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There are plenty of terms for describing the size of a ship. However, most of them are either measures of cargo capacity, or measures of volume. To make things more confusing, most of the volume measures use weight-sounding names such as "ton".

If I were to pick up a ship and place it on a scale, what is the technical term I would use to describe the number I got? Given that seafaring has its own specialized vocabulary, I'm pretty sure I'd get laughed at if I said "the Seawise Giant weighs x million pounds".

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    Generally, "displacement" is used, expressed in tons or tonnes. It is the weight of water displaced by the ship, and hence, as Archimedes will tell us, the weight of the ship: The HMS Hot Licks has a displacement of 150 tonnes.
    – Hot Licks
    Commented Dec 13, 2016 at 1:05
  • Referring to a ship's weight is perfectly acceptable as long as the ship is not floating. A ship in drydock would probably be described as weighing X tons, as long as the subject involves something like moving it with at crane. Referring to it in terms of its normal operation (floating), Hot Licks is correct and displacement is normal.
    – WhatRoughBeast
    Commented Dec 13, 2016 at 1:31
  • Yes, displacement is rightly measured in units of weight or mass rather than of volume, and is equal to the weight or mass of the vessel. The volume of water displaced, even with a given lading, will vary according to the specific gravity of the water in question, which will be greater for salt water than for fresh, by about 2.7 to 2.9 percent (the higher figure for the Red or Med).
    – Brian Donovan
    Commented Dec 13, 2016 at 3:06
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    I think a ship-ton is largely appropriate here ;-)
    – Kevin
    Commented Dec 13, 2016 at 7:43
  • I don't think ship-ton is a thing.
    – Laurence
    Commented Dec 18, 2016 at 18:22

1 Answer 1

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There are two approaches with measuring the weight of a vehicle, one would be with variables such as fuel, hydraulics, coolants, lubricants, passengers etc (gross weight) and one would be without these (dry weight).

The free dictionary defines dry weight as:

Dry weight is the weight of a vehicle without any consumables, passengers, or cargo.

The free dictionary defines gross weight as:

Weight of a vehicle, fully equipped and serviced for operation, including the weight of the fuel, lubricants, coolant, vehicle tools and spares, crew, personal equipment, and load.

For ships in particular it gets a bit more complicated:

According to The Maritime Site there are 6 different ways giving the weight of a ship, which are:

  • Displacement Tonnage:

    Displacement Tonnage is the total weight of the volume of water a ship “displaces” when it is sitting in the water.

  • Standard Displacement Tonnage:

    Standard displacement is the total weight of the volume of water a ship “displaces” without the weight of any fuel and potable water carried on board the ship.

  • Deadweight Tonnage:

    Deadweight tonnage is the weight (in tons) of all the cargo, fuel, dry provisions, supplies, etc. carried on board the ship. (ie. "displacement tonnage" minus "lightweight tonnage")

  • Leightweight Tonnage:

    Lightweight tonnage is best described as the weight of the ship when it was built in the shipyard including all framing, machinery, decking, etc. However, lightweight tonnage does not include the weight of any consumable such as fuel, water, oil, or supplies.

  • Gross Registered Tonnage:

    Gross registered tonnage is a measurement of volume of all enclosed spaces on a ship with 100 cubic feet = to one ton. Gross Tonnage or “Gross Tons” is what you’ll see most often on official ship documents and certificates

  • Net Registered Tonnage:

    Net registered tonnage is also a measurement of volume however you only consider the volume of actual cargo storage areas when dividing the cubic volume in feet by 100 to get your “tonnage”. This includes any tanks, cargo holds, etc. that are normally used for transporting cargo.

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  • So what's the difference between Standard Displacement Tonnage and Lightweight Tonnage? They look to be the same to me.
    – BoldBen
    Commented Dec 13, 2016 at 8:14
  • @BoldBen Standard Displacement Tonnage is the total weight of the volume of water a ship “displaces" without the weight of any fuel and potable water carried on board the ship, whereas Lightweight Tonnage is weight without any consumables such as fuel, water, oil, or supplies on land
    – 3kstc
    Commented Dec 13, 2016 at 10:00
  • @BoldBen From the descriptions above, it sounds like they are measured at different times - LT measured at build time, and SDT at any other time. Presumably, SDT changes over time as bits erode and barnacles / coats of paint etc accumulate. However, look at this reference to Washington displacement as a synonym of Standard Displacement - the definition is quite different from The Maritime Site's SDT.
    – Lawrence
    Commented Dec 13, 2016 at 22:56
  • @Lawrence, if you look at the diagrams on the Maritime Site page, it's actually very similar to the Washington displacement: it includes everything except fuel and water. (Washington displacement is from an arms-limitation treaty, so it mentions things like crew and ammunition, while the Maritime Site page is focused on cargo ships).
    – Mark
    Commented Dec 13, 2016 at 23:24
  • @Mark Thanks for the note. However, with standard displacement excluding potable water and fuel (as defined in the answer above), and Washington displacement including both and more, I don't really see the resemblance. Perhaps it's just that I haven't yet seen the thing that ties the two terms together, but they look quite different to me.
    – Lawrence
    Commented Dec 14, 2016 at 11:36

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