Architecture A bulk carrier's design is largely defined by the cargo it will carry. The cargo's density, also known as its stowage factor, is the key factor. Densities for common bulk cargoes vary from 0.6 tons per cubic meter for light grains to 3 tons per cubic meter for iron ore. The overall cargo weight is the limiting factor in the design of an ore carrier, since the cargo is so dense. Coal carriers, on the other hand, are limited by overall volume, since most bulk carriers can be completely filled with coal before reaching their maximum draft. For a given tonnage, the second factor which governs the ship's dimensions is the size of the ports and waterways it will travel to. For example, a vessel that will pass the Panama Canal will be limited in its beam and draft. For most designs, the ratio of length-to-width ranges between 5 and 7, with an average of 6.2. The ratio of length-to-height will be between 11 and 12. Machinery The engine room on a bulk carrier is usually near the stern, under the superstructure. Larger bulk carriers, from Handymax up, usually have a single two-stroke low-speed crosshead diesel engine directly coupled to a fixed-pitch propeller. Electricity is produced by auxiliary generators and/or an alternator coupled to the propeller shaft. On the smaller bulk carriers, one or two four-stroke diesels are used to turn either a fixed or controllable-pitch propeller via a reduction gearbox, which may also incorporate an output for an alternator. The average design ship speed for bulk carriers of Handysize and above is . The propeller speed is relatively low, at about 90 revolutions per minute, although it depends on the size of the propeller. As a result of the 1973 oil crisis, the 1979 energy crisis, and the resulting rise in oil prices, experimental designs using coal to fuel ships were tested in the late 1970s and early 1980s. The Australian National Lines (ANL) constructed two 74,700-ton coal-burner ships called River Boyne and River Embely. along with two constructed by TNT called TNT Capricornia and TNT Capentaria and renamed Fitzroy River and Endeavor River. These ships were financially effective for the duration of their lives, and their steam engines were able to generate a shaft-power of . This strategy gave an interesting advantage to carriers of bauxite and similar fuel cargoes, but suffered from poor engine yield compared