Ideal model size for scaling?

Discussion in 'Sailboats' started by misanthropicexplore, Sep 23, 2020.

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

    misanthropicexplore Junior Member

    For many years I've wanted to design a blue water sailboat for soloing in dangerous waters. That's a stupid idea for numerous reasons, not the least of which is my lack of sailing and engineering experience. I'm not going to do it, but the subject still fascinates me. I think I can afford to build large scale RC models, however. That seems like a good way to scratch the design itch without risking anyone's life to save my dumb a** or spending medium sized house money on a boat.

    The fun part to me, though would be collecting the data and scaling it to tell me "what would have happened if this was full size boat and I was on it?" I know that scale data can trick you, however because nature doesn't always scale the way you expect it to.

    Is there an ideal scale besides "as big as you can afford?

    The Thunderbird 26 is one of my favorite old boat plans. Would building a 1:4 scale model (Thunderbird 6.5, I guess) and RC sailing it teach you anything about how the real one handled? Would data from accelerometers be scalable? If so, how small can you go and still have useful data? 1:12? (2'2" long)?
  2. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Not realistic at all afa winds and waves go. Models are good to see what you are building. They might look good on a shelf. Might even be fun to operate in the water. But reality is too far off.

    That said, they do hull shape testing in tanks all the time.
  3. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    I think it would tell you a fair bit about what it's sailing characteristics are like.
    If you video it, and then slow the video down (I think it might be in linear proportion to the size, but I am not sure), then the slowed down video should be pretty accurate, in calm water at least.
    The bigger the model, the more effective it will be.
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  4. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    I agree with Bajan.
    Go full size!
  5. Will Gilmore
    Joined: Aug 2017
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    Location: Littleton, nh

    Will Gilmore Senior Member

    Modeling can tell you a great deal. There are factors of scale that don't work well, but knowing what they are means better understanding how they affect the model.

    For instance, the viscosity of water doesn't scale, neither that of the air. A larger vessel is effectively traveling through a thinner medium. Often, scaled models need, proportionally more ballast. However, these differences can be taken into account and contribute effectively to design improvements.

    Decide what specific questions you want to answer. Work around answering those questions and isolate the differences. Do you just want to know if a flat bottom will sail well, or a bilge keel will keep a boat upright and point high enough to make upwind progress, or what is the difference in motion between a skinny deep boat and a wide shallow boat? It can answer rigging questions and trim questions and balance questions.

    I'd go with the largest practical model. Maybe, 1/2 scale will give you a boat you can actually sail around the bay, if not in "dangerous" waters or blue water.

    -Will (Dragonfly)
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  6. sharpii2
    Joined: May 2004
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    Location: Michigan, USA

    sharpii2 Senior Member

    I would go with the 1:12 scale model first.

    It's small enough to be built inexpensively, but big enough to rig with RC equipment and decent servos, without you having the hands of a jeweler.

    I would make everything in proportion to the full-size boat, and even get the Vertical Center of Gravity as close as possible to where it would likely be on the full-size boat.

    The rig would also be in as exact proportion as possible.

    Other than that, I wouldn't work to hard on it. No scaled do-dads which are not absolutely required to make the model sail.

    This way, I'd be able to see if the design has promise as a sailboat. I would find out if it had too much weather helm, or even if it had Lee helm. I would find out if it had enough righting moment to stand up to enough sail to get it to move, let alone the rig I designed for it.

    The inaccuracies would be in the performance numbers, as the viscosity and surface tension of the water would be proportionately much greater for the model than for the full size boat.

    If this model worked out, I would consider building one big enough to ride in, or just go ahead with the full size boat.
    misanthropicexplore likes this.
  7. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    1:12 won't give you a very realistic view of the design characteristics while underway and making way.
    1:1 is, obviously, the most accurate but it's an exponential decay of accurate results from there.
  8. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    When the ratio is very large, there will be gross inaccuracies due to turbulence. A model may have laminar flow the whole length of a keel, while the real boat will be cavitating and cause the rudder to loose steering.

  9. luckystrike
    Joined: Feb 2010
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    Location: Germany

    luckystrike Power Kraut

    I often build a rc-model to test a a new design. This gives me a chance to learn a lot and see the boat sailing. Trim, waterflow around the hull, wavemaking, planing/surfing, behavior in waves can be seen directly and translated into the design.

    I prefere a model length of 1,2m to 1,5m. This is quite a big modell, but still managable for transport and costs.

    If you prefere a certain length in your designs, lets say your designs are always betweeen 20' and 26', choose the same scale every time ( for example 1 : 5,5).

    Wheights must be calculated correctly, but stability must be increased so your model will have a deeper keel and rudder.

    Have fun, Michel
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