design displacement question

Discussion in 'Boat Design' started by spaceboy, Aug 20, 2014.

  1. spaceboy
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    spaceboy Junior Member

    Help appreciated understanding quoted displacement on plans. If a designer of a cargo schooner states a particular displacement, is this just for the boat built as planned minus any cargo? Cheers all
     
  2. Steve W
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    Steve W Senior Member

    It is generally the displacement at the waterline as drawn. A lot of sailboat designers play a little fast and loose when reporting displacement in marketing blurb.

    Steve.
     
  3. jehardiman
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    jehardiman Senior Member

    It should stated as particular displacement at a given draft and trim in salt or fresh water.

    Do not confuse stated displacement (light ship or full in "tons" or "tonnes") with measured tonnage ("tons") or deadweight tonnage ("tons" or "tonnes") .

    http://en.wikipedia.org/wiki/Tonnage
     
  4. gonzo
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    gonzo Senior Member

    Minus cargo would be light displacement. On ballast would be the minimum for safety. Loaded is the maximum including crew and supplies.
     
  5. TANSL
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    TANSL Senior Member

    Gonzo, things are not exactly like that. Among other considerations, what can we say of consumables?
     
    Last edited: Aug 23, 2014
  6. gonzo
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    gonzo Senior Member

    Are consumables supplies?
     
  7. TANSL
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    TANSL Senior Member

    Gonzo, I mean everything that is consumed, or increasing, along the boat trip, gas-oil, fresh water, ice, gray water, food, ... Are gray water supplies?, are the fishing gear supplies? ... Shall I continue?
     
  8. PAR
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    PAR Yacht Designer/Builder

    Published displacement figures can be all over the place. It could mean with half stores, crew and tanks, a dry hull, a full up hull and anything in between. You really have to contact the designer for an exact assessment of what his figures represent. I've seen dry hull delivery weights used as displacement figures and the opposite, full up max load, as it's displacement, so . . .
     
  9. Alik
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    Alik Senior Member

    There is a lot of marketing figures around and designers working in performance boats niche would often give attractive low numbers. I have seen designs where such attractive displacement was unrealistic and did not include any construction margins, it als did not include items such as genset, liferaft, aircons, etc. After the boat is built and fitted with all these essential extras, it will be 1-2 tons heavier and hopefully the structure will still be OK.

    To be fair, on needs to specify displacement according to common standards, say ISO8666. Here we usually clearly show mLDC - loaded displacement condition and mLCC - light craft condition. This is he way we usually present it to the Client, of course when compared with some advertised figures might look 'heavy' but I think good designer should not give any optimistic figures to Client.
     
  10. TANSL
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    TANSL Senior Member

    I totally agree with you, Alik. Fortunately ISO determines exactly what the mLDC. He even adds a "margin for future additions" (which is not a "supplie"). For boats that do not have to comply with the ISO, the definition of full load is much more complicated because it includes many items most of which are not "suplies."
    Indeed, a rigorous designer must provide data to his customers, with integrity and honesty, and should consider the worst case. The "generic" definitions are not acceptable because, among other things, they often hide a lack of knowledge of the designer.
     
  11. DCockey
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    DCockey Senior Member

    Back to spaceboy's question in the first post:
    Help appreciated understanding quoted displacement on plans. If a designer of a cargo schooner states a particular displacement, is this just for the boat built as planned minus any cargo?​
    The person who drew the plans would need to be asked. The answer could range from a precise condition including a good estimate of the weight of the hull, equipment, etc.; to a waterline in an arbitrary location, particularly if the designer relied more on intuition than calculations or if the plans were drawn from an existing vessel or half-model.

    What plans are you interested in?
     
  12. TANSL
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    TANSL Senior Member

    Indeed, you should ask the designer. Typically, however, the displacement shown in the drawing refers to the depth that normally is indicated on that drawing. This displacement / draft may correspond to the situation that the designer wanted to draw, and is not important, at this time, if it refers to full load or any other load condition. Although it should be noted what load condition refers or if it is simply an estimate to begin to determine other properties of the boat.
     
  13. Steve W
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    Steve W Senior Member

    This is what I meant about designers playing fast and loose with the displacement figures in marketing blurb, you often will see some small,sporty sailboat reviewed with a totally unrealistic figure which has to be the dry weight, if the performance ratios are based on this figure they would be meaningless because it cant actually go sailing until the crew which may make up 1/3rd of the sailing displacement steps aboard.
    I would think that for something like a cargo schooner you would base it on the loaded displacement and then add something such as water to compensate when the cargo is offloaded like ocean freighters do but I don't know this.

    Steve
     
  14. PAR
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    PAR Yacht Designer/Builder

    It's usually not the designer so much, as what the marketing team thinks can be used to advantage. I see this a lot on production boats, particularly performance oriented sail. They know they want to feature a light boat, so they offer the delivery weight as it's displacement, which is technically accurate, if the boat was splashed naked, though not ethical (IMO). Most of us actually do have a specific LWL figure, possibly multiples to conform to 8666, but builders and their representatives (sales/marketing) also have a say in the "presentation" of the product.

    The original poster was asking about a commercial craft, which should conform to ISO (at least in this country), so probably not an issue, other than concerns about it by the OP. Lastly (again), the designer should be contacted for clarification, just to be certain, if only to satisfy the OP.
     

  15. Steve W
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    Steve W Senior Member

    If hes talking about a cargo schooner design the designer may be long deceased so, short of a séance he may not be able to get the info there and it likely wasn't designed to conform to anything other than what works.

    Steve
     
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