Sea Land Mercury Panamax Maersk Line Container Ship seen here in the Miraflore Locks in the Panama Canal.
From Wikipedia: Panamax is determined principally by the dimensions of the canal's lock chambers, each of which is 110 ft (33.53 m) wide by 1,050 ft (320.04 m) long, and 85 ft (25.91 m) deep. The usable length of each lock chamber is 1,000 ft (304.8 m). The available water depth in the lock chambers varies, but the shallowest depth is at the south sill of the Pedro Miguel Locks and is 41.2 ft (12.56 m) at a Miraflores Lake level of 54 ft 6 in (16.61 m). The height of the Bridge of the Americas at Balboa is the limiting factor on a vessel's overall height.
The maximum dimensions allowed for a ship transiting the canal are:
Over all (including protrusions): 950 ft (289.56 m) Exceptions:
* Container ship and passenger ship: 965 ft (294.13 m)
* Tug-barge combination, rigidly connected: 900 ft (274.32 m) over all
* Other non-self-propelled vessels-tug combination: 850 ft (259.08 m) over all
Width over outer surface of the shell plating: 106 ft (32.31 m) General exeption: 107 ft (32.61 m), when draft is less than 37 ft (11.3 m) Tropical freshwater.
In tropical fresh water 39.5 ft (12.04 m). ACP uses the freshwater Gatun Lake as a reference. The salinity and temperature of water affect its density, and hence how deep a ship will float in the water. When the water level in Lake Gatún is low during an exceptionally dry season the maximum permitted draft may be reduced.
190 ft (57.91 m) measured from the waterline to the vessel's highest point; limit also pertains to Balboa harbor. Exception: 205 ft (62.5 m) with passage at low water (MLWS) at Balboa is possible
All exceptions are typically allowed only after specific request, an investigation and on a one- or two-time only basis.
A Panamax cargo ship would typically have a DWT of 65,000-80,000 tonnes and a maximum cargo intake of 52,500 tonnes.
The longest ship ever to transit was the San Juan Prospector, now Marcona Prospector, an ore-bulk-oil carrier that is 973 ft (296.57 m) long, with a beam of 106 ft (32.31 m). The widest ships to transit are the two North Carolina class battleships, USS North Carolina (BB-55) and USS Washington (BB-56), which have beams of 108 ft (32.92 m).