A round chine design??

Discussion in 'Sailboats' started by xalexc, Apr 24, 2006.

  1. xalexc
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    xalexc New Member

    I've always come from the school of "big keels are good".

    Can a 45' steel round chine design sailboat with ballast be as stable as a keel boat. Can someone give me the basics on how it works.

    many thanks
    Alex.
     
  2. xalexc
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    xalexc New Member

    What I'm wondering is, are they bluewater boats or can you cross oceans in them?
     
  3. Skippy
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    Skippy Senior Member

    Where's the ballast? Do you mean a steel boat with no keel at all??
     
  4. xalexc
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    xalexc New Member

    round chine design

    it has a small keel, I've attached a pic to see.

    does it mean it will roll a lot more, I'm not sure where the ballast is.
     

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  5. Skippy
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    Skippy Senior Member

    Makes a big difference. Round hull with internal ballast doesn't sound very stable at all.
     
  6. xalexc
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    xalexc New Member

    it has 6000 lbs of internal ballast, the boat has a 6 foot draft
     
  7. Vega
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    Vega Senior Member

    I am not an expert, but I will try to give a little help:

    There are different kinds of stability. Probably the initial stability (the one that goes till around 30ยบ) will be better in a well designed fin keel boat (that means that in two boats with the same displacement, the fin keel boat can carry more sail).
    Final stability and negative stability are normally better in a long keel boat, also because they are normally heavier (this means in a very simplified way that a long keel boat has a little better "safety" stability (final stability). That translates in a higher AVS (the non return point of the boat that leads to a capsizing) and in a better capacity to return to the upright position when capsized.
    The photo you provide is very bad and you don't say the displacement of the boat; without that it is difficult to say more. The 6 foot draft looks on the safe size for this kind of boat. I guess that this mean that it is a really heavy boat and that is good for the stability but bad for the speed.

    For general information about stability and the different types of sailboats take a look at these sites:

    http://web.usna.navy.mil/~phmiller/offshore.ppt#1
    http://www.dickkoopmans.nl/uk/index.htm
    http://www.windpilot.de/en/Ra/rayacen.html
    http://www.boatus.com/goodoldboat/stability.htm
    http://www.naese.com/stabilit.htm
     
  8. messabout
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    messabout Senior Member

    Xalexc:
    Vega has some good comments about initial and ultimate stability. You can learn a lot with Dave Gerrs book; The Nature of Boats. He goes into this subject sufficiently well to allow you to make informed decisions. Gerr also discusses roll cycle timing and accompanying comfort factor. Skene's Elements of Yacht Design has good information about ballast, stability, metacentric heights, etc. Both these books are useful additions to any sailors library and both are pertinant to your question.

    For what it's worth; The Mayflower, Nina, Pinta, and Santa Maria had internal ballast. Not to say they were comfortable, but they evidently made it to where they were going.
     
  9. Skippy
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    Skippy Senior Member

    What kinds of sections did those boats have? I've always thought V bottoms would be best for storing the ballast plus lateral resistance.
     
  10. Vega
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    Vega Senior Member

    No, they were more like U bottoms:p Even better for ballast and giving more form stability.

    Xalexc:
    Take a look at the last post of this thread. It shows that is possible to have a sailboat with satisfactory stability with only interior ballast.
    http://www.boatdesign.net/forums/showthread.php?t=9199&page=2
     
  11. Gilbert
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    Gilbert Senior Member

    The old style sailing vessels like the Santa Maria, Pinta, and Nina are reported to be very comfortable boats.
    Also, stability is a relative thing. You could say there are unstable boats, stable boats, and very stable boats. The first category would include something like a one person rowing shell or wherry. In the middle category you might put a common cruising sailboat. In the last you might put a barge for purposes of illustration. All these boats are suitalble for their purposes. And so it is not for anyone to declare that a round bilged boat with inside ballast is somehow unsafe to be aboard. It is reasonable and fair to compare two boats and say boat A is more stable than boat B.
    And sometimes the boat that is more stable is less comfortable.
    Captain Voss safely sailed around the world in Tillicum, an old northwest coast indian dugout to which he added some freeboard and deck and cabin.
     
  12. Ike
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    Ike Senior Member

    It is difficult to make a comparison based on that photo. There is a difference between stabilty, the ability of a vessel to right itself, and roll (or what some call ride). Some vessels roll a lot but are safe and stable and perfectly able to right themself. Roll is dependent on shape of the hull sections, center of gravity (CG), center of buoyancy (CB) and metacentric height (GM). Given the CG, CB and GM are the same, then the boat with more circular sections will roll more, simply because the reserve buoyancy doesn't change as it rolls. Reserve bouyancy is dependent on the righting arm, that is the amount of force tending to right the boat. What that means is that as a boat rolls the center of buoyancy shifts. On a boat with circular sections the amount of wetted surface doesn't change and the cb moves in an arc the same as the shape of the hull so the righting arm doesn't increase as rapidly. In a boat that is wider than deep, or with U shaped sections, as the boat rolls there is more wetted surface and the cb moves more rapidly to the immersed side, creating a longer righting arm, forcing the boat upright. Where you put the ballast affects the CG, but not the CB, so with everything else held constant as the ballast is moved down into the keel the CG lowers and so theoretically you would have a longer righting arm. But you don't want to overdo it.

    A boat that has a really quick snappy roll will rapidly fatigue it's passengers, whereas a boat that rolls over and very slowly rolls back gets them all wet and scares the H out of them. What's best is somewhere in between.

    Let me give you some examples. Icebreakers have almost perfectly circular sections. They roll badly, and are very uncomfortable, but they travel all over the wolrd in a perfectly safe manner. The ships I was on had round bilges but were almost flat on the bottom with some tumble home. The could roll 45 degrees, come right back up and didn't knock you around. They were very comfortable. A newer ship I was on had about the same sections but had a lot of tophammer (superstructure, sail area, whatever you want to call it.) To compensate the designers put the engines and tanks as low as possible and kept as much weight down low as possible. Those ships would snap quickly to one side and throw you overboard, meanwhile rolling quickly back upright. I always thought they were dangerous.

    So, to know how stable your boat would be we need to know some things like where is the ballast CG. What are the sections like in shape, Length, depth beam, amount of ballast.

    If it has keel, whether ballast is in it or not, the keel will help resist rapid rolling as well.
     
  13. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Stability is primarily a product of waterline beam and vertical center of gravity. Chine type is a very minor factor. The boat in the photo looks very seaworthy to me as long as the overall center of gravity is kept low.
     
  14. Gilbert
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    Gilbert Senior Member

    I have spotted the site where your picture is from.
    I would suggest that you not pass up the opportunity to aquire this boat.
     

  15. Mikey
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    Mikey Senior Member

    ...and it has enough roll damping. What about the lateral area of the keel? Does it look lonely down there?
     
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