Originally Posted by Flatballer
Gross Tonnage is a confusing thing. It's actually related to internal volume, nothing to do with actual weight.
You are quite right. I used to do a lot of admeasurment (tonnage) work. If you use all the tricks to reduce measured tonnage with the U.S. system, now only applicable for vessels that do not make international voyages, the upper length limit for a 100 G.T. vessel is 5000 feet!
Normally, a 100 G.T. vessel is under 100 feet long. The U.S. Statutory Tonnage Admeasurement system is one of the silliest regulatory regimes ever devised. This was one of the reasons for the establishment of an international tonnage system based on external measurements which is almost immune to any cute tricks.