Sailboat stability...

Discussion in 'Boat Design' started by ErikG, Oct 23, 2003.

  1. ErikG
    Joined: Feb 2002
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    Location: Stockholm, Sweden

    ErikG Senior Member

    I'm having an argument around sailboat stability issues in sweden at the moment and would like your input.

    On a boat w. LOA/Beam at around 3 what gives the largest effect at normal sailing angles at 15-25°? Increasing form stability or adding a bulb?

    Adding a bulb lowers CG a bit, but G_Z does not change to dramatically with a lowered CG. On the other hand increased form stability is not just added without drawbacks, a very wide boat is probably not what they want either.

    Or is there a point in the design spiral where it becomes inefficient to try to inrease stability with either a bulb or increased form stability.
    The argument is that on the newish Bavaria Match they don't have a bulb, I believe that is a result of trying to make her rate okish under IMS, and effectively lessening her stability for rating reasons, or does a keel like that give stability enough for any and all situations?

    Opinions and thoughts please...

    Link to Bavaria Match
     
  2. SailDesign
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    SailDesign Old Phart! Stay upwind..

    ErikG,
    If LOA/Beam is about 3, you certainly wouldn't want to incresae beam much. That is Open Class territory, and could make her uncomfortable as all get-out. A bulb may not do much to help, but it may be the only option, and you should take all you can get if it really has to be done.
    Also look at decreasing mast weight or rigging weight if possible. 10 lbs on the mast is worth 100 lbs (or kg if you prefer) on the keel.
    Are we looking at this because the boat is tender, or because you want to carry more sail?
    Steve
     
  3. ErikG
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    ErikG Senior Member

    Hey Steve, I was hoping that you'd come to my rescue :)

    Nah, just an argument for the arguments sake really, and I hoped to be able to learn something more while I'm at it...

    Bavaria Match is LOA:10,50m Beam:3,25m Displ:4500 kg, Keel 1750 kg

    LOA/Beam approx. 3
    Open boat style ? NO WAY :D just an ugly IMS boat IMHO

    I'm just curious whay she doesn't have a bulb keel, and then we entered into an argument where my enemy :) was saying that having a bulb is meaningless, more or less, and I don't really agree. Sure having a bunch o guys on the rail does good for performance but it sucks for safety...

    am I right?
     
  4. Fco.Lopez
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    Fco.Lopez Junior Member

  5. nemo
    Joined: Apr 2002
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    nemo Naval Architect

    Yes the only reason for that is the IMS rule... IMHO it leads to boats that are not seaworth... take a look at the new IMS 600 Siemens Mobile, it's the ugliest boat I've ever seen, it has no bulb, and the lower part of the keel is filled with foam, instead of lead!
    Anyway, even IOR boats didn't have bulb, but they had a trapezoidal keel made of lead..
     
  6. Stephen Ditmore
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    Stephen Ditmore Senior Member

    You've hit upon the two things that make the most difference, CG and waterline beam. People get focused on displacement, but displacement only matters in as much as it effects CG (it can be shown mathematically that displacement mostly cancels out). Any form of movable ballast or hiking crew matters, of course.

    Adding waterline beam will add more drag unless you plan to reduce displacement. If you reduce ballast in order to reduce displacement you're lowering your ultimate stability in the event of a knockdown. The bulb keel is the better way to go as long as your structural floors and keel attachment are strong enough to support the greater righting force.
     
  7. SailDesign
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    SailDesign Old Phart! Stay upwind..

    And here we see the real difference btween boats built to a rule, and boats built to do what boats should do. I personally hate rule boats, and have never had to design one (wonder why......) purely because I refuse to believe that the way to win races is to design a slow boat that somehow has an even slower rating because it is not safe to sail anyway.
    Biased? Me? Hah!

    Steve
     

  8. gonzo
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    gonzo Senior Member

    A keel with a bulb has more wetted surface than a keel in a hull with slack bilges. I think designs, both shapes and materials, are heavily influenced by formulas and fashion; often to the detriment of real performance. I owned a 34' Wharram. They are the ultimate plywood el cheapo boats. However, sailing at 14 kn was usual. In "The Common Sense of Yacht Design" Herreshoff developed a formula to compare boats that included: cost, draft, real speed, and other factors that affect performance and handling. Ultimately, a rating does not indicate sailing performance or actual stability. Perhaps you can jump out of the spiral and introduce new elements. For example, a dory with side decks and no ballast will float happily at 90 degrees of heel.
     
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