Appropriate Scale for Modeling

Discussion in 'Sailboats' started by MastMonkey, Jan 10, 2011.

  1. BATAAN
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    BATAAN Senior Member

    Of course 1/8 inch gives the same info, it's all proportional. Smaller scale is cheaper, larger can possibly be more precise.
     
  2. Paul B

    Paul B Previous Member

    You should plan to double that again if you want to be in the neighborhood of meaningful data collection.
     
  3. MastMonkey
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    MastMonkey Junior Member

    I didn't realize the answer was so contentious. I honestly was expecting that a scale had been established for modeling accurate performance, I just hadn't been able to find it. That may be because there is room for debate. I would appreciate if I could learn the merits of each. I assumed bigger was better, but I want to keep it as small as possible so that I can make more experiments.

    Bataan, thank you for the information. I will try and find a copy of that book. It sounds fascinating. I like practical, simple methods. That kind of testing was what I had in mind. I love the idea of pulling a model boat on a string. It reminds me of that scene in the movie "Wind" where Matthew Modine's character and his designer are sitting in a pool with snorkels as they push a model boat back and forth across the surface.

    I also want to be scientific about it though. Using asymmetric foil shapes seems an important component to improving twin keel performance. So the model must be big enough that the asymmetry of the keels is meaningfully expressed as lift producing surfaces. So I do need a testing method that is somewhat exacting. For further testing I was planning on building an RC model of the most promising hull and keel configurations.

    Paul, could you elaborate further at all. What do you feel is appropriate. Why is a testing tank necessary?

    Thank you all for the reminder that there is still a lot to learn about boats.
     
  4. gonzo
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    gonzo Senior Member

    Herreshoff used models of about the scale Bataan suggests. He would make several and towed them from a small launch with a towing arm. That is a bar with an attachment in the center from which the tow line is attached. A model gets attached to each end. When they get towed, the one with more resistance pulls more and stays behind. It does not give absolute numbers, but compares models with small modifications really well. For example, you can test different keel configurations on the same hull.
     
  5. Paul B

    Paul B Previous Member

    The answer is not contentious.

    Towing tank data tells us anything less than 1:5 is probably giving nothing more than anecdotal evidence. Of course even the best tanks using the best calibrated equipment might be considered little better. Remember, all the AC teams use 1:3 scale models with the most sophisticated methods and years of CFD and practical knowledge to back them up. All of them come up with a "best" solution, most are failures in competition.

    If you think you can gather meaningful data on a project like this without the use of sophisticated (expensive) tank time then good luck to you.

    Of course knowledgeable people who have been in this game for many years already know the answer you are looking for.
     
  6. gonzo
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    gonzo Senior Member

    You don't need a test tank for meaningful testing. The models can be towed behind a boat or from a fishing rod and a bicycle.
     
  7. Paul B

    Paul B Previous Member

    This method tells you nothing about the lift/drag of the keel(s) in sailing mode. That is what the original poster wants to compare.

    By the way, just what was the year Herreshoff last designed a boat? Do you think he would still use the same method today?
     
  8. Paul B

    Paul B Previous Member

    Or behind a Swallow.
     
  9. gonzo
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    gonzo Senior Member

    I think Herreshoff used the simplest possible method. This method of towing side by side give the difference in lift at various speeds. The standard attachment points for towing sailing models work in this system.
    Paul: once more you resort to sarcasm instead of supporting your position.
     
  10. MastMonkey
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    MastMonkey Junior Member

    Paul, I see what you are saying, but I also wonder whether the simple methods described above still have some validity for some test. The tank test completed by the AC boats seem like they are looking for very small incremental improvements in performance. If there is still lots of room for improvement in a design could these basic methods offer some insight? Lift versus drag of the keel is but one of what needs to be determined. There are still unknowns for the twin keel arrangement regarding placement and camber. In addition to resistance, wavemaking would be observed. I have read some reports that twin keels can actually slow or trap water between them, creating extra resistance. So maybe the test that Herreshoff conducted as described in Gonzo's post would be applicable?

    Hypothetically, if I built two RC sailboats, modeled after the same hull but with differences in keels, would the advantage of either arrangement be apparent and applicable to a the full sized boat?

    I can't afford tank test or 1/3 sized models but I do not want to give up on the idea.

    I think that sicne there is so little data to begin with for a purposefully twin keeled boat, there has to be something useful in the basic methods above. But again, there is still the question of scale.
     
  11. Paul B

    Paul B Previous Member

    Has to be?

    Good luck.
     
  12. Paul B

    Paul B Previous Member

    This method tells nothing about lift. Based on your comment I seriously question whether you understand what lift is.
     
  13. MastMonkey
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    MastMonkey Junior Member

    What I mean is that even antcedotal evidence can be useful, especially when there is nothing at all to begin with. I accept that I know nothing about fluid dynamics, but I have seen numerous example of models being tested for aerodynamic and hydrodynamic behaviour and some of them have been quite small in comparison to the full sized object.
    I assumed that there was some scaling factor that was applicable to these models that was determined by a some principle of FD. I guess there isn't.
     
  14. Doug Lord
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    Doug Lord Flight Ready

    It is basically true that the larger your model the less chance of error. I'm building a 5' model to test an 18' multihull. But you can get meaningfull data from a model that can be built to a weight that is exactly scaled from the full size. The displacement varies as the cube of length , the SA varies as the square of length and the RM varies as the 4th power of length. So you need to try various lengths of model to make sure the scaled version can be physically built at that weight and it is helpful if it fits a conventional scale like 3/4", 1" , 1.5" or 3"=1'. Sailing models "lose" RM as they are scaled down so ,again, the larger the better. Generally, testing sailing models with scale SA won't work(except in very light air) because of how dramatically the RM decreases as the boat is scaled down. Models between 3' and 5' have been used to test in real world conditions by people such as Team Hydroptere, the CBTF(Canting Ballast Twin Foil) Company, Yves Parlier, Dr. Sam Bradfield, Greg Ketterman and others who all added radio control and tested many aspects of their designs; tacking, gybing, balance, foil placement and area(not sections). Model testing, as was pointed out earlier, is a damn good way to learn about the basic characteristics of a potential design. Be careful not to try to get too much info......
    For basic calculations see this:http://www.boatdesign.net/forums/mu...-self-righting-trimaran-test-model-36058.html posts 12 and 13 most especially....
     

  15. BATAAN
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    BATAAN Senior Member

    The method would compare two versions of the same hull, allowing quick and easy answers to relative performance, not absolute. The difference between a twin keel and conventional version will be so slight as to be very difficult to measure by model in any other way.
     
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