Tonnage and Displacement

Warship design and construction, terminology, navigation, hydrodynamics, stability, armor schemes, damage control, etc.
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Karl Heidenreich
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Tonnage and Displacement

Post by Karl Heidenreich » Sun Jul 18, 2010 5:29 pm

This maybe seem ridiculous but there is something I don't get here. While studying the characteristic of several ships I came to notice the ambiguity of some terminology and now I am confused.

Let's see: displacement is the measure of water displaced by the ship. And tonnage or weight is the actual weight of the ship. And I have always considered one dependant of the other and will vary in small measure because of geometrical issues of the hull form, etc. This implies that a relationship formula exists.

Now:

TITANIC:
Tonnage: 46,328 tons
Displacement: 52,310 tons

Displacement > Weight

QUEEN MARY:
Tonnage: 81,237 tons
Displacement: 81,961 tons

Diplacement > Weight (slightly)

Now:

QUEEN MARY II:
Tonnage: 148,528 tons
Displacement: 76,000 tons

Tonnage >>Displacement

OASIS OF THE SEAS
Tonnage: 225,282 tons
Displacement: 100,000 tons

Tonnage >> Displacement

How on Earth that can be?
An appeaser is one who feeds a crocodile, hoping it will eat him last.
Sir Winston Churchill

Byron Angel

Re: Problems with tonnage and displacement

Post by Byron Angel » Sun Jul 18, 2010 7:04 pm

Karl Heidenreich wrote:This maybe seem ridiculous but there is something I don't get here. While studying the characteristic of several ships I came to notice the ambiguity of some terminology and now I am confused.

Let's see: displacement is the measure of water displaced by the ship. And tonnage or weight is the actual weight of the ship. And I have always considered one dependant of the other and will vary in small measure because of geometrical issues of the hull form, etc. This implies that a relationship formula exists.

Now:

TITANIC:
Tonnage: 46,328 tons
Displacement: 52,310 tons

Displacement > Weight

QUEEN MARY:
Tonnage: 81,237 tons
Displacement: 81,961 tons

Diplacement > Weight (slightly)

Now:

QUEEN MARY II:
Tonnage: 148,528 tons
Displacement: 76,000 tons

Tonnage >>Displacement

OASIS OF THE SEAS
Tonnage: 225,282 tons
Displacement: 100,000 tons

Tonnage >> Displacement

How on Earth that can be?

..... Hi Karl,

In the merchant trade (I used to be in the international ocean freight forwarding business), tonnage and displacement are a little more complex. This is a good site to explain the differences and nuances -

http://www.gjenvick.com/SteamshipArticl ... ained.html


Byron

boredatwork
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Re: Problems with tonnage and displacement

Post by boredatwork » Sun Jul 18, 2010 7:22 pm

Karl Heidenreich wrote:Let's see: displacement is the measure of water displaced by the ship. And tonnage or weight is the actual weight of the ship.

How on Earth that can be?
Because the two aren't related.



Displacement is the actual MASS of the ship - which is identical to the MASS of the water displaced by the ship.

Tonnage on the otherhand usually refers to a series of measurements, usually in relation to merchant ships, of the ENCLOSED VOLUME of the ship from Keel to Funnels. Used for tax, insurance, and harbour dues purposes, it's measured in Register Tons - a unit of VOLUME = to 100 cubic feet. If filled with water 1 register ton would weigh 2.8 tonnes.

As such there is no direct relation between the two in the way for example that Full load Displacement and Standard Displacement are related. Your statement that Queen Mary's "Diplacement > Weight (slightly)" is the quivalent of saying my 184cm height > 182 pound weight (slightly).

You can increase or decrease either without necessarily affecting the other. For instance putting fuel into a ship increases it's displacement but does not affect the total internal volume in any way. Likewise putting up the shutters to enclose the Queen Mary's promenade deck will increase her tonnage substantially but not affect displacement at all.

Likewise consider a 35,000 ton Battleship KGV and a 21,000 ton Aircraft Carrier Ark Royal - the Ark Royal by virtue of her large enclosed hanger probably has a greater tonnage than the battleship, despite displacing considerably less.
Last edited by boredatwork on Sun Jul 18, 2010 7:56 pm, edited 1 time in total.

