What size model should represent a vessel design in life testing?

Discussion in 'Boat Design' started by kvsgkvng, Mar 27, 2012.

  1. kvsgkvng
    Joined: Jan 2012
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    kvsgkvng Senior Member

    I understand that the ultimate test would be building the actual size boat and sailing it. But being a bit cautious, I would like to build a model first.

    I could see that a small 5-inch model would not work well. On the other hand, building a quarter scale boat is also out of question.

    What would be sufficiently accurate model of a LOA 12.5 ft x B 5 ft (LOA 3.81m x B 1.52m) shallow sailboat with sails in order to check if this boat would sail predictably fine (or the opposite of it)?
     
  2. tom28571
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    tom28571 Senior Member

    Many designers who do towing tests say that 4' is the minimum size for getting any reasonably accurate data. That is the size I use and it makes it easy to get the desired weight and CG distribution.
     
  3. Submarine Tom

    Submarine Tom Previous Member

    1/3 or 1/4 would be good.

    Larger the better of course.

    -Tom
     
  4. Jeremy Harris
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    Jeremy Harris Senior Member

    The simple answer is to use a scale that is practical in terms of being able to test it at the same Froude number as the full size boat. This makes small models of limited use, as they need to be operated at speeds that are too low to be practical for testing, and in very smooth water in the test tank or lake.

    Anything less than about 3 to 4 ft overall is going to be pretty difficult to test in a meaningful way, IMHO.
     
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  5. yachtie
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    yachtie Junior Member

    As a engineering patter-maker I can only stress out that accuracy and fussiness in every detail is a mast, when making a model for testing. That's more difficult then it firstly appears. The testing is even more difficult without experience and professional equipment.
    You'll end up with a pretty toy nearly useless.
    Stick to the letter with proven plans, is my suggestion. It will safe you money and time.
    Good luck.
     
  6. Leo Lazauskas
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    Leo Lazauskas Senior Member

    W.C. Webster found that surface tension was important for models
    shorter than about 1.5m and advised against using anything
    shorter. See:
    "The effect of surface tension on ship wave resistance", 1966.

    The recent ITTC found that the median total uncertainty for
    resistance coefficients for two models (one with L=3.048m
    and the other with L=5.720m) tested at many tanks around
    the world were:

    Fr, 3.048m , 5.720m
    0.10, 10.1%, 3.6%
    0.28, 3.7%, 1.4%
    0.41, 6.4%, 1.8%

    Expect larger uncertainty for shorter models.

    Sorry about the columns not lining up.
     
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  7. PAR
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    PAR Yacht Designer/Builder

    Considering the full size yacht is only 12' 5" I'd say a model is an extravagant expense that's wholly unnecessary. Then the question is: how would you know if it does sail "predictably fine" or not? Given that you are stumped by balancing out a 12.41' dinghy on paper (screen), do you have the skills necessary to ascertain the model's balance?
     
  8. kvsgkvng
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    kvsgkvng Senior Member

    Thnak you all for informative suggestions.
     
  9. JRD
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    JRD Senior Member

    A model of any scale will only be useful for trialing if you have the equipment, facilities and engineering knowledge to test it with the appropriate scale factors (Froud, Reynolds etc) and understand if it meets your design criteria when scaled up.
    However a 1/4 to 1/3 size model may still be a good way to get a better visualisation of the boat layout and test the practicality of some of your ideas.
     
  10. messabout
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    messabout Senior Member

    Model building is fun for its' own sake. Approach the model in that way and you'll be fine. A good model, especially an RC sailboat, can easily cost as much as the full sized (12.5 ft) boat. If you build a first class model, it can also take as much time as the full sized dink. Best plan is to build the full sized boat first, and then go sailing.

    Matter of fact you need to be a reasonably good sailor in full sized boats before you can get much satisfaction from a model, let alone meaningful data.
     
  11. yachtie
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    yachtie Junior Member

    Too right
     
  12. Submarine Tom

    Submarine Tom Previous Member

    Just build a proven design that meets your needs.

    Better yet, buy one and confirm it,

    then sell it and build one.

    Oops, I meant, build one and then sell the other one.

    -Tom
     
  13. PAR
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    PAR Yacht Designer/Builder

    A 12' 5" boat is a model . . .
     
  14. Submarine Tom

    Submarine Tom Previous Member

    A 12' 5" boat is a model.

    But, a 12.5' boat is a yacht!

    -Tom
     

  15. BMcF
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    BMcF Senior Member

    I've been building and testing models for various HPMVs (mostly SES but including the odd semiplanning monohull, high-speed cat and hydrofoil thrown in) in the range of 1/4 to 1/20 scale over the last 25 years or so.

    Oddly enough, the 1/20 scale model was physically the largest of them all..by far..at almost 17m LOA and the one at the opposite end of that spectrum was the second largest, a hydrofoil at 1/4-scale and 13.5m LOA.

    You have to be cognizant of every scaling factor at whatever scale you are using and account for them accordingly. In the realm of the HPMV, acheiving anything close to a plus/minus 5% estimate of speed/drag/power with a reasonable degree of confidence is considered pretty good. Not so with other ship types where more certainty is not only desired but economically imperative.
     
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