Talk:Displacement (ship)

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Bad metric conversion[edit]

I believe the English to Metric conversions in this article are badly incorrect. No time to check it now, though. Lou Sander (talk) 13:02, 2 September 2008 (UTC)

They WERE badly incorrect. I fixed them and included a link to a Metric to English converter. Lou Sander (talk) 12:10, 5 October 2008 (UTC)


Somebody changed some units, which is fine. But they included the unit "ga," which links to an article on several kinds of gallons. Please, somebody, fix this ambiguity. Otherwise, somebody will probably revert your edit. Lou Sander (talk) 11:52, 22 October 2008 (UTC)

Weight vs. Mass[edit]

I've found many definitions of ship displacement that refer to weight, but none that refer to mass. IMHO, unless there are some authoritative definitions referring to mass, the article should use weight. I'll wait for comments. If none are persuasive, I'll change it to weight. Lou Sander (talk) 12:16, 22 October 2008 (UTC)

Since this has not been commented on in over three years, I changed all instances of "mass" to "weight", and included a specific reference in the first mention. Lou Sander (talk) 17:33, 12 November 2011 (UTC)

Are Displacement and Deadweight correctly stated in ship’s papers?

A vessel would be assigned with a Displacement and a Deadweight for each of her Load Lines, i.e. Tropical, Fresh, Summer and Winter, early at a new building stage and I do not expect that somebody will be surprised to learn that.

It appears however, that there is a rift between the values stated in the ship’s papers and the actual Displacement and respectively Deadweight, which can lawfully be achieved. The reason for the rift is due to the fact that Load Lines are assigned for a standard Specific Gravity 1.025 (no units, as it is a ratio), while a vessel’s hydrostatic tables also use the same number of 1.025, however this number here is Density (kg/ltr), which is used for Displacement (mass) determination.

In my personal opinion, this oversight on the part of the shipping community has occurred sometime in the middle of the last century, probably related to the adoption of the latest Load Lines Convention 1966, and it went on undetected for some time, as general cargo still had the larger share of the total cargo volumes transported by sea. Later on, with the rise of the bulk cargoes trade and the increase of volumes transported in bulk, naturally there was a need to improve the accuracy of Draught Surveys, which still remains the most reliable (and often the only available) method for determining the amount of cargo loaded or discharged on a bulk carrier vessel. People started to look more closely into the laws of physics, and realized something which has always stared them in the eyes, that a Specific Gravity of 1.025 does not equal a Density of 1.025 kg/ltr, where Density value is always by 0.002 kg/ltr less than the Specific Gravity value, for the same sample of sea water.

Here I would hurry up to say, that if someone thinks that this article will be yet another article on Draught Surveys accuracy, they will be disappointed and they may wish not to continue reading. Although the problem between what is stated in ship’s papers as Displacement and Deadweight, and what a ship can actually achieve, without breaking the law, only becomes apparent on a bulk carrier vessel, during a Draught Survey, the problem is equally applicable to any type of vessels, VLCC and post panama container vessels also included. I will try to present the problem in a nut shell below.

Each assigned Load Line states a Free Board, which easily translates into Draught. Having obtained e.g. the Summer Draught by the above method, one can make a reference in the Hydrostatic Tables using the Summer Draught as an argument, and can obtain a Displacement, which goes into the ship’s papers. The problem is that the so obtained Displacement will be for Density of 1.025 kg/ltr. As explained earlier, Load Lines are assigned for the standard Specific Gravity of 1.025 not Density. If a vessel is floating in water with the standard SG of 1.025, then the Density of that very same water will be 1.023 kg/ltr. Let’s assume that a bulker has to load a complete cargo of iron ore, in a loading port located in the Summer Zone, having a sea water with the standard SG of 1.025. In this case, the vessel would only be able load cargo up to her summer marks by the law. However on completion of loading, a Draught Surveyor would come with his Zeal hydrometer and will find that the dock water Density is only 1.023 kg/ltr, then he will rightly apply a Density correction to the Summer Displacement, which was derived from the Hydrostatic Tables, for the Summer Draught. The Master will rightly complain, saying something of the kind:

.... But this is my Summer Displacement?! The vessel is on her summer marks, the sea water is of standard SG 1.025, why on Earth you are reducing my Summer Displacement!? This way we will be short of our pre-declared cargo and I have to explain to the Charterer why did we load less cargo than we declared! In such case, may I load some extra cargo to submerge the Summer marks, for dock water allowance, as obviously you found the Density to be less than 1.025?

And the typical reply from the attending Surveyor below, would shatter Masters, not giving them any chance to lift their declared cargo:

Unfortunately Captain, a Density of 1.023 kg/ltr equals a SG of 1.025. The vessel is on her Summer Marks, we are at a port located in the Summer Zone, and the dock water is of the standard SG of 1.025. You can’t load even a ton more of this cargo.

Sounds familiar? At least to me it does. This is more or less the scenario which takes place on a daily basis, in many bulk cargo loading ports, where Draught Surveyors do proper Draught Surveys. The consequences are that vessels’ cannot reach their official Displacement and Deadweight, and as a result they are short of their declared cargo, because Masters are compelled to use the vessel’s official Deadweight for their cargo lift calculation.