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Karl Heidenreich
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Re: Problems with tonnage and displacement

Post by Karl Heidenreich » Sun Jul 18, 2010 7:34 pm

boreatwork:

Excelent explanation and it makes all the sense of the world! Thanks for the link.

Regards,
An appeaser is one who feeds a crocodile, hoping it will eat him last.
Sir Winston Churchill

Neoconshooter
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Re: Problems with tonnage and displacement

Post by Neoconshooter » Sun Dec 02, 2012 9:10 pm

Karl Heidenreich wrote:This maybe seem ridiculous but there is something I don't get here. While studying the characteristic of several ships I came to notice the ambiguity of some terminology and now I am confused.

Let's see: displacement is the measure of water displaced by the ship. And tonnage or weight is the actual weight of the ship. And I have always considered one dependant of the other and will vary in small measure because of geometrical issues of the hull form, etc. This implies that a relationship formula exists.

Now:

TITANIC:
Tonnage: 46,328 tons
Displacement: 52,310 tons

Displacement > Weight

QUEEN MARY:
Tonnage: 81,237 tons
Displacement: 81,961 tons

Diplacement > Weight (slightly)

Now:

QUEEN MARY II:
Tonnage: 148,528 tons
Displacement: 76,000 tons

Tonnage >>Displacement

OASIS OF THE SEAS
Tonnage: 225,282 tons
Displacement: 100,000 tons

Tonnage >> Displacement

How on Earth that can be?
Your premis is not true.Tonnage is the usable volume/weight availible for pay load. The Displacement is the volume of water with a Mass equal to that of the ship.

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Herr Nilsson
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Re: Problems with tonnage and displacement

Post by Herr Nilsson » Mon Dec 03, 2012 2:08 pm

I thought the question was already answered. Anyway. Perhaps it's interesting to know that Tirptz's tonnage was 28,195.89 GRT
Regards

Marc

"Thank God we blow up and sink more easily." (unknown officer from HMS Norfolk)

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José M. Rico
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Re: Problems with tonnage and displacement

Post by José M. Rico » Mon Dec 03, 2012 7:27 pm

Herr Nilsson wrote:Perhaps it's interesting to know that Tirptz's tonnage was 28,195.89 GRT
Thanks for pointing that out Marc.
I's not surprising considering the low profile of Bismarck class battleships (and of almost all battleships for that matter).
Just for comparison purposes, do you know what was the GT of the Graf Zeppelin?

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Herr Nilsson
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Re: Problems with tonnage and displacement

Post by Herr Nilsson » Thu Dec 06, 2012 4:03 pm

José M. Rico wrote: Just for comparison purposes, do you know what was the GT of the Graf Zeppelin?
No, unfortunately not.
Regards

Marc

"Thank God we blow up and sink more easily." (unknown officer from HMS Norfolk)

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Re: Problems with tonnage and displacement

Post by José M. Rico » Thu Dec 06, 2012 9:24 pm

Herr Nilsson wrote:
José M. Rico wrote: Just for comparison purposes, do you know what was the GT of the Graf Zeppelin?
No, unfortunately not.
I would bet her GRT was greater than that of Bismarck. :D

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Re: Tonnage and Displacement

Post by orange6330 » Tue Jul 14, 2015 12:04 am

May I beg to differ.
Tonnage is a measure of the cargo-carrying capacity of a (merchant) ship. It is a measure of volume.
Displacement is the weight of a ship, used for warships and only as a secondary fact for merchant vessels. It is a measure of mass or weight.
In order to estimate (since obviously volume can't be converted into weight) the displacement (weight) of a (merchant) ship when you have the tonnage in Gross Tons, you have to divide by 3.4, although this factor is variable. This is an approximation, since the relation or ratio volume vs. weight obviously depends on the ship's design. (For the Queen Mary II it appears to be 2).

