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Tonnage versus Displacement

Discussion in 'General Yachting Discussion' started by Oscarvan, Nov 19, 2015.

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  1. Oscarvan, Nov 19, 2015
    #1
    Oscarvan

    Oscarvan Senior Member

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    Trying to get my head around the 199 ton limit for owner operation.....tonnage not displacement as has been pointed out elsewhere on this forum.

    While I understand the difference, ie one is a volume , the other the actual weight of the vessel, I'm trying to form a relationship between the two.

    Example, a Hatteras 56MY weighs roughly 85.000 pounds, or 42,5 US tons...... what would it's tonnage roughly be? Very dull chainsaw math..... 56x18x(wild guess)20=20160 cubic feet = 201 ton x 60% for the pointy parts = roughly 120 tons? Does that sound right?

    So the largest I can operate without a Masters would be, say a 70 footer?

    Thanks.

    Edit: After finding this

    https://www.uscg.mil/hq/msc/tonnage/docs/TG-1_Current.pdf

    I come up with a number of 110 GRT..... for a 56' Hatt and about 210 GRT for a Hatt 70? Sound about right?

    Edit2: Figured out how to put names off Yachtworld into the documentation search..... an 80's 70' Hatt comes in at about 76GRT.

    Guess I answered my own question..my budget is the limiting factor, not the GRT... thanks though. ;-)
    Last edited: Nov 19, 2015
  2. Capt J, Nov 19, 2015
    #2
    Capt J

    Capt J Senior Member

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  3. Oscarvan, Nov 19, 2015
    #3
    Oscarvan

    Oscarvan Senior Member

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    Yes, I did mention that. I got that part. I also posted the exact same link you did.... But thank you.
  4. Capt J, Nov 19, 2015
    #4
    Capt J

    Capt J Senior Member

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    There's no ryhme or reason to it. A 2001 76' Lazzara I ran was 101 GRT, a 1988 75' Hatteras MY 75 GRT, a 62' Sunseeker predator 72 GRT,
  5. Oscarvan, Nov 19, 2015
    #5
    Oscarvan

    Oscarvan Senior Member

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    So who makes the call? A naval architect/engineer? The CG?
  6. chenkl, Nov 20, 2015
    #6
    chenkl

    chenkl New Member

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    The tonnage measurement (or calculation) is based on "International Convention on Tonnage Measurement of Ships".
    http://www.imo.org/en/About/Convent...nvention-on-Tonnage-Measurement-of-Ships.aspx
    In the convention there's very detailed description telling you how to measure and calculate the volume, and how to exempt some space from total volume, and how to calculate gross tonnage. You can have a classification society to issue you a tonnage certificate so you can use the tonnage all over the world.

    There's no direct relation between gross tonnage and displacement.