Bruce in Bangkok wrote:
....
If you use a simple calculation with port, starboard and bottom as
flat plates (which isn't accurate worth a damn, but will serve to
illustrate my point) then the Emma Maersk has an underwater surface
(loaded) of 371,733.5 square feet. Using the same method, my sailboat
has just about 1,000 square feet of underwater area. Thus for every
one day in port for the Emma Maersk she leaches out the equal amount
of TBT that my boat does in 371.7 days.

I'm sure that Roger could refine these numbers with his computer but
they do serve to indicate that perhaps politics played some part in
banning pleasure boats use of TBT first since pleasure boats seldom
belong to any pressure groups and commercial shipping companies have
tremendous clout in maritime affairs.

With the exception of a few major ports, and especially Navy ports
(which often have dozens of idle ships) I think you might find that
small boats have more surface "in port" than large ships. What I don't
know is where the dividing line is ... where 100 tons ships exempted?

In my home port, Boston, there are very very few 1000' ships. More
typically, there are a small number of 500' ships, most of them turning
around within a day. I was quite surprised the last time I went through
the inner harbor (fall haulout) and there were as many as 8 ships
coming, going, or docked. These ships are at most the equal of 200
pleasure boats, so this would be the equal of 1600 pleasure boats. I'm
guessing there are at least that many boats in the inner harbor, and
maybe three times that number in the extended harbor. Then we can look
at nearby harbors (Scituate, Plymouth etc to the south, Marblehead,
Salem, Manchester, Gloucester to the north) which have many more boats,
but even less ship traffic.