Tonnage

[happy daze john in oz]

How then with cruise ships that carry no cargo.
When I once asked the skipper of one I was told the gross tonnage was based on total space within the ship
But having read the bit by Ivan about the 22 pages I just wonder.

[j.sabourn]

Yes but would hate to see others pull the cork first . JS

[j.sabourn]

#41.. John if you are referring to canal dues , every ship has a Suez Canal,certificate which declares its tonnage as per the amount of dues. You will probably find if look it up somewhere that passenger ships also pay on the heads of passengers also. Like most things everything isn’t as easy as some seem to think. Years ago on this site it surprised me that how many didn’t know why the focsle head was cleared out before transit. This was because the space under the focsle was only exempt from dues if empty you would need a Suez Canal cert. to properly answer your question . Unless anyone has read one recently. As like most rules and regulations regarding money and cash flow change quite often. Cheers JS

Apart from dates and canal dues I would imagine they were the two biggest incomes for Egypt. JS

[happy daze john in oz]

Ivan and JS, thank you for all the education I am getting, much of which I should have learned when at sea, but catering never needed to know.

Ivan you mention the 22 pages regarding tonnage.
My question is in relation to displacement tonnage.
I was once told that all Royal Navy ships are measured by displacement tonnage, not sure if this is correct.

But displacement tonnage must vary, a ship fully laden will displace more than one without.
But during a voyage there must be a change in displacement tonnage as fuel, water and other suplies are used.
Is this correct or is the change so small that it is not accounted for?

[j.sabourn]

Ivan can answer that one if he likes , RN ships when being described is usually on their total displacement fully armed and stored , so when you say , a 10000 ton warship that’s what she weighs. A merchant ship Is usually described by seamen as their deadweight tonnage that the amount of cargo weight wise the can carry, this of course will vary according to what marks you are loading to, so is usually referred to as Summer load line. Ivan’s got the hard work to do if he wants to carry on. Cheers JS.

[Ivan Cloherty]

John Oz, John JS has more or less covered it, displacement is calculated when fully laden with ammo, stores fuel etc, As RN are not commercial vessels their tonnage is not as critical as MN vessels where-in every revenue tonne is important. So tonnage is used as a 'Class' rather than a true factor as in the old days where a battleship would be classed as a 36000 ton class, but in fact would displace anywhere from 33000/36000 tons, it was also a political factor where-in an opposing faction would declare they had an 38000 tonner hoping that would give them a psychological advantage. Although built on the same hulls, displacement of Battleships could vary by about 2/3000 tons depending upon number and size of guns as a vessel with 14" guns would need heavier shells than a battleship fitted with 12" guns also the varying weight of the guns would be taken into account. But it really was a Class thing as in a 9000 tonner as opposed to 12000 tonner etc. But I will study my 1919 edition of my RN seamanship book in case they can offer further clarification.

[j.sabourn]

It might be worth mentioning as regards class for naval vessels , after the WW1 , and the treaty of Versailles , Germany was restricted to the tonnage of their naval vessels, they got past that by building the pocket battleship to comply with the tonnage rules, they packed as big a punch as a conventional battle ship however which our Lords of the Admiralty either failed to notice , or they decided to turn a Nelson’s blind eye to. Cheers JS.

[Ivan Cloherty]

Oz John further to my #46 I have digested my 1909 (updated 1917) Royal Navy Oracle and there is not much more to add, it confirms that the RN uses 'Displacement' as a measure of comparison for their vessels and it includes fully armed, crewed and fueled. Each vessel has a calculated Gross and Nett tonnage for harbour dues and Suez canal transits. note :- this oracle was written before Panama opened, which has its own dues measurements. Like MN vessels all RN ships have a Gross and Nett UK Certificate as well as a Suez Gross and Nett Certificate

These are 1909 examples, but the formulaes (too detailed to go into) are still relevant today

Battleship 14000t displ Suez gross 8184t Nett 4359t UK gross 8105t nett 4689t

1st Class cruiser 13550t displ Suez gross 7814t nett 3516t UK gross 7754t nett 3550t

Destroyer 495t displ Suez gross 495t nett 223t UK gross 487t nett 17t

As you can see there are differences the biggest difference being in the destroyers nett tonnages.

If you need more let me know.

[happy daze john in oz]

But what about a tanker or a container ship.
When fully loaded they would displace a different amount to when unladen, and displacement as a measure of tonnage is what I was wanting to know.
From your figures so far I think I now understand how the difference between the rating for RN ships.
But a container ship may drop off containers with out taking more on at that port.
How much would such actions effect the displacement tonnage?

[j.sabourn]

John if you want to measure the weight of a ship you first learn the rule of Archimedes who’s the reason your ship floats and doesn’t sink , that a body immersed in water will sink until it displaces its own weight .you have draft figures and plimsol marks to enable you to do this also the mate has an intrument for measuring the specific gravity or the density of the water you are floating in and will probably get a clean bucket from somewhere and get it from over the side , and it is not magic to work out the total weight of ship and everything on it. The small Instrument he uses for this is called a hydrometer and not a hygrometer that is inside your Stevenson screen at the bottom of your garden , if I have got them the wrong way round put it down to it being a long time since have done. You can always find your displacement if in still water and can read your drafts and plimsolls. Your dues for canals however unless otherwise specified are on measurement tonnage and not usually weight and this is why you have a certificate for the Suez and probably a different one for the Panama. Depending how hard up the dictators of those two countries are. Gross and net tonnage are purely a measurement of space of different parts of a vessel. To go into open open and closed shelter deck tonnages you would first have to understand what these are.If you can find a small paperback on tonnages and keep on reading it , you will get there. Kemp and Hughes used to have one I believe. I am just as liable to tell you the wrong thing today so don’t take me as gospel. Cheers JS