In the following values, we must consider that by convention 100ft3 equal 1 ton when calculating tonnage (volume) . I have been on both the Queen Mary I and II, and also on the USS Nimitz. It would appear to me that here is no way in which the Queen May II may weight less than the Queen Mary I, however "hollow" (voluminous) the QM II it may be. The Nimitz is a wholly different thing, a little bit shorter than the QMII, much lower and compact, it obviously looks (and is) much heavier than either of the QMs, although its two hangar decks are obviously hollow enough.

Thus, even though tonnage (volume) is a complicated thing to estimate and there are complex rules to do so, I can see no way in which the QMII is lighter than the QMI.

Furthermore, the below data would make the QM1 much heavier than battleship Bismarck (displacement = 50.00 tons), something apparently ridiculous. I could accept that for the QM2 and for the Nimitz, not for QM1.

I would appreciate further input from forum members, since displacement bigger that tonnage is apparently ridiculous.

QUEEN MARY:
Tonnage: 81,237 tons
Displacement: 81,961 tons

Diplacement > Weight (slightly)

Now:

QUEEN MARY II:
Tonnage: 148,528 tons
Displacement: 76,000 tons

Tonnage >>Displacement

VinnyL
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Re: Tonnage and Displacement

Post by VinnyL » Sat Sep 05, 2020 6:41 am

There seems to be a lot of confusion with the terms Tonnage and Displacement. The last entree in this forum was by orange6330, back in July 2015, which was 5 years ago. He had some of it right, but not all. There was a lot of comparing the Bismarck battleship with the QM 1 and QM II. I found this comparison a bit strange, since this is comparing apples to oranges! The battleship should be compared to other battleship, and not commercial ocean liners.

At any rate, I will proceed. First, military ships are all measured in tonnage, and this tonnage is its weight. The weight of any ship is EXACTLY its displacement in water, and not roughly. Archimedes proved this over 2,000 years ago. All non military ships, while still using the term 'tonnage' is just a measure of its volume, and has very little to do with its actual weight. This is why you see such large number of tonnage for commercial ships, such as the larger cruise ships. The big US aircraft carries probably has a large tonnage value in volume as well, but this is never measured or given. Comparing commercial tonnage with military tonnage is futile and should never be done.

Now for some facts:
Military - Battleships
1) The Bismarck's tonnage (displacement=weight) was 41,000 long tons, and when fully loaded (people and ammo) was 49,500 LT. Its length was 823 ft. overall. It could move at 30 knots.
2) The comparable British battleship was the King George V was 38,031 LT, and fully loaded was 42,237 LT. Its length was 745 ft and could attain a speed of 28 knots.
3) The comparable US battleship is the Iowa Class (4 ships) which are 45,000 LT, and fully loaded at 57.500 LT. Its length is 887 ft, and could travel at 33 knots.
4) The Japanese monster battleship Yamato was 64,000 Lt, and fully loaded was 70,527 LT. Its length was 862 ft., and could travel at 28 knots.
Military - Aircraft Carriers
1) WW II Essex Carrier was 27,100 LT., and when fully loaded was 36,380. Its length was 872 ft, and could go 33 knots.
2) Today's US super carriers Carl Vinson (CVN-70)is 101,300 LT and is 1,092 ft long. Its speed is given as 30+ knits (not real limit).
3) Today's US super carriers Gerald R. Ford (CVN-78) is 100,000 LT-and is 1106 ft long and will go in excess of 30 knots.

Commercial
1) The QM i had a weight (displacement) of 77,400 LT, and has a volume tonnage of 81,237 (1947). Its length is 1019 ft., and could travel at 30 knots.
2) The QM II has a weight (displacement) of 79,287 LT, and has a volume tonnage of 149,215. Its length is 1,132 ft. It can travel at 30 knots.

All of these facts are from Wikipedia. Notice that QM II is both heavier and bigger (no volume) than QM I. It is hard to find the real weight of most cruise ships today. It is like they try to keep that info a secret! But the cruise ship 'tonnage' keeps going up, and in general, they are just making the ships taller, and thus they have more volume. Of course, they are getting really bigger also (more actual weight), but this is hard to find out.

