The Cargo Ship


Here are a few notes on the characteristics of a typical cargo ship of just after 1945, and things have probably not changed greatly. The terms are often very confusing if you are not familiar with them, so I have explained them as best I can. There are some variations, but I have chosen what appears to be the most common practice. The technical information is taken from Kent's handbook.

The knot is a unit of speed, one nautical mile (6080.22 ft) per hour. Multiply speed in knots by 1.15 to get speed in mph. The nautical mile is one minute of arc on a great circle of the earth, assumed to be a sphere of radius 6371 km. It is an annoyance that also appears in wind velocities from official sources, but it does make it easy to find the distances along great circles, which is why it was originally defined.

Sea water weighs about 64 pcf, or 1026 kg/m3. Buoyancy is discussed in Hydrostatics. The action of propellers, though mainly for air, is treated in Fans.

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.

The Victory ships, of which the AP2 is an example, were a little larger than the Liberty ships that preceded them, but much less spartan. The first, SS United Victory, was launched 28 February 1944, and there were 534 in all. Like the Liberty ships, they were all-welded. Instead of the 2000 hp triple-expansion reciprocating engine of the Liberty ships, they had a 6000 hp (or 8500 hp) cross-compound steam turbine geared to the propeller axis. This raised the speed from 11 knots to 15.5 knots, lessening the danger from submarine attack. They were armed with a 5" stern gun, a 3" bow gun, and 8 20mm machine guns. The fuel in both cases seems to have been Bunker C fuel oil. The relation between horsepower, speed and displacement is given approximately by the Admiralty Coefficient C = (displacement)2/3(speed)3/hp, where displacement is in tons and speed in knots. For a typical cargo ship of the period, C was about 400. For the AP2, it was 427, using the slightly different figures 14,800 tons and 16.2 knots. The displacement and the speed could differ under different coditions of loading, so the figures are not always consistent.

The AP2 had a single propeller of 18.25 ft. diameter and 4 blades, rotating at 100 rpm. The good-practice diameter of a propeller in feet is given by d = 50(hp/rpm3)1/5, and this propeller agrees with the formula. The pitch of the propeller was 17.5 ft. The power plant was amidships, with a long shaft to the propeller in the stern.

This ship carried approximately as much as a heavy U.S. railway freight train (7000 tons), using about the same horsepower, but the freight train will move at twice the speed of the ship on the level, or more, up to about 48 knots. The train will require a crew of three at the present time, or nine considering 24-hour operation, while the Victory ship had a crew of 54. There are no tracks in the sea, nor water on land, but the comparison is interesting. Ships today seem to carry most of their cargo in containers on deck, which must affect their stability; it would be nice to know how this is managed. A cargo ship generally proceeds at a constant full speed, about 80% of the full horsepower available, when at sea, and is most efficient at this speed.

References

J. K. Salisbury, ed., Kent's Mechanical Engineer's Handbook, 12th ed. (New York: John Wiley & Sons, 1950). Power Volume, pp. 15-69 to 15-83

US Maritime Museum has a photograph and a cross section of a Victory ship. SS Lane Victory is on display in San Pedro, and SS American Victory in Tampa.


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Composed by J. B. Calvert
Created 27 July 2003
Last revised