Tonnage/Displacement

Hi!

On another thread, while correctly acknowledging Titanic's displacement of 52,310 tons at 34 feet 7 inches, Dave Gittins expressed astonishment at the 'registered displacement' figure of 77,780 tons that I quoted, which appears on the British Registry entries for Olympic and Titanic, and a slightly higher figure for Britannic. H esaid that he had never heard of such a term.

It is clear that Titanic did not displace nearly 80,000 tons, but then: what exactly does 'registered displacement' signify? Or has there been an error in all the British registers?

Best regards,

Mark.

 

The builders notebook says the displacement is 34 feet 7 inches at 52,310 tons with a net carrying weight of 12,770 tons.

I have never heard registered displacement. To me the term is familiar but only to the Great Lakes where vessels are required to register there loaded displacement for entrance into the Sarnia and Soo traffic systems. I would assume that in Titanic's day that was the way of keeping records on how much the ship weight when loaded and how much water she displaced. But I defer to Dave Gittin's on the English system.

 

Hi Mark.

I've found a document (PDF format) on the net called: "EU Ship Emissions to Air Study." Although it doesn't deal with the subject you've raised, a footnote provides information on the different categories by which tonnage of a vessel is calculated, none of which appears to be "Registered Displacement".

You probably know this information already. However, should it prove useful, the URL is as follows: http://europa.eu.int/comm/environment/enveco/taxation/ship_emissions/app5final.pdf

Regards,
Paul.

 

Displacement is applicable to warships and certain other non-registrable, non-revenue-earning vessels and structures. As regards merchant vessels it is primarily of interest to naval architects (and, at some prevailing condition of lading, salvors) for obvious reasons.

'Registration' implies primarily a taxation parameter deriving from revenue earning capability in various forms and is therefore restricted to a commensurate volumetric, i.e. 'gross or nett register'. Different registries and toll gatherers, notably the Panama Canal authority, have different parameters for assessing these taxation 'tonnages'.

Deadweight, insofar as it is a registration parameter, derives from the displacement at extreme conditions of lading and is primarily applicable to tankships.

In other words, "registered displacement" apropos merchant vessels is a misnomer. This is not to say that some obscure registry somewhere does not demand it!

If my memory serves me right, displacement does not feature on the certificate of registry of general cargo and passenger vessels.

Noel

 

In 1912, "registered displacement" indicated that the merchant vessel complied with load-line legislation. It was only in 1890 that an Act of Parliament made compulsory the adherence to a set of load-line tables framed by a Committee of Lloyd's Register of Shipping. There was much controversy concerning these regulations, because the marking of the load-line affected the reported displacement of a vessel, which in turn affected the assessment of the earnings of ships. Dock and harbour dues were paid on the earning power of a particular merchant hull, not the service rendered. Before regulation, competitive shipbuilders sometimes tended to increase the depth of loading at the expense of available freeboard, which endangered the vessel's seaworthiness. Moorsom's rules for under-deck tonnage, embodied in the Merchant Shipping Act of 1854 and which served as the basis for Board of Trade acceptance for the following 40 years, was made obsolete by technological advancement, especially in the business of steam-powered shipping. Many schemes to standardise a fair assessment of merchant hulls fell apart, due to the diverse nature of the trade. Passenger ships tended to be undertaxed, while ships carrying heavy cargo were overtaxed, with respect to their potential earnings. The field was equalised by enforcing adherance to a set of displacement tables (which fixed the load-line for each particular type of vessel) developed by Lloyd's chief surveyor, Mr. Martell, and by requiring that each vessel carry an officially guaranteed "Curve of Displacement." Of course, this requirement affected only British shipping, so in order to remain competitive in the world market, additional legislation was required to restrict foreign-flagged vessels from British ports that did not comply with Lloyd's displacement tables.

This evolution in merchant ship legislation was still in recent memory by Titanic's time. The term, "registered displacement," indicated a standardised measure of the ship's earning potential, as documented in a certificate that was issued by Lloyd's (or the Bureau Veritas or the British Corporation) and recognised by the Board of Trade. Over time, I assume that use of the term faded as standardised load-line fixing became universally accepted, which probably explains why it is not found in general use today.

Parks

 

Hi!

Thanks Paul, Erik, Noel and Parks, for your replies. As usual, they were detailed and appreciated. I will have to check back with my notes of the registers (they are too big to photocopy) in order to find further details that might help us clarify the situation.

In the last post, Parks wrote:

My problem here is the units -- 77,780 tons would not seem to respond to any earning potential. Could you measure it in tons or have I misunderstood? I will also try and post back Mauretania's figures which might tell us more. I am sure you all know far more about it than me.

Apologies for my ignorance.

Best regards,

Mark.​

 

The way that is worded it makes it sound as if there is some formula (which there could be) for determing earnings based on revenue and some how combining it with tonnage. I am completely lost and a little worried. Not to mention a little woozy.

 

Mark and Erik,

Do you have access to any edition of White's Manual of Naval Architecture published before 1912? Chapter 3, Tonnage of Ships, explains in comprehensive detail the relationship between tonnage and displacement -- and therefore earnings potential -- but I don't know when I will have time again in the near future to work another summation of the material. I'm sure that if you can find a copy of White's manual in your library, you will find satisfying answers to your questions before I am able to collect enough free time to gather all the information that you need.

