A ship's 'tonnage'?

I've noticed over the years that many of the travel publications, ship reviewers, internet cruise websites, etc., refer to GRT (gross registered tonnage) as 'the ship weighs in at xxxxx tons'. Let's be clear: gross tonnage is NOT a measurement of weight, it's a measurement of SPACE. One 'ton' on a passenger ship is equal to100 cubic feet of revenue-generating space (5'x5'x4') measured on the interior of the vessel (cabins, bars, boutiques, casino, gift shops, shorex, dining room, etc.). It does NOT include crew accommodations or outdoor space. This total measurement of tonnage divided by no. of passengers (2 per cabin) will determine if the ship is high or low density, i.e. crowded or uncrowded). I'm a 38 yr veteran of the cruise ship industry so I can assure the reader of this post the information to be accurate & correct! I welcome any comments or questions.

Do you have a list of low to high density ships?

I've noticed over the years that many of the travel publications, ship reviewers, internet cruise websites, etc., refer to GRT (gross registered tonnage) as 'the ship weighs in at xxxxx tons'. Let's be clear: gross tonnage is NOT a measurement of weight, it's a measurement of SPACE. One 'ton' on a passenger ship is equal to100 cubic feet of revenue-generating space (5'x5'x4') measured on the interior of the vessel (cabins, bars, boutiques, casino, gift shops, shorex, dining room, etc.). It does NOT include crew accommodations or outdoor space. This total measurement of tonnage divided by no. of passengers (2 per cabin) will determine if the ship is high or low density, i.e. crowded or uncrowded). I'm a 38 yr veteran of the cruise ship industry so I can assure the reader of this post the information to be accurate & correct! I welcome any comments or questions.

Well, as a 42 year merchant ship's officer, I can tell you that your post is not quite correct. You are correct that the ship does not "weigh" what the Gross Tonnage is, it weighs what the displacement tonnage is, and most cruise ships do not publish the displacement, as it is not important to the vessel's usage. "Gross Registered Tonnage" is an archaic term that is no longer used. "Gross Tonnage" has replaced it, and "Gross Tonnage" does not refer to 100cuft of space, it is a modified, unitless number that is derived from the gross volume inside the vessel, the modifying factor is based on complex formulas of naval architecture.

Another incorrect use you made of Gross Tonnage is the fact that Gross Tonnage does include crew spaces and machinery spaces. What you are referring to as the total of "revenue generating" space (i.e. passenger volume) is Net Tonnage, just like the Net Tonnage of cargo ships does not include tankage, machinery space, or crew accommodations.

And yes, the GT divided by the number of passengers does give the "space ratio", but it is a somewhat skewed number, in that larger ships have larger machinery spaces and larger crew spaces, and these are included in the Gross Tonnage number.

Other than that, I agree with what you say.

I would just add to chengkp's comment that displacement tonnage is important mostly to stability. The ship cannot "weigh" more than it displaces, otherwise it will sink. That's it in a simplistic form.

Also, military ships are described by displacement. The navies of the world don't worry about net tonnage at all, and frankly couldn't care less about how much space the crew has after some minimum point. Different function, different terms.

I have a feeling that the OP might be using a shorthand conversion of tonnage to space.

So where is the question? :cool:

Good work Chief. So GTs include all inside space? That makes sense.

Good work Chief. So GTs include all inside space? That makes sense.

Yes, GT is all internal volume of the vessel, hence "gross". Net tonnage is the space for cargo (in this case passengers), like net weight being without the packaging.

Interesting thread!

Sent from my SGH-M919 using Forums mobile app

just to add some confusion i would say that for cargo ship such as tanker and bulk carrier the significant measurement is the deadweight (DWT) which is the max "weight " that the vessel can load

just to add some confusion i would say that for cargo ship such as tanker and bulk carrier the significant measurement is the deadweight (DWT) which is the max "weight " that the vessel can load

Yes, gross tonnage, aside from bragging rights of cruise ships, is used mainly for port fee, registration fee with flag state, and canal tariffs. Displacement is important to naval vessels, mainly again for bragging rights or treaty limitations. The important figure used in maritime commerce is the deadweight tonnage (which also includes the fuel and water the ship carries, so not just cargo). To make the comparison between what is important even more visual, the Oasis class cruise ships are touted as the largest cruise ships in the world, with a gross tonnage of 225,000 and a displacement (how much the ship weighs) of about 100,000 tons. The Seawise Giant, which is only 6th on the all time largest gross tonnage chart at 260,000 (just a bit over the Oasis class), but Seawise Giant had a deadweight capacity of over 560,000 tons, so she could lift 5 Oasis class ships and still float.

