Panama Canal - Panamax

The Panama Canal was designed and built to accommodate the World War I battleships, Arizona and Pennsylvania. These vessels were 106 feet in beam and had drafts of 34 feet with displacements of 34,000 tons. By comparison, during World War II, larger military vessels, battleships, and aircraft carriers with beams of up to 108 feet, drafts of 38 feet, and displacements of 53,000 tons transited the Canal. These World War II vessels barely fit the 110-foot-wide lock chambers with less than 12 inches between the ship's sides and the concrete lock walls.

For instance, the fourth Missouri (BB-63), the last battleship completed by the United States, was laid down 6 January 1941 by New York Naval Shipyard and launched 29 January 1944. After trials off New York and shakedown and battle practice in Chesapeake Bay, Missouri departed Norfolk 11 November 1944, transited the Panama Canal 18 November 1944 and steamed to San Francisco for final fitting out as fleet flagship. Ships of this class had a Full Displacement of 57,271 tons, an Overall Length of 888 feet, an Extreme Beam of 109 feet and a Waterline Beam of 108 feet, with a Maximum Navigational Draft of 38 feet.

The successful transiting of these vessels set precedence for the passage of the larger commercial vessels of today. Known as Panamax, these vessels have displacements of over 70,000 tons, which is more than double the size of the designed lock capacity. The largest ship capable of transiting the canal is called a "Panamax." The size of a ship transiting the canal is limited by the size of each lock chamber. Vessels must measure less than 32.3 meters / 105 feet at the beam (width), 294.1 meters / 956 feet in length for containerships and 289.6 meters / 991 feet in length for other commercial vessels, and 12 meters / 39 feet in draft [the portion of the vessel submerged below the water line].

Ships too large to transit the canal are called "post-Panamax." Transits of Panamax vessels are increasing, representing one-fifth of all transits in 1983 and one-quarter of all transits in 1988. By the mid-1990s, one out of every three ships was a Panamax transit. The average size of a ship in the world oceangoing fleet is increasing and more ships are being built as post-Panamax vessels. During the 1980's, about 92 percent of the world cargo fleet could use the Panama Canal, but by 2000, it was estimated that only 82 percent were able to use it.

Panamax-size ships make a more effective transit through the canal by carrying more cargo, but they diminish the efficiency of the canal because they are limited, by canal policy, to daylight transits, require extra pilots and line handlers, take longer to traverse a set of locks, and are restricted to single passage through the narrowest portions of the canal in the Gaillard Cut. In fact, the lockage of a laden Panamax vessel requires 40 percent more time than a vessel with a beam under 30 m because the narrowest passageways through the Gaillard Cut prevent the largest vessels from safely passing one another.

Water will not compress. It takes an enormous amount of energy to force the oversized Panamax vessel into a lock chamber. In order for a pilot to get a Panamax into a lock chamber, the vessel's engines must be placed at full speed ahead and the electric locomotives operated at maximum towing capacity. In some cases tugboats are directed to assist with the lockage by pushing on the stern of the vessel. Each time a Panamax vessel is forced into a lock chamber, the whole structure begins to vibrate. Cracks can be observed in the concrete lock walls, and the steel miter gates leak.

Most vessels now built for the world's container fleet are too large for the canal. These larger vessels are built to carry more containers and make fewer port calls. In practical terms, larger ships mean the operating costs of a vessel can be distributed across more containers. These larger ships, however, require ports with deeper drafts and more shoreside services, and are in port longer to unload and load-back containers. As such, these ships are scheduled to call on fewer ports and use alternative inland modes to transport containers to final position, rather than transit through the Panama Canal.