Ideal model size for scaling?

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

  1. misanthropicexplore
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    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
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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
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    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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    BlueBell . . . _ _ _ . . . _ _ _

    I agree with Bajan.
    Go full size!
  5. Will Gilmore
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    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
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    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.
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  7. BlueBell
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    BlueBell . . . _ _ _ . . . _ _ _

    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
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    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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    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
  10. messabout
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    messabout Senior Member

    To choose an appropriate scale for a model the choice is heavily influenced by weight. Suppose you are building the model to one to twelve scale, (one inch to the foot = 1/12) . Use the denominator cubed to learn the scale weight of the model. 12 cubed is 1728. Let us imagine that your full sized boat is carefully estimated to weigh 3500 pounds all up. Divide the cube of the scale into the estimated weight of the full sized boat..........3500/1728 = 2.02 pounds. That is all the weight the model can have if it is to be a true scale model.

    No way to build a model at that scale and make it actually RC sailable. Why? You will need a fin and bulb to keep it reasonably upright. The bulb will use up most or all of the allowed weight. Try six inch scale for example That is 2 inches to the foot. Now the model will come closer to scale weight (displacement) Six cubed is 216 divide the 3500 pounds by 216 and you get 16.2 pounds which is entirely do able with the model. The boat and its weighted keel of fin and bulb will easily fit inside that weight limitation. You might even have to add ballast. Choose another scale if you like but the weight deal works as described in whatever scale you choose.

    Bottom to a larger scale so that you can have some room to work with. That is not all........If you are to build to an accurate scale then your allowable variation of dimensions on the model becomes a mater of interest. If your full sized boat is to be built to a tolerance of plus or minus a half inch at the sections, then a one on twelve model forces you to work to an accuracy of 0.5/12 = 0.o41 inch.....about one millimeter. Can you see where I am going with this argument?
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  11. misanthropicexplore
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    misanthropicexplore Junior Member

    That makes sense and was my concern, thank you! It also highlights somethings that confuse me about scaling.

    If I make a half scale model, half the linear measures would make 1/8th the displacement, but 1/4 the sail area, so a half scale model would have 2x the SA/D. It seems like the "to the 2/3rds power of D" in the SA/D would correct that one, but aren't there other design rations that would get out of wack and need to be modeled "wrong" to be right for a particular ratio?
  12. messabout
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    messabout Senior Member

    Similitude is difficult to achieve with a sailing model. The shape and size of the ballast keel or fin gets in the way of fair comparisons. The ballast to displacement ratio is less than realistic when compared to the full sized boat. Another item is the SA/D as mentioned. The model will have a sail that is pretty close to the water surface and is in many cases less efficient than the relatively high altitude sail of the full sized boat. Sure enough you need a lot of sail on the model. With a lot of sail you need a a lot of righting moment which means that the bulb must be really deep if the bulb weight is not going to upset the design displacement ..............A one meter model might have a fin that is 12 to 14 inches deep. The rudder will also have to be deep. Those features are not realistic for any but the most extreme full sized boat.

    The model can give you a SWAG idea of how the full sized hull might behave. Even that has its' limitations on account of relative Froude numbers, skin quality, the surface condition of the water in which the model is sailed. and a whole basket of other variables. Never mind all that technical stuff......The model is fun to sail and for many of us worth the time and cost used to build the model.
  13. Doug Halsey
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    Doug Halsey Senior Member

    Since the Froude & Reynolds numbers scale by different functions, the testing that's done in tanks involves making nontrivial corrections to the data.

    The process is roughly like this:
    1-In the tank, tow the model at a speed that corresponds to the full-scale Froude number.
    2-Calculate the viscous drag at the model's Reynolds number & subtract it from the measured drag to get the model's wavemaking drag.
    3-Scale the wavemaking drag to full size.
    4-Recalculate the viscous drag at the full-scale Reynolds number & add it to the wavemaking drag to get the full-scale total drag.

    If the flow is sufficiently well behaved and/or the model isn't too small, this procedure works well, but for shapes involving large regions of separated flow it doesn't always. Even top designers can be fooled by the tank tests, as in the notorious case of Britton Chance's 12-Meter Mariner.
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  14. Dolfiman
    Joined: Aug 2017
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    Dolfiman Senior Member

    ITTC reports gather the state of the art about model tests for scaling. Here are some of them about :
    ** Ship models preparation (inc. about devices for turbulence simulation) :
    ** resistance tests (in line with Doug #quote above) :
    ** about yacht testing technique :

    And there are many others.... , just Google ITTC + your key words.
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