Seawaymax is a ship type. Like the Panamax and Suezmax, this classification refers to a navigation requirement - in this case, the maximum size a ship can be and still fit through the St. Lawrence Seaway and its canal locks, to travel between the Great Lakes and the open ocean (initially, the Gulf of St. Lawrence).

Seawaymax vessels have the following absolute maximum dimensions:

  • Length: 225.6 m (740 feet)
  • Beam: 23.8 m (78 feet)
  • Height (measured from waterline to top of superstructure of 35.5 m (116 feet).
Although larger ships can and have operated on the Great Lakes, most ships now being built to operate there are limited to Seawaymax for reasons of flexibility, in case they ever should be required to operate elsewhere.

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