Posted by Andrew Craig-Bennett on December 06, 2002 at 10:33:05 from 195.93.49.13 user ACB.
In Reply to: Tonnage posted by John Lambert on December 06, 2002 at 08:35:05:
Be warned that I have a slight professional involvement here and can get tiresome about it....
In medieval Europe, Kings wanted some means of assessing the size of ships, in order to collect harbour dues, to know what ships might be handy in the event of a war, and so on. Since the principal trade of the time involved pickled herrings in barrels (if you were members of the Hanseatic League or Dutch) or Bordeaux wine in barrels (if you were English) the idea of measuring the number of barrels a ship could carry commended itself.
The method fixed on was to measure the number of barrels of a standard size, called a tun, that could be carried under a ship's deck . Hence "tunnage", which became tonnage. Over time, rather than lug barrels in and out of a ship to measure her, calculations were adopted instead. To make this easier, it was accepted that a "tun" barrel holds 94 cubic feet.
The most important method of calculation, used until the nineteenth century was called "Builder's Old Measurement", and it is, (I think!):
length on keel x beam x depth in hold in feet
________________________________________ = tons
94
A variant of this was adopted by the Royal Thames Yacht Club in the 1840's for rating racing yachts, and is still in use as "Thames Tonnage":
length on deck from stem head to rudder post x beam x half beam
________________________________________________________________ = tons
94
One of Patrick O'Brian's anachronisms has Aubrey and Maturin discussing Thames Tonnage, cooked up in the 1840's, when they should be using Builder's Old Measurement.
British yachts were measured, for informal purposes, in Thames Tonnage until quite recently, when the glass fibre boat factories switched over to overall length in feet. Some of us die hards, like Ian Wright and myself, still use Thames tonnage. The "Goblin" for example, is a seven tonner.
Merchant ships still need to be measured for port dues, and a variation on the above system, called Register Tonnage, is used for them. This is a complex subject, because money is involved, but essentially the volume under the weather deck in cubic feet is calculated and divided by 100 to give the gross tonnage; the volume occupied by crew accomodation, propelling machinery and navigation spaces is then deducted to give the nett tonnage.
This tonnage is carved, with the ship's official number, on the main beam of all British ships registered under Part One of the Merchant Shipping Act of 1894. The tonnage that John gives the Customs officers at Harwich is "Goblin"'s Nett Register Tonnage, which is less than her Thames Tonnage, but John is right - it is her Nett Register Tonnage that the Customs men want for their form. The "Welcome" of Rochester has an approved deduction for accomodation space carved over the bunk that Port and Starboard sleep in - "Cert. To Accomodate 1 Seaman"
This is not the end of the story. We also need to know how much cargo a merchant ship will carry, and since we don't shift an awful lot of barrels of herring and wine these days, we measure the weight of cargo and stores, including fuel, that a ship can load before she is down to her Plimsoll line. This is her Deadweight tonnage, and unlike any of the tons we have looked at so far, it is a real weight. Some of the deadweight will be used for fuel and stores and the rest can be used for cargo.
The volume of the available cargo space is still measured in actual cubic feet, and there is another phoney ton, used for charging freight, called a Bill of Lading Ton, or Bacon Lettuce and Tomato Sandwich, which is usually 42 cu.ft. - a figure close to the weight of water, or course.
Now when a ship is being built, and when she comes to be scrapped, we need to know the actual weight of the structure, (or how much water she displaces when empty, which is the same thing, of course) and this is called the Light Displacement Tonnage, or LDT. You will notice that this is the very first time that we have considered the actual weight of the ship!
Warship's don't carry cargo, or pay port dues, so they are simply measured by their displacement - their actual weight.