simple question I can't seem to find an answer to re published displacement numbers

Discussion in 'Boat Design' started by Vineet, Jun 8, 2021.

  1. Vineet
    Joined: Apr 2021
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    Vineet Junior Member

    Are published displacement numbers on sites like Sailboatdata.com based on empty or fully loaded boats?
     
  2. Alik
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    Alik Senior Member

    Published data for small craft should be based on ISO8666. All other data, without reference to this standard is just about cheating ;)
     
  3. gonzo
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    gonzo Senior Member

    Sailboatdata has published data provided by manufacturers or dealers. They usually don't specify empty or loaded.
     
  4. DogCavalry
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    DogCavalry I aim to misbehave.

    Per Dave Gerr, if they don't specify, it's as empty as possible. Seating cushions removed, no anchor or chain. Like that. Lighter has become a selling point, like maximum number of berths, even if the berths can't be used, and manufacturers must have sales to survive.
     
  5. philSweet
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    philSweet Senior Member

    And to put some perspective on how weird this can get, I was once present when a "25,000" pound boat was hauled out and the load cell read 56,000 pounds, and we had emptied it as much as we could - no water and very little fuel. It did have a bunch of options, but still ...
     
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  6. fallguy
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    fallguy Senior Member

    Displacement is supposed to be volume below design waterline.

    Weight is something else.

    But if a boat is designed to displace X pounds and it weighs Y, then the available loading is X-Y to the waterline. And after Y is greater; the boat goes below her lines.

    For example, bringing 100 gallons of freshwater is 800 pounds which could take a small boat below her lines. A watermaker is lighter, but uses power, etc.

    There is a metric called pounds per inch or kg/cm that defines how much the boat goes below for added weight. My boat, for example has a ppi of about 750#. So for loading above designed displacement, 750# lowers the boat an inch.
     
  7. DogCavalry
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    DogCavalry I aim to misbehave.

    Weight and displacement are supposed to be the same. In fact they literally are the same thing since a boat can't displace more water than it weighs, nor less for that matter. If in design load state it's 6" below it's lines, the NA is incompetent or dishonest.
     
  8. sharpii2
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    sharpii2 Senior Member

    I think it is safe to assume that the displacement number quoted is the empty boat unless otherwise noted.

    This may not be just because of less than honest builders.

    It may also be because of the racing community. When racing a boat, it is common practice to load it as lightly as possible (except maybe for beer). This means just enough food and water for the crew to complete the race.

    Since most races are a few hundred miles or less, this means a relatively lightly loaded boat--especially considering the number of crew on board. So, if the boat is to be raced at all, the prospective owner is going to want to know the weight of the empty boat for comparison reasons.

    And since most 'modern' boats are designed with at least some racing in mind, the builders have a lot of incentive to quote this lower number for the displacement.

    Add to this the tradition of long-distance cruisers taking any boat that is remotely up to the job (it once was old sailing work boats when they were plentiful) that they can get for a relatively low price, and you get a lot of long-distance cruising boats that were originally designed for medium distance racing, or were of designs highly imfluenced by such.

    And since most of these voyages on these boat were successful (as the ones on the old work boats were), these boats came to be seen as the more seaworthy types. Their vertues were accentuated and their vices were accepted (just as with the old work boats).

    And one of these vertues was light displacement, because typically that meant higher potential speed--especially in light wind conditions.
     
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  9. The Q
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    The Q Senior Member

    I've been using Sailboat data a lot over the last few weeks, for very small boats.. If they were the crewed up displacements then the bouyancy was filled with helium!!
     
  10. fallguy
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    fallguy Senior Member

    But a boat empty might sit above her lines and there might be some loading capacity. So, the design displacement may be more!!! than the boat's dry weight.
     
  11. DogCavalry
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    DogCavalry I aim to misbehave.

    Certainly. That's why "design displacement ", as distinct from "displacement".

    Design displacement is whatever the designer claims he was working towards. Displacement is current reality.
     
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  12. DCockey
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    DCockey Senior Member

    I doubt that many US builders or importers are aware of ISO8666.

    Many, if not most, US builders are members of ABYC which has Standard S.8 "Boat Measurement and Weight". That standard includes:
    8.4.6 Displacement - the weight of the water displaced by a boat at a specified loading condition. Presumably the loading condition should be described but it could be anything.
    8.4.13 Weight, Base Boat - This is essentially a bare boat without outboards, optional equipment or any equipment which is not permanently attached.
    8.4.14 Weight, Trailerable -essentially the heaviest the boat should weigh with fuel and other fluids, outboard engines, options, etc while conforming to load capacity limitations.

    In reality displacements and weights without explaination could be almost anything, and displacements and weights from third-party databases are notoriously unreliable.
     
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  13. Alik
    Joined: Jul 2003
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    Alik Senior Member

    To my knowledge, most of ABYC standards are very close to ISO Small Craft standards, since 2010? Anyway most of European boats imported to USA would have ISO8666 figures.
    I could have checked ABYC definitions but frankly speaking they charge too expensive for being able to read it ;)
     
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