Vincent (09-04-2020)
PS A foot note about the battleships of WW II
a)The Bismarck had 15" guns where each armor piercing shell was 1764 pounds and produced nearly 200 million ft. lbs. of muzzle energy a(ME) and could shoot this 22.5 miles.
b) The US Iowa class battleship shot a 2,700 lbs armor piercing shell which produced 262 million ft. lbs. of ME over a distance of 24 miles.
c) The Japanese Yamato shot a 3219 lb shell which produced 327 million ft. lbs. of ME over a distance of 26 miles.
Note that 1 ft. lbs. is the amount of energy it takes to move 1 lb. the distance of 1 foot. 1,000 ft. lbs. is the energy needed to move 1 lb. 1,000 feet, or 1,000 lbs 1 foot, and everything in between. And so on.

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Re: Tonnage and Displacement

Post by pgollin » Mon Sep 21, 2020 12:15 pm

.

And, just as a gentle reminder, the displacement will vary with the density of the water that the ship is floating in (see, for instance "plimsoll line")

.

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Re: Tonnage and Displacement

Post by Mostlyharmless » Sat Sep 26, 2020 12:39 pm

The issue of the displacements of warships in WW2 has a few other sources of confusion. If we have reliable documents, we find a simple "light" displacement which is the weight of the ship without any stores, ammunition, boiler water or fuel. We can add in weight of everything except the fuel and reserve boiler water to get the Washington Treaty defined "standard" displacement. We can load in the reserve boiler water and perhaps 2/3 or 3/4 of the total possible fuel load and get a "trial" or "normal" displacement. If we fill up the fuel tanks, we get a "full load" displacement except that sometimes it was necessary to warn captains not to carry the maximum oil load possible (the commanders of the North Carolina and South Dakota Classes were instructed not to bring the main armour deck to less than 5' of freeboard in WW2).

Because the standard displacement had to be published when ships were laid down according to the Washington and London Treaties, newspapers could give that displacement and so that value tends to be remembered. However, the standard displacement as published was seldom the true figure. For example, Iowa was laid down as a 45,000 ton standard battleship but was about 48,110 tons standard displacement http://www.bb62museum.org/stats.html as completed.

The published numbers for German and Italian ships were intended to mislead whilst the Japanese Navy had withdrawn from the treaties after 1936 and no longer even calculated standard displacements. The earlier declared displacements for Japanese cruisers were too low as a result of some combination of over optimistic calculations and deception (it was actually quite hard for anyone to predict the weight of a ship before building it but the British were possibly over careful and the Nelson Class came in 1,100 tons light). The Japanese heavy cruisers were found to be unstable and structurally weak but with Japan's withdrawal from the Treaties they were reconstructed after 1936 to be far over their earlier declared displacements.

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Re: Tonnage and Displacement

Post by paul.mercer » Thu Oct 01, 2020 4:31 pm

Gentlemen,
Many thanks for the info - although its all a bit technical for me!
Given the figures quoted, could it be that the Washington treaty and the ones that followed it were actually a waste of time as itwould appear that no-one (except possibly the RN) intended to keep to it or at least interpret the rules it in their own way?
in any case, who was going to monitor the building of these ships as I doubt that many navy's would want people from other countries inspecting their new ships and even if they did, who is going to judge whether the ship is over the allowed weight - it wouldn't be easy to put a battleship on a pair of weighing scales so the odd thousand or two tons would surely be hardly noticed on a completed ship>

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Re: Tonnage and Displacement

Post by Steve Crandell » Thu Oct 01, 2020 7:11 pm

I think it's pretty obvious that the US "Tin Clad" cruisers would not have been built but for the treaty limits, considering how inadequate their protection was. The British put 14" guns on the KGVs and the North Carolina class were designed with 14" guns.

If you read one of Fischer's books which cover the design of these ships you will see that all sorts of compromises were made to keep them under the treaty limits. Where there was fudging I think was what exactly was counted in a ship's displacement. But the ships were in general pretty close to the displacement limits when completed. Of course, as the war progressed they all got heavier primarily because of additional AA armament.

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