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

orange6330
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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

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