I don't think that he means cast iron, but there are a few 19th century low carbon iron ships around. Iron behaves a lot differently than steel, especially when it comes to rust and ductility.
There is a very big difference between the lifespan of steel ships and small steel yachts. No matter how thick a steel plate is, for any given alloy it rusts at a pretty constant rate. If you lose a 3/32" per decade of 1" thick steel plate used on a small ship, that is far less consequential than losing an 3/32" of 3/16-1/4" steel plating used on a yacht.
Back in the 1980's when I was working designing steel yachts, we generally thought of yacht quality steel boats as having somewhere around a 20 to 30 year lifespan with proper maintenance. It was not that we considered these boats beyond salvage at the end of this period, because steel hulls can almost always be replated and restored, but we felt that the cost of doing the major rebuild involved would way far exceed the value of the boat in question once restored.
Last edited by Jeff_H; 07-24-2006 at 02:29 PM.