Comparison of hulls and keels

Discussion in 'Sailboats' started by misanthropicexplore, Oct 11, 2019.

  1. misanthropicexplore
    Joined: Apr 2018
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    Location: Upper middle Missouri River

    misanthropicexplore Junior Member

    Three designs interest me: The Thunderbird 26, The Barchetta Class, and the MaxiMOOP. (Also a 3D model of the MaxiMOOP here, if you are interested.) Despite being 26', 20', and 4', all of them have successfully crossed oceans.

    If you scale them up to identical lengths, they are all very comparable in basic measures, with the exception of the keel on the MaxiMOOP: it's absolutely massive compared to the other two.

    Why?

    The designers say it's to help it move in light winds, but I thought deep heavy keels made it harder to move in light winds.

    Is it an issue of the speed? The MOOP only goes 2.5 knots.

    It is the fact that waves don't scale?

    Is it a practical thing? While the MOOPs keel is large in scale, it's very shoal in real life inches (about 16")

    Thanks all, look forward to hearing your thoughts.
     
  2. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    The robots keel is massive to keep it upright at the minimal cost of increased drag.
     
  3. gggGuest
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    gggGuest ...

    Basically boat designs don't scale very well, especially to such an extreme.
     
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  4. jehardiman
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    jehardiman Senior Member

    Yep, stability scales with the cube, make a model 1/10th the size, the sail and keel area are 1/100th, but the stability is only 1/1000th of the original. So you have to up the keel volume/mass by 10 times.
     
  5. misanthropicexplore
    Joined: Apr 2018
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    misanthropicexplore Junior Member

    When you say stability increases as a cube, do you form stability, because the buoyancy increases a cube with every linear change in dimension? Or because the ballast mass increases as a cube with every linear change in dimension? Or am I missing something. (I'm usually missing something.)

    The MaxiMOOP (assuming I did the math right) had D/L of about 460, a SA/D ratio of about 13, and ballast ratio of 174. The sail is a Balestron that feathers rather than reefing, so I figured that was the cause of the massive ballast, but also those number look about right for any size boat that has to trudge through storms in blue water at 2 - 3 knots, instead of going around them or waiting them out, don't they?

    Other than draft and sheer mass to lug around, would would be the disadvantage to such a high ballast ratio?
     
  6. jehardiman
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    jehardiman Senior Member

    Stability, period, decreases with the cube of the scale. It doesn't matter if it is form stability, or ballast stability. FWIW, you proposed non-geosims, so it doesn't exactly track, but the reason that smaller vessels have a higher B/LwL coefficient unless they have massive keels is for this reason. Every vessel that works, looks like it is supposed to. Comparing dimensionless factors between radically different sized vessels is meaningless.
     
  7. gonzo
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    gonzo Senior Member

    I think that there is a difference in scaling between form stability and that provided by ballast. If the volume increases by the cube, but the waterplane area by the square, stability should change by a different factor. For example, if the length and beam double, the waterplane will be four times larger. The ballast will be eight times heavier. If you consider the force required to heel the boat at the same angle as the stability comparison, the form stability will increase by four. The stability provided by the ballast will be 8 (weight increase of ballast) * 2(depth of keel increase), or I am missing something fundamental?
     
  8. TANSL
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    TANSL Senior Member

    I believe there are several fundamental things that you have forgotten. Stability is not measurable, it cannot be said that it is four times greater or things like that. But, to understand us, when you talk, for example, that "stability increases by four", what do you understand by stability?.
     
  9. jehardiman
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    jehardiman Senior Member

    Remember that actual stability is comprised of two overlapping parts, form stability and absolute stability. The issue is that you forget that form stability is based upon the waterplane inertia, not area. So while the waterplane area increases/decreases by a factor of 4 (2x2) and the volume increases/decrease by 8 (2x2x2), the waterplane inertia increases/decreases by a factor of 16 (2 x2^3). For most near vertical situations, form stability dominates when scaling.
     

  10. TANSL
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    TANSL Senior Member

    The waterplane inertia is used to calculate the metacentric radius: I / V ("I" is the waterplane inertia and "V" is the submerged volume). When changing the length and the breadth, the area of the flotation changes, the inertia of the flotation changes, but also the submerged volume changes, so giving a figure on how much the metacentric radius changes seems somewhat risky.
    In any case, I suppose you are talking about the initial stability that is usually assimilated to the value of the initial GZ. As can be easily deduced, this value depends on the position of the center of gravity of the ship, which will also, probably, change if we change the length and beam. In summary, and in my opinion, the figures that are being considered are totally incorrect.
    If there is any incorrectness in what I have said, please indicate it.
     
    Last edited: Oct 16, 2019
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