IRON ore is one of the most difficult cargoes in the world to carry, because it is so dense and puts a ship down to her load marks without actually filling the hold. For various technical reasons the higher the iron ore in any given hold can be raised above the bottom of the ship, the better she is in a rough sea, particularly when rolling. On the other hand, since a large number of iron- ore carriers do not take any other kind of cargo, but run light from their discharging berth back to loading berth, it is necessary to provide adequate immersion for their screws.
Big ballast tanks are necessary for this purpose and, as the modern ore carrier has developed, there has been a tendency to raise the bottom of the hold, placing at the same time ballast tanks underneath. The drawing above shows one of two ships owned in Sweden, built in Germany and operating on a long-term time charter to the Bethlehem Steel Corporation to carry iron ore from an open roadstead in Chile, known as Cruz Grande, to Sparrows Point in Maryland, U.S.A.
They spend their working life running between these two terminal ports, through the Panama Canal, and spending fewer than twelve hours in loading and not much more than twenty-four hours in discharging. This is done by mechanical grabs of special type originally invented for work on the Great Lakes, where iron ore is carried in considerable quantities.
The ships, known respectively as the Svealand and the Amerikaland, are really three huge boxes, surrounded by water-ballast tanks, with a ship-shaped forward end and a propelling motor-room at the after end. The drawing shows the arrangement of holds, ballast tanks and hatch-lifting gear. The hatches are of steel plates rolled semi-circularly in a thwartship direction. These vessels are two of the biggest in the world with machinery aft.
The Svealand was built at Hamburg in April 1925 and the Amerikaland in June 1925, also at Hamburg. Either has a deadweight tonnage of 21,000 and a gross tonnage of about 15,355. They have a length of 561 ft. 4 in., a beam of 72 ft. 3 in. and a depth of 44 ft. 1 in. With their three big box cargo holds fully loaded with ore they draw just over 34 feet of water. Propulsion is on two screws by means of two four-cycle diesels built in Germany. Either engine has eight cylinders of 29.12 in. diameter and 47.25 in. stroke. The engines develop a total of 4,850 brake horse-power at 115 revolutions. When fully loaded the ships can motor at about 11 knots burning 20 tons of fuel a day.