Sailboat Hull Design

Discussion in 'Boat Design' started by C Khaleeli, May 18, 2017.

  1. C Khaleeli
    Joined: May 2017
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    C Khaleeli New Member

    What kind of hull is better for a sailboat: Flat bottom or multi-chine? What are the benefits? What kind of hulls are best for offshore/imshore racing/cruising?
    Last edited: May 18, 2017
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Welcome to the forum.

    Buy the book "The Nature of Boats" by Geer, which will give you an overview to your questions. Ted Brewer also has a good beginners book, which can be found in the online book store here. The questions you're asking aren't answerable, except in the most basic sense and there's no such thing, as the best hull form for any of the places you've listed. Some are better than others, but all will have good and bad points to consider. Essentially, your questions amount to asking what car is best for driving, which is so broad and undefined a set of questions, that no real answers are possible.
    Last edited: May 18, 2017
    terrnz likes this.
  3. C Khaleeli
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    C Khaleeli New Member

  4. fredrosse
    Joined: Jan 2005
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    fredrosse USACE Steam

    There is a previous thread titled "Displacement Speeds - Number of Chines", but it was a typo, so look up "
    Displacement Speeds - Number if Chines"
  5. terrnz
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    terrnz Junior Member

    Have you done much sailing?
  6. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    Flat bottoms are almost always resorted to as a cost saving means. It cost far less money to make a flat bottom hull than it does any other type, especially if the boat is not made out of fiberglass. The reason it cost less is simply because it takes less labor and often less skill to build. It also thakes less labor and skill to fit one out with a deck and interior.

    Flat bottom boats usually have either shallow or deep underwater hull sections.

    The shallow ones give the greatest amount of initial stability which, with a sailboat, means better sail carrying ability per displacement. That's the good. The bad is that such bottoms tend to slam and pound ferociously in any kind of a chop. They slam harder than any other bottom type, and can slam to the point of destruction. For this reason, flat bottom boats with shallow sections (which are 1/10th as deep as they are wide, or less) are best used in protected waters, such as rivers and small lakes.

    Flat bottom boats with deeper underwater sections, which are, say 1/5th as deep they are wide, tend to pound and slam far less, but have much lower initial stability. For a sailboat, they are likely to need ballast, to have adequate stability to carry sails. Even then, they are likely to heel rather sharply, which is good, because then the corner formed by the bottom and the side form a "V" which tends to slice the water rather than slam it. Boats with this type of flat bottom can be ocean worthy. But, compared to boats with with more sophisticated bottom shapes, they tend to be mediocre or even poor performers. This is for two reasons: One, this type of bottom has more surface area than more sophisticated types, so has more friction drag per displacement. Two, this type has sharper corners for the passing water to get around, when the water is not moving parallel to the water line, and even when it is. Never the less, sailboats with such bottoms have crossed oceans. Flat bottom boats almost always has flaired sides, to mitigate the two disadvantages mentioned above.

    Multi-chine bottoms are almost always used to approximate a round bottom, but while using materials, such as steel, aluminum, and plywood, which don't like to take compound curves. The true round bottom does have less surface area, but it's pennies-on-the-dollar less. Multi-chine bottoms with shallow underwater sections are generally stronger, per material used, and slam far less than shallow flat ones. Such slamming may be uncomfortable but is almost always survivable. Most high-performance mono hull sailboats have shallow underwater sections.

    With shallow underwater sections, the initial stability is slightly less than with a shallow flat one with the same underwater volume. With a deeper ones, it is probably more.

    A "V' bottom is generally a compromise between a multi-chine one and a flat one.

    The shallow "V" bottom has the least initial stability of the three, but is somewhat stronger than the shallow flat one, and slams and pounds somewhat less. However, one has to be careful when designing a sailboat with a "V" bottom hull. If the half-angle of the "V" is close to usual heel angle of the boat under sail, The "V" bottom version will slam and pound even more ferociously than the flat bottom one.

    With both shallow and deep sections, the "V" bottom will have less surface area than the flat one, but more than the multi-chine one. It should be kept in mind, though, that with very shallow underwater sections, the surface area differences will be very slight, even between a flat bottom and a multi-chine one.

  7. C Khaleeli
    Joined: May 2017
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    C Khaleeli New Member

    I have done quite a bit of sailing, most of it in the past 5 years of my life. Most of the sailing I have done has been in dinghies like the CFJ. Recently I have started sailing keelboats (mostly the J24, I have crewed a little bit on a J109).
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