See post submitted by LuvBowt. There is a vast difference between displaced tonnage ((cargo ships, freighters, aircraft carriers,) & GRT (gross registered tonnage). It is a measure of space or area & has NOTHING to do with WEIGHT. Different ships are measured in various ways: passenger vessels are measured in' enclosed revenue-generating space', not in weight! (See my post June 2017)

Nobody has mentioned that most of the gross tonnage is at the buffet..

See post submitted by LuvBowt. There is a vast difference between displaced tonnage ((cargo ships, freighters, aircraft carriers,) & GRT (gross registered tonnage). It is a measure of space or area & has NOTHING to do with WEIGHT. Different ships are measured in various ways: passenger vessels are measured in' enclosed revenue-generating space', not in weight! (See my post June 2017)

See my post. All ships, whether cruise ships, cargo ships, or aircraft carriers have 4 basic measurements: Gross tonnage (again, GRT has not been used since the 1980's), Net tonnage, deadweight tonnage, and displacement tonnage. All ships are measured the same way, some just use different measurements as important. In my post #3, I did not say anything about "weight", and I agreed with you that Gross Tonnage (not GRT) is a measure of enclosed volume. It was only in subsequent posts that deadweight and displacement were brought up. But guess what, if you look at the plans for a cruise ship, any cruise ship, you will find the GT, NT, Deadweight, and Displacement listed. Why? Because while different tonnages are more important to different types of ships than others, all of them need all 4 types of tonnages defined so that port tariffs, canal tariffs, flag state registration fees, the amount of fuel and water a cruise ship can carry, and the cruise ship's stability (which uses both displacement and deadweight tonnages to calculate) can be determined. This is one reason I pointed out that the Oasis has a GT of 225,000, while only displacing 100,000 tons. (You will note that I don't add a unit like "tons" to GT, since it is defined as a unitless number, while displacement is defined as the weight of water displaced, in tons.

You are correct that GT is a "generally" volume measurement, to quote the everpopular Wikipedia:

"Gross tonnage(often abbreviated asGT,G.T.orgt) is a nonlinear measure of a ship's overall internal volume. Gross tonnage is different from gross register tonnage.[1]Neither gross tonnage nor gross register tonnage should be confused with measures of mass or weight such as deadweight tonnage or displacement.

Gross tonnage, along with net tonnage, was defined by The International Convention on Tonnage Measurement of Ships, 1969, adopted by the International Maritime Organization in 1969, and came into force on July 18, 1982. These two measurements replaced gross register tonnage (GRT) and net register tonnage (NRT). Gross tonnage is calculated based on "the moulded volume of all enclosed spaces of the ship" and is used to determine things such as a ship's manning regulations, safety rules, registration fees, and port dues, whereas the older gross register tonnage is a measure of the volume of only certain enclosed spaces."

Note the reference to GT as "overall internal volume", not "revenue generating volume", which is Net Tonnage.

If you want to see the exact calculation of GT from the ship's volume simply google "gross tonnage" and go to the Wiki page. To show the non-linearity of the relation between the ship's volume and GT, here is a little chart from Wiki, as well:

Gross tons Volume (m3)

2-------------------10

24-----------------100

260----------------1000

2800---------------10,000

30,000------------100,000

320,000----------1,000,000

I was merely pointing out some inaccuracies in your statements regarding GT.

Thou shalt not question the chengkp!! I love that guy! :)

My question is I have listed all the ships I have cruised on, against a ship I'm considering.  I have the GT and GC.  My ships range from 34.xxxxx to 41.xxxxx.  So which ship is less crowded?  The higher number or the lower number?

8 hours ago, kasmith2 said:

My question is I have listed all the ships I have cruised on, against a ship I'm considering.  I have the GT and GC.  My ships range from 34.xxxxx to 41.xxxxx.  So which ship is less crowded?  The higher number or the lower number?

Not sure what "GC" is, but I'm going to assume that is the "nominal capacity"?  And, I will assume you are referring to a ratio that is "gross tons per passenger" (GT/GC)?  If so, then the higher number is "considered" to be "more spacious" or "less crowded".  I use "considered" since as I stated before, GT is not an accurate measure of public or passenger space.

They should simply put all ships on a set of scales & quote what it actually weighs, same as a bag of potatoes.

Cuts out all the nonsense about displacement, gross weight, net weight, etc.

Or is there a flaw in my argument? 

JB 

11 minutes ago, John Bull said:

They should simply put all ships on a set of scales & quote what it actually weighs, same as a bag of potatoes.

Cuts out all the nonsense about displacement, gross weight, net weight, etc.

