10.ship tonnage

The sketch at the right shows the principal dimensions of a cargo ship. The length can be the length overall (LOA) or the length between perpendiculars (LBP) at the water line. The depth is measured from the keel to the upper continuous deck. The draft is measured from the keel to the water line of the loaded ship. The beam is the width of the ship. The front of the ship is the bows, the rear the stern. The starboard side is the right side, facing the front of the ship, while the port side is the left. Our example ship, the AP2 (or VC2) Victory ship, has a LOA of 455 ft, a LBP of 436.5 ft, a beam of 62 ft, a depth of 38 ft, and a draft of 28.5 ft.

The tonnage of a ship is not a weight, but a volume. One ton is 100 cubic feet. The total internal volume of a ship is its gross tonnage, and if we subtract all the volume not used for cargo, we get the net tonnage. The AP2 had gross tonnage 7850, net tonnage 4850. This means that the cargo occupied 485,000 cubic feet and fuel, engine, crew quarters, etc. occupied 300,000 cubic feet. From the ship's dimensions, we find that LBP x beam x depth = 1,028,000 cubic feet, which, of course, is somewhat greater than the gross tonnage, but is consistent with it.Formulas were created to estimate the tonnage of a ship from its dimensions. For wooden ships, Builder's Old Measure was instituted in 1773, in which tonnage = (L - 0.6W)W²/188. Applied to the AP2, this formula gives 8540 tons, somewhat less than the actual tonnage, indicating that the modern steel ship is less "blocky" than the wooden cargo ship. Of course, formulas applying to the newer ships have also been developed. The tonnage of a ship was used for assessing port dues and other charges.The total weight of the ship and everything in it is the displacement, measured in long tons of 2240 lb. A long ton is only a little larger than a metric ton of 1000 kg, but is considerably larger than the U.S. short ton of 2000 lb. The displacement of the AP2 is 15,200 tons. This weight displaces 532,000 cubic feet of sea water weighing 64 pcf, and loads the ship down to the normal water line. The deadweight is the difference in displacement when the ship is completely unloaded, of cargo, fuel, crew and so forth. The cargo deadweight is the weight of the cargo alone. For the AP2, the deadweight is 10,800 tons. Therefore, the weight of the ship alone is 15,200 minus 10,800 tons, or 4400 tons. The reference does not give the cargo deadweight of the AP2, but if it is in the same proportion to the deadweight as the net tonnage is to the gross tonnage, it would be 6673 tons. Most of the difference would probably be fuel.