How did they know how much a ship weighed?

rebelatsea

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There was no standard system of measurement until the introduction of "standard", and "light and deep draught" displacement calculations.
BOM (Builders Old Measurement" had been used for sailing vessels for centuries based on the number of casks which could be fitted in the hold. Problem with that is that casks varied depending on contents and size.
By our time, with steamships BOM was irrelevent, and was being replaced by "Tons Burthen", which has it's own problems as it is based on a dimensional calculation - and length, breadth, and depth of hold measurements varied wildly .
The other problem with our era and particularly (but not exclusively) a USA problem was that the definition of a ton/tun/tonne varied from port to port and country to country. Imperial, American and Metric weights were only just being accepted into common use.

Weights and sizes are often quoted with no definitions which is no help at all. This has often resulted in authors drawing plans which were accurate but wrongly scaled, and modelers doing the same in three dimensions

So when I wrote my book on the Southern Iron Navy, out of sheer self defence I evolved a calculation to show relative size in approximate displacement tonnage. This was the result of hours of going through published figures and plans !

Length overall x maximum beam x mean draught, divided by 70 = equals approximate displacement tonnage.

It may not be scientific, but it seems to work well enough .
 

Mark F. Jenkins

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So when I wrote my book on the Southern Iron Navy, out of sheer self defence I evolved a calculation to show relative size in approximate displacement tonnage. This was the result of hours of going through published figures and plans !

Length overall x maximum beam x mean draught, divided by 70 = equals approximate displacement tonnage.

It may not be scientific, but it seems to work well enough .

I did something very similar when I put together the stats for my old website. (I did the calculations nearly twenty years back and can't recall the specific formula I used, but my methodology was the same.)
 

Mark F. Jenkins

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You calculate the amount of water displaced by the ship and convert that to weight.

Generally, yes, but it must be remembered that tonnage is never a cut-and-dried dimension... actually, there are very few truly cut-and-dried dimensions when it comes to measuring a ship. The actual displacement will vary between when it's empty or loaded, fully fueled or running low on fuel, etc.... which in turn can affect measurements such as waterline length! There are some agreed-upon assumptions as to percent of fuel loaded and so forth that are conventionally used to produce a 'standard' figure.

But, as John noted, those conventions weren't in use yet at that time.
 

Patrick H

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Mar 7, 2014
When I was much younger I used to wonder the same thing. Then I saw a reference to displacement ("she displaced ____tons") and figured out they were always calculating the weight of water displaced.
 

Bruce Vail

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In reference to cargo ships or passenger ships, tonnage can be confusing for the non-professional. Wiki does a pretty good job of explaining the main elements -- see https://en.wikipedia.org/wiki/Tonnage

Tonnage does NOT refer to the weight of the ship, unless you are talking about lightweight tonnage, which is a term used pretty much exclusively in the ship scrapping business to estimate the scrap metal value of a vessel to be demolished.
 

DixieRifles

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Interesting, but it still raises more questions. I can understand how to calculate buoyancy of a hot air balloon but this seems more complicated.
For one thing, the deeper into the water the boat sits, the higher the pressure is acting on the hull. The same is true for air ships but the delta pressure is negligible.
 

Mark F. Jenkins

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The best way to think of displacement tonnage is as a general, relative index of ship size, and to not get too hung up on specific numbers... a thousand-ton vessel is a lot more substantial than a hundred-ton vessel, but smaller than a ten-thousand tonner (and at that time, there weren't a whole lot of ships anywhere substantially larger than that... trying to remember what the Great Eastern clocked in at, but that was definitely unusual for the time).
 

rebelatsea

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Generally speaking the deeper a vessel sits in the water, the more water it displaces, which is reflected in my formula. There is another problem , in that weight of water displaced is affected by relative salinity. The same vessel will not displace the same everywhere, which the designers for nations with "salt water navies" always have to bear in mind.

Bruce Vail is absolutely correct in what he says, but constructional weights have to be calculated very carefully to ensure stability and correct flotation at all states. HMS Captain showed what happens when weights are not carefully controlled, and marks a sharp dividing line between the old "instinct and rule of thumb" and modern controlled construction.
 

Waterloo50

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That’s what the draught marks are for, I’d originally thought that draught marks were for ensuring that a ship was loaded evenly but I’ve since learned that the draught marks are for measuring water displacement, there’s apparently 6 locations of draught marks on a ship which helps to give an overall average weight. Apparently a ships hull can sag or flex when loaded and one draught mark alone could give a false reading. The draught marks are essential for customers that have their goods transported by cargo ships, customers only want to pay for the weight of their cargo and not have the weight of the ship included in the cost, the known weight of the ship before it is loaded is obviously essential information.
 

georgew

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I did something very similar when I put together the stats for my old website. (I did the calculations nearly twenty years back and can't recall the specific formula I used, but my methodology was the same.)
One of the things that can make you use colorful language is trying to make sure you understand whether a citation is using overall dimensions. A good example of this problem is trying to make an estimate of the converted displacement of the Manassas. We have some fairly accurate dimensions of her earlier incarnation (Enoch Train), but the addition of the ram and its supports, the hull blisters (were they solid or hollow?), etc, plus an estimate of her ironing and loaded draft (anywhere up to 17 feet). Some of the newspaper accounts of Enoch Train seem to be using dimensions as measured on deck. Then you have the issue of how much of the vessel was above the waterline, so you can estimate the ironing. The only solid number I ever found about this vessel was the weight of a replacement smoke stack put aboard after Head of Passes (1600 lbs).
 
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