Purpose of a centerboard/daggerboard on a small sailboat?

Discussion in 'Sailboats' started by CardboardKing, Oct 12, 2014.

  1. CardboardKing
    Joined: Mar 2013
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    CardboardKing Junior Member

    Who is the ignorant one?

    Okay, I need an expert opinion on something. I already know the answer, but my friend said, "Ask any expert," so I am.

    My friend, who claims to have over a decade of sailing yacht-building experience, told me that the only purpose of a centerboard/daggerboard on a small sailboat is to keep the boat from heeling over 90ยบ when hit with wind abeam.

    I told him that the main function of a centerboard/daggerboard on a small sailboat is to provide a surface for the water to apply a force to so that the force of the wind is vectored into the desired direction of boat travel (i.e. in the direction that the bow is pointed). Otherwise, the boat would just float downwind.

    We argued about this for several minutes. He adamantly insists that there is practically no force from the water against the center/daggerboard on the leeward side of the board. He claims that all of the force against the board from the water is from the same side as the wind because, due to the presence of its hard chine alone, the boat wants to rotate around the point where the leeward side of its hull meets the water and (according to him) the opposing force of the water on the centerboard prevents that.

    Even when I presented him with the following diagram, he informed me that Fwater 2 is so small as to be non-existent and that Fwater 1 is the only force that exists on the centerboard. And, in fact, that Fwater 1 is the only reason for the centerboard's existence in the first place.

    Now, again, he told me, "Ask any expert on saliboats, sailing or sailboat design, and they will tell you that the only purpose of a dagger or centerboard is to keep the boat from capsizing when the wind hits the sails."

    Okay, experts. Whay say you? Who is the ignorant one? My friend, or me? :?:

    Thank you, even if I turn out to be the one who's wrong. (Although I'm pretty sure I'm not.)

    Attached Files:

  2. rcnesneg
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    rcnesneg Senior Member

    Uh... A daggerboard or centerboard (unweighted) do one thing: Stop leeway. A weighted keel or centerboard does two things: Stops leeway and keeps it upright. A canted keel does one thing: Keep it upright. An unweighted daggerboard or centerboard is useless for balance, it actually makes it a tiny bit worse because the daggerboard wants to float.

    Chines have nothing whatsoever to do with any of this.
  3. gonzo
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    gonzo Senior Member

    Agreed. The only thing a daggerboard does is to provide a surface that produces a force (technically a reaction to a force).
  4. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    If we speak with technical rigor, a surface can not produce any force. A body immersed in a moving fluid undergoes forces produced by the fluid. The shape of the profile of the body may change in magnitude and in direction, the forces produced by the fluid.
  5. upchurchmr
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    upchurchmr Senior Member

    I don't think the boat cares about the verbal description, no matter how accurate. Certainly the friend won't get the technical rigor.

    Cardboardman, you win and you knew it. I assume your friend is in a non technical occupation?
  6. Fanie
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    Fanie Fanie

    Yes it can - if it's magnetic. Getting it to do something continuous is something else ;)
  7. Ike
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    Ike Senior Member

    This magnetic stuff has nothing to do with the posters questiion.

    You are right and your friend is wrong. Without a centerboard/daggerboard the boat would sail sideways.
  8. Steve W
    Joined: Jul 2004
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    Steve W Senior Member

    Actually the centerboard or daggerboard does 2 things. As everyone has already pointed out, it prevents leeway, allowing the boat to sail upwind, but it also provides balance, as in center of lateral plane to balance the center of effort of the sail plan which is also necessary to allow the boat to sail upwind.

  9. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Cardboard, I hope there was some beer on this bet, because you can have a good drunk tonight. The appendage doesn't prevent a capsize, even if it's ballasted. Hell I've capsized plenty of ballasted boats, so your buddy owes you a few beers.
  10. messabout
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    messabout Senior Member

    I will cheerfully second Pars answer. Your friend is insufficiently schooled in the dynamics of sailing boat, the principals of physics, and trigonometry.. Enjoy your beer and your victory.
  11. Petros
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    Petros Senior Member

    all correct. I think your friend is confusing the fuction of a weight keel boat with an unweighted dagger board or centerboard. in an unbalanced boat the crew has to use their weight to counter the force of the sail. In a large weighted keel sailboat, the weight on the keel does that. both prevent lee way, so he was wrong in both circumstances.

    Tell your friend not to build any more boats unless they were designed by someone else that knows what they were doing.
  12. Waterwitch
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    Waterwitch Senior Member

    Is this by chance for the cardboard Puddle Duck Racer you were going to build? Not to drift your thread but was curious as to what ever happened with that project.
  13. CardboardKing
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    CardboardKing Junior Member

    No, actually. What happened with that project was that I didn't have a totally enclosed indoor space to work in and the cardboard got rained on a few too many times before I was able to waterproof it. The hull I originally built still exists, and is surprisingly intact considering the number of times it's been rained on, but it is not worth repairing and/or finishing at this point. As soon as I can get myself and a nice large vehicle out to the furniture store again, I am going to pick up some new cardboard and start over.

    I only had a couple of weeks' worth of work in the first one, and the experience I gained will help me make the second one even better. And this time I intend to avoid rain.

    And, although it usually feels pretty good to be right, this time I am made uneasy because I just gave this friend over $1,000 in materials to build a boat for me. Until that conversation about centerboards, he sounded like he knew exactly what he was talking about. Now, I wonder how much else he doesn't know...
  14. upchurchmr
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    upchurchmr Senior Member

    What boat is he going to build for you?

  15. Ike
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    Ike Senior Member

    He may be a good boat builder, but he just doesn't understand some of the physics that the designs are based on. That doesn't necessarily mean he can't build a good boat. I have met quite a few boat builders over the years who could build a really beautiful superior boat, but didn't know the basic engineering that went into the design, but they could read plans and knew how to put things together, and were master craftsmen. The best thing for you to do is keep track of what he is doing, and ask a lot of questions.
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