Displacement vs weight

Discussion in 'Stability' started by regulus, Jan 21, 2008.

  1. regulus
    Joined: Jan 2004
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    regulus New Member

    What is the difference between displacement vs weight for a sailboat. My sailboat have a displacement of 15500 pds, +, I presume 1000 or 1500 pds, oil, water and others. On the travel lift balance, the weight indicator is at 18000pds. Whats?
  2. charmc
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    charmc Senior Member

    I think you must now presume 2500 + pds for oil, water, and others. :)
  3. alan white
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    alan white Senior Member

    Displacement is the weight required to sink the boat to what the designer felt were its ideal lines. Usually the designer anticipates an average payload in crew, tankage, and stores will be needed to achieve the design displacement.
    Weight is a measurement on a scale. Displacement is a volume of water in the case of a boat. The word displacement is usually in reference to an ideal, where "light" or "heavy" are added to indicate which side of the ideal the boat falls to, if that is the case. In your case, the weight indicates a "heavy" displacement if the boat is to be used at that weight.
    Ideally, you should compare the designer's elevation (side view) drawing to the waterline of your boat in the water. See if the boat is a few inches below the designer's ideal waterline. If so, the boat may have been more robustly built than anticipated by the designer. Or, it may be that the designer incorrectly calculated the displacement based on the scantlings specified. In any case, if the boat appears to be sitting close to her lines when normally loaded, all is well.

  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I have never seen a sailboat, of that displacement range, of any real age (more then a couple of years) that didn't have all sorts of stuff aboard, that didn't need to be or is even remembered to be.

    Take everything off your boat when the travel lift gets ready to splash her again. I mean everything that's not nailed, screwed , glued or through bolted to a substantial hunk of boat and place it on the dock.

    All the engine spares, extra bits of line, those old fenders you're still hanging onto for some unknown reason, the old dock lines that never did get tossed, the anchor(s), dinghy, cleaning supplies, the old broken winch handle that apparently has a similar bond with you as the old fenders, everything. Look in every cabinet, locker and floor board for stuff that you've likely forgotten you still had aboard and toss them on the dock.

    I'll bet you a six pack you will have about a ton and a half of gear, that you didn't realize you had.

  5. Jratte
    Joined: Jul 2007
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    Jratte Junior Member

    Also be advised that some manufacturers will low ball the displacement to make the boat seem lighter than it will ever be in real usage. They may not allow for full or partially full tanks, crew weight, crew gear, food and other stores etc. Boats do tend to get heavier with age not just because of the gear they acquire over time but they may also be absorbing some of that precious water they spend so much time in. If you haven't already, I would suggest reading Dave Gerr's "The Nature of Boats". In it he discuss how to account for tanks, crew, gear etc. It would be good to compare his method to whatever you used to arrive at 10000# or 15000 #. Hope that helps.

    Josh Ratty
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