 

Or is there a flaw in my argument? 

 

JB 

As noted earlier, the "displacement" tonnage (your bag of potatoes) is less meaningful to certain types of ships, and for certain types of transactions.  And, it's "gross tonnage" not "gross weight", since gross tonnage is not a measure of weight at all.  And, the derivation of the term "tonnage" comes from the past when a ship's size was measured in how many "tuns" (casks of beer or wine) that it could carry.  Tuns were a standard size, so they were useful for calculating volume in those days.  So, tonnage has always been about cargo volume, and not weight.  Displacement is really only cared about by the navies of the world, or when a ship needs to enter a floating drydock.  Naval architects care about displacement simply because that determines the shape and size of the hull beneath the water, which controls the power needed to propel that hull, and the speed it can attain, and it has direct effect on the stability of the vessel.  But these considerations have nothing to do with the operation of the vessel, only it's design and construction.  Gross tonnage and deadweight tonnage are used in ship's operation.

Buffet. Dave that was funny.

1 minute ago, chengkp75 said:

As noted earlier, the "displacement" tonnage (your bag of potatoes) ........

Thanks.

As always, I'll bow to your expertise - my experience of commanding a vessel is limited to being second-officer on pedalos in a park lake.

I had thought that "displacement tonnage" was the weight of water displaced by the ship.

As in measure the height of water in dry dock with & without ship to figure the difference in the volume of water & convert  that to the weight of the water displaced by  the ship. So much easier than my tongue-in-cheek suggestion of putting a ship on a set of scales.

Is there such a measurement?

JB 

3 minutes ago, John Bull said:

 

Thanks.

As always, I'll bow to your expertise - my experience of commanding a vessel is limited to being second-officer on pedalos in a park lake.

 

I had thought that "displacement tonnage" was the weight of water displaced by the ship.

As in measure the height of water in dry dock with & without ship to figure the difference in the volume of water & convert  that to the weight of the water displaced by  the ship. So much easier than my tongue-in-cheek suggestion of putting a ship on a set of scales.

Is there such a measurement?

 

JB 

 

 

That’s pretty much what displacement tonnage is.... the weight/mass of the ship measured in tons or tonnes.

uh.. this is all so confusing,  😵  I need a drink 

This info is all wrong. The GT is a number that you take and divide by how many people are on the cruise so you can prepare yourself with how bad your want your attitude to be because the "ratio" isn't what you want it to be. If that ratio was higher, you would clearly have a shorter line in the buffet at lunch time or 20% more free chairs on the popular deck during peak hours.

26 minutes ago, John Bull said:

 

Thanks.

As always, I'll bow to your expertise - my experience of commanding a vessel is limited to being second-officer on pedalos in a park lake.

 

I had thought that "displacement tonnage" was the weight of water displaced by the ship.

As in measure the height of water in dry dock with & without ship to figure the difference in the volume of water & convert  that to the weight of the water displaced by  the ship. So much easier than my tongue-in-cheek suggestion of putting a ship on a set of scales.

Is there such a measurement?

 

JB 

 

 

Yes, displacement tonnage is the actual weight of the ship (actually the weight of the water displaced by the ship when placed in the water).  But, as I say, displacement is not that relevant in ship operations, so it is not used much.  A cargo ship does not care how much it weighs, it cares about how much cargo it can carry (net tonnage if light, less dense cargo like ping pong balls), or deadweight tonnage (for lead ingots or a tanker full of oil).  A cruise ship doesn't care how much it weighs, it cares about the volume of space it has for passengers, so again net tonnage is important, but net tonnage figures are not generally published, so gross tonnage is used.

To show why displacement tonnage doesn't really matter in operations, I'll repeat my comparison of the Seawise Giant and the Oasis of the Seas:

Gross tonnage:

Oasis:  224,000

Seawise Giant:  260,000 (so about 16% larger in volume)

Displacement tonnage:

Oasis:  100,000 tons

Seawise Giant:  81,000 tons (no cargo) (so weighs 19% less)

Deadweight tonnage:

Oasis:  15,000 tons

Seawise Giant:  564,000 tons (3700% more cargo capacity)

So, a ship (Seawise) that is only 16% larger in volume, but weighs 19% less can lift 3700% more cargo.  So, comparing the displacement of the two ships is worthless, since the two ships use their displacement differently, and obtain two widely different capacities (Seawise is very poor on the passenger comfort point). It's simplistic, I know, because the Seawise has a displacement of 646,000 tons when loaded, but it gets the point across about the different tonnages being important to different types of ships.

Joebucks:  so if you look at my chart, the miracle is the least crowded, followed by the Sun?