailing craft.pdf

    The small boat guys are so used to getting beat up for their size, that I assumed the 2nd half of the formula was a coefficient relating increased stability to LWL. It is actually FSSL, a factor for speed stability loss.

    Shows what little I know.

    I always thought a sailboat was MORE stable with way on, with pressure on the sails. But the article explains that a displacement boat will sail in a wave trough and there is no water in that moving hole to support the hull. The boat is more likely to tip over.

    And this is cool: the longer the LWL the bigger the trough, so the big boats are LESS stable than the little guys. Woohoo!

    The first half of the formula is the static stability approximation. But being perfectly sinusoidal implies that every boat has an AVS of 180 degrees. This is not as practical as I had imagined. Apparently the formula may be close for angles of heel less than 30 degrees.

    Happy New Year.
     
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