The term Seawaymax refers to the maximum size of vessels capable of passing through the canal locks of the St. Lawrence Seaway. There are a total of fourteen locks in the St. Lawrence Seaway that include six locks in St. Lawrence River and eight locks in Welland Canal. The locks are 233.5 m (766 ft) long, 24.4 m (80 ft) wide, and 9.14 m (30 ft) deep. The size of ships that can pass through the seaway is limited by the size of these locks. Taking this into account, a typical Seawaymax ship is designed to be 225.6 m (740 ft) long, 23.8 m (78 ft) wide, and 35.5 m (116 ft) in height, with a draught of 7.92 m (26 ft).
The St. Lawrence Seaway allows ships to travel between the Atlantic Ocean and the Great Lakes, as far as the western end of the Lake Superior. But the limitations of locks’ size and channel depth allow only 10% of the sea going vessels to traverse the entire seaway. A number of large bulk carriers built on the lakes in the past are larger than the specification of Seawaymax, and so they cannot travel downstream beyond the Welland Canal, and thus can’t enter into the Atlantic Ocean. But now almost all new lake freighters are being constructed according to the specifications of Seawaymax to allow these vessels for use beyond the Great Lakes.
At present, the St. Lawrence Seaway is primarily used for carrying bulk cargo such as iron ore, limestone, coal and salt from the mines to the industrial areas down the lakes. For sometimes, the possibility of using the seaway for container shipping is also under consideration. In the past, proposal to expand the seaway had been rejected on the grounds of higher cost and environmental issues. In recent years, Seawaymax vessels have also been facing some problem due to lower water level in the Great Lakes.