Do Great Lakes Ships Need More or Less Ballast Than Ocean-Going Ships?

Ships on the Great Lakes require less ballast than ocean-going vessels of comparable size.

Reason: Freshwater vs. Saltwater Density

• Freshwater density: ~1,000 kg/m³
• Seawater density: ~1,025 kg/m³ (2.5% denser)

Buoyant force = weight of displaced fluid. In denser seawater, a ship floats higher (displaces less volume) for the same weight. In less dense freshwater, it sits deeper. To maintain safe draft, stability, and propeller immersion when transitioning from sea to lake, ships must take on ballast. Conversely, lake ships designed for freshwater have lighter scantlings and carry less ballast capacity because they don't need to compensate for the density difference.

Additional Factors

• Lake ships ("lakers") have boxier hulls (maxizing cargo in locks), lower freeboard, and operate in less severe waves than ocean ships ("salties").
• Ballast water management regulations (invasive species) apply to both, but lakers are exempt from some exchange requirements.