I was told that my 41 foot steel cutter weighed about 26,000 lbs. or 13 short tons, "half-load" (full diesel tanks, empty water tanks). I thought this was a little light, as a friend with a similarly sized steel ketch weighs 36,000 lbs.

The "official weight" on the mandatory federal registry plaque is "18.18 tonnes", which is just shy of 40,000 lbs. Yikes! But I believe this is an arcane number relating to displacement, not actual mass. Look up "Thames measurement"...it's crazy and arbitrary.

http://en.wikipedia.org/wiki/Thames_measurement
So are more general measures of "tonnage":

http://en.wikipedia.org/wiki/Tonnage
So I had a word with the crane operator, saying that while I estimated the weight as 30,000 lbs., would he be so kind as to give me the "crane estimate"?

His figure was 29,500 lbs. This may not be super-accurate, but it conforms closely with what I expect is the "real weight". That is what a truck driver will care about, and only accurate sling or crane scales can give that figure, unless you want to drive into a tight lock and, using Archimedes' principle, figure out how much water of a given volume you are displacing.

Strangely, there are fibreglass boats in my yard considerably heavier than mine. For a steel hulk, it's just a middleweight.