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  • Tonnages at sea

    Originally posted by Gaelsail View Post
    You've got me interested in these calcs now. Can you please explain this further? what do the different weights mean?
    Gross and Net tonnage is a measure of Volume, not weight at all. Lots of rules and regulations, incl. manning, are based on Gross Tonnage, as is Port Dues etc. in most countries, while net tonnage is much less used these days. Basically, Gross tonnage is the volume of the entire vessel, but with a lot of exceptions and "tricks" to keep it low, which would take too long to get into here. Net Tonnage is volume of the Cargo Holds/Tanks or whatever spaces is "revenue earning".

    Now the actual tonnages, as a measurement of weight:
    Displacement is, as the name implies, the weight of the water that the ship displaces at various times, which equals the weight of the ship, incl. cargo etc.
    Loaded displacement is the weight of the ship when loaded to her max. allowed Load Line, while Light Ship displacement is the weight of the hull and machinery only.
    The difference between them is the Deadweight Tonnage, or the max. weight the ship can carry in the form of Cargo, Bunker Fuel, Water, Stores and the weight of Crew and Pax and their belongings, especially if a Cruise ship.

    For obvious reasons the Deadweight of a Cruise Ship is not a meaningful measure of her capacity. Neither is Gross/Net Tonnage, or even Displacement in most cases, but it is the best we got, aside from max. number of Pax and her length over all.

    Hope this helps, but it probably should have been under a different thread, as it doesn't do much for the QE 2.

  • #2
    I copied this interesting post from the QE2-thread, page 5.
    With best regards from Jan-Olav Storli

    Administrator and Owner of CaptainsVoyage.
    Main page: http://www.captainsvoyage.com
    Old forum: http://captainsvoyage.7.forumer.com/
    Join us: Save the "Kong Olav" on facebook

    Surround yourself with positive, ethical people who are committed to excellence.

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    • #3
      Before my cruise through the Panama Canal last year I had fun looking into how tonnage is calculated. I found it really interesting that on a passenger cruise ship the balcony space is not calculated when the dividers between cabins are moveable/temporary. If the partitions are fixed in place the balcony space would count in tonnage calculations.

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      • #4
        Originally posted by pilotdane View Post
        Before my cruise through the Panama Canal last year I had fun looking into how tonnage is calculated. I found it really interesting that on a passenger cruise ship the balcony space is not calculated when the dividers between cabins are moveable/temporary. If the partitions are fixed in place the balcony space would count in tonnage calculations.
        The tonnage measurement rules most used is the 1969 version. Before that the rules were even more ridiculous, which resulted in what was known as "Paragraph Ships". (199, 299, 499, 1199 and 1599 GRT respectively)
        This had to do with the number of crew required to sail her, among other thing.
        In the US Offshore Supply Vessels under 300 GRT did not need to have a licensed Master, hence the early ones were all 299 GRT.

        The difference was whether "temporary" (Bolted) or "permanently" (Welded) closing of the "Tonnage Hatch", which was a small space between Main deck and the Shelter deck.

        I remember visiting a shipyard in Ulsteinvik when at the Maritime Collage in Aalesund, (1962-4), where they were building a Newsprint Carrier that had two sets of tonnages, depending on whether as "Open" or "Closed" Shelterdecker. As "Open" she was 499.98 GRT, as "Closed" she was 1199,.. GRT.

        In this case they had made a little mistake in the design so she would have become too "big". This was solved by welding some stipps onto the deep frames where the transverse measurements would be take. (Frame to frame amidships)

        This particular ship had three deck. They had cut out a small section of the second deck so it would not be "continuous", and thus not the measuring deck. It could easily be put back. (I think it was probably "temporarily" put back after measuring and before leaving the yard with the smallest tonnage)

        In the accommodations the hallways were wide and the lockers in the cabins were large, as they were not measurable in the gross tonnage, while the crew cabins were tiny, as they were.

        These "tricks" eventually got so stupid, and at times dangerous, that they were changed and the "two tire" tonnage certificates were done away with, but there are still a number of rules that is being circumvented by "innovative" methods, as mentioned by Pilotdane.

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        • #5
          Perhaps other find these to be of interest?

          http://www.nauticalia.com/uk-info/sp...lass/2184.html
          With best regards from Jan-Olav Storli

          Administrator and Owner of CaptainsVoyage.
          Main page: http://www.captainsvoyage.com
          Old forum: http://captainsvoyage.7.forumer.com/
          Join us: Save the "Kong Olav" on facebook

          Surround yourself with positive, ethical people who are committed to excellence.

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          • #6
            Another few term used on ships;
            "Bending moment". I.e. how much stress the ship is exposed to from uneven distribution of weight along the length of the vessel.
            - Too heavy in the middle the ship is said to be sagged, or sagging. Here is a bad case of sagging:


            - Too heavy in each end, too light in the middle, the ship is said to be hogged, or hogging. Here is a bad example of hogging:

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            • nari
              nari commented
              Editing a comment
              Isn't sagging also associated with the bow and stern caught in the trough between large waves?
              I recall the first mate stating at breakfast years ago as the ship approached Sydney (where the seas can get very rough indeed) that "we have back breaking weather". As he spoke, there was an enormous thud and his eyebrows went up. Fortunately the ship was small (1760 tons).

          • #7
            That is correct Nari. That is why static hog or sag has to be controlled and bending moment kept within allowable limits at all times.
            Dynamic hog and sag is induced when a ship is exposed to long swells especially.

            Wrongly loaded cargo, or change of static bending moment caused by consumption, change of ballast etc. while underway can cause the ship to be subjected to excessive bending during a voyage. That was the probable direct cause of the MOL Comfort sinking, although wrong skanteling during construction was also a factor.

            For example, large ore carriers with 7 or 9 holds carry heavy cargo only in every second hold to control bending moments, both during loading/discharging and during the voyage. It is fully possible to break a ship by wrong loading, even in port, as the Trade Daring in 1994, seen here:

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            • nari
              nari commented
              Editing a comment
              Do you know where that port is? Looks vaguely in WA - but there are no elegant bridges there, for sure.

            • ombugge
              ombugge commented
              Editing a comment

            • nari
              nari commented
              Editing a comment
              Thanks for that. Were the very long tankers of the 80s and 90s reinforced in their midriff to add strength? Did the history of collapsing hulls change anything?

          • #8
            Another measurement is the displacement tonnage of vessels.
            Here is a simplified explanation on the subject: https://gcaptain.com/watch-a-ships-d...Captain.com%29

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