[sailaway21]

There is in fact a huge difference in longevity of steel hulls in fresh versus salt water. Many people wonder at how "old fashioned" some of the ore boats on the Great Lakes look. They look that way because they are old-fashioned with many of the hulls approaching 75-100 years old. (that's not a misprint) The only thing that has done many of them in is the trend towards larger ships.

Most ocean-going ships now utilize a system called cathodic protection. This electrical system sets up a milliamp current through the hull which causes it to resist galvanic corrosion. I've seen plates abraded to bare steel that did not rust with such a system. Bottom painting used to be an annual event for these ships and it now is only done every two years, and then only as necessary upon inspection. Normal red lead or other bottom paints do not work with such a system; a special paint is required.

A passing thought based on Jeff's remarks on topside weight in a steel boat. The SS United States, the "Big U", was constructed with an aluminum superstructure and was one of the first, if not the first, to be built this way. (she still resides in Norfolk) The cost is prohibitive for average merchant ship construction but the weight savings are undeniable. It may surprise some that most modern warship's houses are constructed of aluminum for the weight savings. (the modern warship is no longer designed to be able to resist gunfire or missile strikes, the enemy is not supposed to get close enough for guns and missiles are to be shot down. the weakness of this theory in extremis was discovered by the British in the Falklands War when one of their frigates was struck by an Exocet missile. Aluminum burns extremely hot, especially when an aluminum-magnesium alloy is used.) While expensive due to the construction techniques required, an aluminum topsides could be entirely feasible. (i probably should have said, prohibitively expensive!)