With regard to “weight,” ships are rated by the weight of the water they displace. But how about actual weight? Does anyone know how much a particular ship actually weighs? I find it hard to believe there could be correlation between the displacement and the actual weight. Do ship builders track the weight of the materials used?
It’s the same thing. According to Archimedes’ Principle, the buoyant force is equal to the weight of the fluid displaced. And since the ship is afloat and stable (not accelerating up or down), the buoyant force is exactly the same as its weight. (Or, if you dropped the ship into the water, the ship sinks until it has displaced enough water that the buoyant force equals its weight, at which point it stops sinking.)
At which point you shout “Eureka” and run naked down the street:)
But ship tonnage generally isn’t really about “tons” as in 2000 avoirdupois pounds.
For warships, it generally really is “tons” as in 2000 avoirdupois pounds. An aircraft carrier that is advertised to displace 90,000 tons of water actually would weight 90,000 tons = 180,000,000 lbs. on an oversized bathroom scale in a drydock.
But for civil ships, “tons” is often actually a measure of enclosed cubic footage. See Gross tonnage - Wikipedia and the related articles for more about “tonnage” as applied to commercial shipping.
Yes, commercial ships have a displacement figure as well, which really is the same idea as that used for warships. But almost any time you encounter a tonnage figure for a commercial ship, it’s one of the other kinds, not displacement.
I’m sure they have a very good idea of how much ships weigh now based on the materials used. In times past they may not have cared much. It’s the carrying capacity that mattered, and as I learned here on the Dope, that capacity was based on the volume of material that carried be carried instead of the weight since much cargo could be lighter than water.
A war ship is DWT, dead weight tonnage. What the ship weight fully armed and ready for sea.
A merchant ship is GWT, gross weight tonnage. which is weight of the cargo she can carry. The weight is calculated by volume.
The first mate and captain do need to know what the weight of the ship, cargo, and fuel are. And where the metabolic center of an empty ship is(I believe that is the right term). The cargo needs to be loaded in a manner to keep the ship from being top heavy or have a list while at sea. Now days there are computer programs that keep track of all the numbers. Also the Chief Engineer needs to keep the Mate informed on fuel consumption and which tanks he is pumping from.
Everything that goes into the ship during the construction process goes across the weighbridge (or should) and everything that comes off it, (scrap, waste, paint cans etc.) is supposed to go across the weighbridge as well (and not just over the side). So, they should be able to calculate how much weight is in the ship.
Before computers were available to do the number crunching, stability calculations were very lengthy and tedious. From about 1890 a mechanical computer called Amsler’s Integrator became available to help. The metacentric theory allows designers to think about the ship’s stability in a useful way, although idealised way.
Hoo boy. It’s a bit confusing: Deadweight tonnage is the weight of the cargo carried (sometimes expressed as the weight difference between two hull marks). Gross tonnage is simply an approximation of the cubic volume of the ship, where one ton (or tonne) is 100 cubic feet. Net tonnage is the volume of the passenger/cargo spaces, thus the earning power of the ship, in a way. Displacement tonnage is the actual weight of the ship under a given set of circumstances, say empty, or with fuel and ammunition, or with fuel and stores but not crew.
An interesting story: The Titanic when launched was the world’s largest liner (gross tons). It’s somewhat older sister ship, ostensibly identical, was a few score tons ‘lighter’. The ships truly were just about identical, but the promenade deck on the Olympic was open for most of it’s length and didn’t count toward gross (or net) tonnage, which only counts enclosed volumes. The promenade deck on the Titanic was enclosed for much of it’s length, and was thus added to the gross tonnage, making it fractionally larger then the Olympic. The later sister Britanic (sp?) was like the Titanic.
Wiki has an article (look under ‘ship tons’) which clarifies it quite a bit.
You go too far here. GT is a volumetric measure and is used by cargo vessels, particularly for calculation of government imposts, and capacity to carry high stowage factor cargos (ie light weight/high volume cargos). However DWT - a meaure of weight - is used all the time for calculation of carrying capacity for low stowage factor cargos. By tradition, the GT figure is often mentioned as a key vital statistic of a particular ship. But to people actually working in the industry, DWT is very important.