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The allowable size is limited by the width and length of the available lock chambers, by the depth of the water in the canal and by the height of the Bridge of the Americas. Ships that do not fall within the Panamax-sizes are called Post Panamax. The limits have influenced those constructing cargo ships, giving clear parameters for ships destined to traverse the Panama Canal.
"Panamax" has been in effect since the opening of the canal in 1914. In 2009 the Canal management published the "New Panamax", that will be in effect when the third lane of locks, larger than the current two, are operational from 2014.
The increasing prevalence of vessels of the maximum size is a problem for the canal as a Panamax ship is a tight fit that requires precise control of the vessel in the locks, possibly resulting in longer lock time, and requiring that these ships transit in daylight. Because the largest ships traveling in opposite directions cannot pass safely within the Gaillard Cut, the canal effectively operates an alternating one-way system for these ships.