This is not a blow-off...with my wife opening a new restaurant next week (knock on wood...construction delays have postponed the opening date three times now), the loss of our nanny during the holiday season and the looming deadline for "Ghosts of the Abyss," not to mention the demands of my paying job (where we're readying another aircraft carrier for deployment), I don't have much time right now for in-depth research/analysis. The only reason why I was able to summarise two chapters of White's last night was because of a cancelled appointment that left me waiting at home with a couple hours' worth of unexpected free time. If you can't find a copy of White's, I'll get to your questions as soon as I can but I can make no guarantees on when that will be.

Parks

 

Hi Parks,

Thanks for your time. I can see from your post how busy you are, so thank you for taking the time to respond in depth. Thanks also for your first reply on the other thread, 'Aftermath: Who Do You Think Was To Blame?' There's no hurry.

I do appreciate this, thanks again.

Best regards,

Mark.

 

Mark,

As soon as I can, I'll answer your question, one way or the other. If the answer proves to be too involved/lengthy to post here, I'll post it on my website and notify you here that it's up. I read through the material quickly the other night and will have to figure how to condense a chapter and a half into a concise Web-friendly explanation.

Parks

 

Thanks for the info Parks. I look forward to your post.

 

I reviewed the earlier thread; unfortunatley, it's all beyond me. Most books say the ship weighed 46,000 tons and displaced 66,000 tons. but then there are different numbers, and net vs. gross.
For us beginners, can someone provide a primer on the differences.
Also, Archimedes Principle of Buoyance (if I recall basic physics from many years ago correctly) says, essentially: You have a block of wood, 1 cu. ft. in volume, which weighs 10 lbs. Water weighs (let's posit) 20 lbs. per cubic foot. So, the block of wood will sink until it displaces (pushes aside) its own weight, or 10 lbs., which means in this case it will sink down half way. If we drill and whole, fill it with lead, and make the wood weigh 21 lbs, it will continue to sink deeper and deeper, never pushing aside 21 lbs, because it's equivalent volume of water is 20 lbs.
NOW, as to the ship, how to you measure it's volume for displacement purposes? I assume it's a bit more complex than my block of wood example.
As usual, I anticipate pages of responses. You all get an "A+" in advance.

 

From the Glossary of Nuatical terminolgy at http://www.seagrant.wisc.edu/shipwrecks/glossary.html

gross tonnage
The overall volume of a ship's hull, including crew cabins, storerooms and machinery spaces. A ton equals 100 cubic feet. The calculation of tonnage is complex, and a major revision in tonnage calculation laws occurred in 1864. The term "old measurement" reflects measurements before this change.

Net tonnage
The volume of cargo a ship could carry, equal to gross tonnage minus the crew cabins, storerooms and machinery spaces. One ton equals 100 cubic feet.

From Wreck Commissioner's Report, Description of the Ship, The Steamship Titanic:The "Titanic" was a three-screw vessel of 46,328 tons gross and 21,831 net register tons,...

It also points to the ship's actual displacement (Which would be the actual weight of the ship) as being 52,310 tons when fully loaded to a navigation draft of 34 feet 7 inches.

If memory serves, the 66,000 ton figure comes from an ad man confusing the issue by adding the gross tonage to the net tonnage. Unfortunately, it became a part of the Titanic mythos by being repeated uncritically by a number of writers over the years.

I hope this helps.

 

I believe this error was first noted here: http://groups.google.com/groups?q=66,000+group:alt.history.ocean-liners.titanic&start=30&hl=en&lr=&ie=UTF-8&scoring=d&selm=6j8l3i%24jed%40bgtnsc03.wor ldnet.att.net&rnum=31

 

Here is an image purportedly showing the dimensions for Titanic and Olympic. Note that their displacements are given as 52,160 tons instead of the commonly accepted 52,310. Was a change made to increase their displacement which I am unaware of?

More interesting tidbits there: Titanic's GRT is actually given as 46,329 instead of 46,328 tonnes.

 

I can't comment on the displacement. Titanic's Registration Certificate gives GRT as 46,328.57 tons. It seems somebody rounded it up.

While I'm at it, tonnes have nothing to do with it. In the context, tons were a measure of volume at 100 cubic feet per ton.

Today Gross Tonnage is only an index number and is not expressed in tons or tonnes. The correct usage is RMS Emetic's Gross Tonnage is 55,123.

 

Darn it, should've known something was misspelled/misused. Speaking of discrepancies, your site (still up on Web Archive) mentioned Titanic's hull depth to be 64'3", while in that pic it is given as 64'6" -- is this due to difference in measurement methods or something else? That's the only other noticeable difference on the dimensions book aside from the 34'6" load draft (which I have been informed was the load draft at the bow, 34'7" being load draft at the stern).

 

On a related note. The Titanic was nearing the end of her maiden voyage and many tons of coal and a heavy amount of food stocks would have been used up and substantially lightened her tonnage by the time she struck the iceberg. Not to mention the weight of her lifeboats, passengers and crew would lighten the ship once they had left the ship. Not sure how much lighter the ship was during the latter stages of the evacuation. Perhaps the Titanic sank much slower than Thomas Andrews had estimated because they were continually removing lifeboats and people off the ship which may have lifted the ship up at the same time the water flooded in and weighed her down, which created the impression she was settling down very slowly.


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