I thought I might repost "A drinking man's tunnage" from several years back:

Tonnage has nothing to do with the weight of your boat. It is a measure
of how much wine you can carry.


The word 'tun' was originally a size of a cask used to ship wine from
Spain & Portugal to England. In 1347 a tax of 3 shillings per tun was
imposed and this was called 'tonnage'. A ship's size became known by the
number of casks it could carry, and the word tonnage started being used
to describe a ship's size.


It was found that if you took the length x the breadth x the depth of
the hold under the deck and divided by 100 it was close to the number of
casks. That is where we get the "Measurement ton" of 100 cubic feet per
ton.


There are several kinds of tonnage: The first two are used by the tax
collector. The next two are used by designers. The fifth and sixth are
used by freight salesmen and canal operators and the last one is used by
the USCG for documenting boats.


Gross Tonnage - is the internal volume in cubic feet of the vessel
minus certain spaces above the main or tonnage deck like stacks and
ventilators which are called "exemptions" .


Net Registered Tonnage - is obtained by deducting from the gross tonnage
the volume of space that can't be used for paying cargo or passengers,
that is to say the space occupied by the engines, the crew's quarter,
the stores, etc.


Displacement Tonnage - is the actual weight of the water "displaced" by
the ship and is usually quoted in long tons of 2240 lbs.


Light Displacement Tonnage - is the weight with nothing in it.


Loaded Displacement Tonnage - is the fully loaded weight to the maximum
and is on her Summer draft in Salt Water..


Deadweight Tonnage - is the difference between Light and Loaded
Displacement Tonnage....the Actual carrying capacity of the vessel.


Panama & Suez Canal Tonnages - these are different to the international
ones. There used to be a lot of variations between countries and they
thought they were being conned, so they came up with their own for
everyone.


Simplified Measurement System - The USCG decided that all this was to
much for the bureaucrats to deal with for yachts so they came up with
this formula.
(Now pay attention Capt. Schlemiel).


Take the horizontal distance between the outboard ends of the boat not
including rudders and bow sprits.
Multiply that by the maximum beam outside to outside.
Multiply that by the distance from the sheer line not including bulwarks
or cap rails to the outside bottom of the hull not including the keel.
Add the volume of the deck house/cabin top.
Multiply by .5 for sailboats and .67 for power boats.
Divide by 100.


This will give you the "Gross Tonnage". Net tonnage is 90% of gross for
sailboats and 80% for power boats.


It should be obvious to those of you that have gotten this far that
"tonnage" has nothing to do with anything except in the mind of some
government bureaucrat.


Enough to make you want a drink ain't it?


--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com