The Tonnage act of 1789 was a tax levied on ships that were coming into ports within the United States. The tax was based on the weight of the ship.

what is the burthen tonnage

navy ships are clasified by weight or tonnage, total tonnage,

[all the ships]

The 48 in the serial number denotes the tonnage. Divide that number by 12 to get the tonnage of 4.

it is the expession given in terms of volume for the total capacity of vessel(gross tonnage) and for the cargo carrying capacity (net tonnage).