Book of the Sea: Ship Tonnage

[Bewildered Badger]

Maybe I'm just being dense but I can't work out how to calculate a ships tonnage at all.
In the section on deckspace (page 21) it says
"The amount of space on board
is measured in tonnage. The
deck space in five-foot squares
is calculated by dividing the
ship’s length in squares by the
following value. Each five-foot
square equals one ton."

What following value? If it is the Width given on the same page then we get bizare results. A sixty foot long vessel works out at 12 five foot squares long. Divided by three (the value for a medium size ship) results in a total tonnage of four.
Where am I going wrong?

 

[Mongoose Gar]

Simple answer there - it should be multiply, not divide. 12x3=36 ton ship.

 

[Bewildered Badger]

Right, that makes more sense, but I'm still a bit confused. According to the table at the top of page 21, a Medium size vessle (such as a 60ft one) has a minimum tonnage of 51. Does this mean that our hypothetical ship must now have a lower deck, also of 36 tons (for a total of 72 tons)?

 

[Mongoose Gar]

Extra levels can be no larger than the main deck, but can be smaller. The lightest 60ft long ship you could build would be 51 tons, with 36 tons of space on the main deck, and another 15 tons below in a very cramped space beneath.

 

[Bewildered Badger]

Ah, I see. Thanks, that's very helpfull.