The situation is highly annoying to all players, i.e. Masters, Owners, Charterers, Shippers, Receivers as there is a lack of understanding of the issue. It is in the interest of all concerned that there is a solution to the problem soonest possible.

Worked Example

Cape Size Bulk Carrier: MV Cape NoName Summer Draught: 18.00 m Summer Displacement: 205,000 mt Light Ship Weight: 25,000 mt Deadweight: 180,000 mt Bunkers, FW, Constant, etc.: 3,000 mt Declared Cargo to load: 177,000 mt

The vessel has just completed loading at a port located in the Summer Zone, with sea water standard SG 1.025, and vessel’s draught is 18.00 m.

Draught 18.00 m  Displacement 205,000 mt Displacement corrected for Density = 1.023∗205,000 = 204,600 MT 1.025

Less Light Ship Weight: 25,000 mt

Less Bunkers, FW, Constatnt, etc.: 3,000 mt

Cargo Loaded as per Draught Survey: 176,600 mt

Declared Cargo to load: 177,000 mt

Shortage: 400 MT — Preceding unsigned comment added by (talk) 06:55, 5 February 2014 (UTC)

Is a ship's displacement the same as its weight?[edit]

The lead includes the sentence "Displacement measures weight of a volume of water displaced, not the vessel's absolute weight in pounds put on a scale." This seems to contradict the earlier sentence, "Displacement or displacement tonnage is the weight of water that a ship displaces when it is floating, which in turn is the weight of a ship (and its contents). ". Furthermore, it seems wrong. How does a ship's "weight in pounds put on a scale" differ from its displacement (in pounds)? TypoBoy (talk) 15:46, 17 April 2015 (UTC)

We could make this article clearer by organizing the subject into its logical layers:

  • A ship displaces its weight in water, in conformance with Archimedes' principle.
  • A ship's load lines make it possible to calculate the volume of water displaced.
  • The displaced volume can be used to calculate the weight of the water displaced (and, therefore, of the ship). The calculation requires knowledge of the density of the water, which is a function of its temperature and salinity.
  • Named, conventional, displacements (such as "loaded displacement" and "Washington displacement") give a ship's displacement under specified conditions.

Toward this end, I propose the following specific changes:

  • Striking the second sentence of the lead, "It is usually applied to naval vessels rather than commercial ones, and is measured when the ship's fuel tanks are full and all stores are aboard." The first part is wrong; every vessel has a displacement (and the "calculation" section mentions merchant vessels specifically). And the second part is misleading; though there are named, conventional displacements, a vessel has a displacement at every moment.
  • Changing the sentence "The traditional method of determining a vessel's displacement uses draft marks (also known as "load lines")." It seems worthwhile to make it clear that this is how the operators of a ship measure its displacement, once the naval architects have done the hard work of figuring out where to paint the load lines. (Also, the word "traditional" seems misplaced here; this is a matter of mathematics, not culture.)
  • Changing the sentence "Displacement, however, is not absolute for a given vessel and load: it varies by the density of water being displaced. " Since displacement is a measure of a ship's weight, it is absolute for a given vessel and load. And it's misleading to say that "a ship with a given mass (i.e. dead weight in kg) will displace less seawater than fresh". That's only true of the volume of water, and the reader has understood since the first sentence that we're talking about weight. I propose replacing that entire paragraph with something like "The weight of the displaced volume of water varies with its density; Cold water is denser than warm water and saltwater is denser than fresh."
  • Changing the first sentence of the "definitions" section. It now reads, "A variety of terms are used to describe varying degrees of displacement under specific loads:". I propose avoiding the term "degrees of displacement", and changing that to something like, "There are terms for the displacement of a vessel under specified conditions:" TypoBoy (talk) 18:38, 17 April 2015 (UTC)

I made a version of the page with the edits I'm suggesting in my user space, at User:TypoBoy/Displacement. TypoBoy (talk) 19:57, 17 April 2015 (UTC)

I am not any sort of subject matter expert, but your rewrite looks pretty good to me, at least at first glance. BZ! It seems to be accurate and well-written, and a great improvement on the current version. Pretty good work for a fellow who usually just corrects misspellled words. ;-) One thing I noticed is the mention of hydrostatic tables. There doesn't seem to be an explanation of that term in Wikipedia. IMHO there should be some sort of explanation of it in the Displacement article, or, even better, a separate article on the subject. You seem to have enough background, or at least to have done enough research into Displacement, to create something about hydrostatic tables. Their might also be something brief in the article cautioning about the difference between the concept of displacement (a.k.a. displacement tonnage) and the very different concept of Tonnage. IMHO just mentioning Tonnage in the See also section doesn't do the trick for these deceptively confusing terms. Lou Sander (talk) 01:56, 18 April 2015 (UTC)
Much of the confusing language was added recently. Take a look at the article as it was back in early February. Kendall-K1 (talk) 20:38, 18 May 2015 (UTC)