……..Another theory posed to explain the sinking of the Fitzgerald is the grounding of the ship on Six Fathom Shoals. Captain McSorley reported damage to the ship's fence rail and vents as it sailed near the shoals, only minutes after Captain Cooper of the Anderson had expressed concern about the Fitzgerald's position with respect to shallow water. The Fitzgerald , like many Great Lakes vessels at the time, was thought to be particularly vulnerable to damage from groundings because of decreased freeboard. Concern for such groundings had been cause validated by a reported increase in the number of groundings during docking operations due to decreased freeboard. Moreover, repeated groundings tend to reduce the thickness of the ship's hull, decreasing its strength. As vessels typically were dry docked only once every five years, there would be ample time for the effects of these incidents to cumulate before being discovered and repaired.