Scaling Down a Sharpie

Discussion in 'Boat Design' started by FromMystic, Mar 14, 2017.

  1. TANSL
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    TANSL Senior Member

    All this has nothing to do with the original question but it is an interesting subject.
    In my opinion, it can not be said that model testing has limited success because it is not true. What happens is that the environmental conditions in which they are going to be made, for example, the sea trials are totally different to those existing in the tank at the time of the test. That is why correction coefficients are established, which are updated with experience, to extrapolate the results of the tank test to realty.
    And, as in any human activity, there are always bugs.
  2. Ilan Voyager
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    Ilan Voyager Senior Member

    99% success when you're testing well known shapes, and that you have correlating coefficients with hundreds of similar ships. The boxy ships have no problems. The coefficients are a mix of hard science and empiricism.

    On very new shapes, with no hard boiled correlation coefs and a known behavior in full size, it's better to be cautious. In this case you'll find in the report a commentary with lots of "seem", "possibly" and "probable" words.

    For example Shuttleworth tested trimaran amas around 1978, and the best shapes in terms of drag, had pointy slim sterns that proved to be disastrous in real life.
    On the other side the Queen Mary 2 had a better speed than expected from the model testing and calculations with a rather significant margin.

    I have been told, when drinking too much wine with some guys from the "Aerospatiale", that also in planes there has been surprises, bad and good.

    But testing models is an invaluable tool, even at the age of computer simulations. At least you can eliminate at 95% the false good ideas cheaply compared to a full sized failure.
    There are series of hulls born from tank testing which are an immense help for the designers.
  3. TANSL
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    TANSL Senior Member

    I agree. For me to eliminate what is not good enough or get a 95% approximation to what will be the final product would be a great success, never a limited success.
  4. CT249
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    CT249 Senior Member

    Because, for example, you will end up with a vessel that is very narrow for its length, which will lead to stability issues. It will also lack sufficient volume, because skin thickness does not scale with length - whether 27 or 18 feet it still has to take punctures and the weight of a person. The scaled down boat will therefore float too low.

    Even issues such as the crew weight cannot be directly scaled in small sailing craft; the crew work on a three person boat can be done in a different manner to that of a two person boat. If someone has to go down to leeward in a three-person boat to grab a line or some other minor task, two people are still sitting to windward. If the same task is done in a two-person boat, a greater proportion of crew weight is moving to leeward. That has effects on design, as racing skiff designers know.

    Freeboard will also be too low - the freeboard that was sufficient for a 27 footer is not sufficient when the boat is scaled down. The fact that waves do not scale down means that a big boat, if directly scaled down, may lack sufficient volume forward and therefore may nosedive. There's a reason that smaller boats have more beam, and therefore fuller bow angles, than their longer cousins.

    Some designers have also noted that since small boats tend to travel at different speed/length ratios (as hull speed does not scale in a direct linear fashion) the flow around a smaller hull is different. A shape that works in a larger boat may cause turbulent flow in a smaller boat (vis Rob Humphrey's remarks on some old Ron Holland boats).

    The above are just some of the factors. Put it this way - if hull lines could be directly scaled up and down, why don't the top designers just do it like that? The answer is that they know it doesn't work.

  5. PAR
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    PAR Yacht Designer/Builder

    We, nor the OP are talking about models and their vices, but a 40% reduction of a 27' boat, which if proportional, will not succeed. It'll flop over the moment it hoists its sails, assuming it can somehow remain upright on launch day, as the crew boards. Now, if you don't make a proportionate reduction: one the boat no longer is a New Haven, but a distortion with possibly some New Haven aestedics and two: what's the point of a reduction, if it can't be an accurate, just smaller rendition of the origional, that actually functions similarly to the New Haven?

    Simply put, just because you can do something (proportionality scale her down), doesn't mean you should or that it's a viable solution, though certainly a drafting possibility. A futile exercise in scaling, with the results being an unstable, unsuitable and unattainable craft seems pretty pointless, even if the drawing exercise can be achieved.

    Lastly, there are lots of sharpies and skiffs that can have New Haven attributes and aestedics applied. The OP needs to decide what he wants and make his design selections based on these goals, the aesthetics, notwithstanding.
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