Weird Sailing Factoid

I'm reading bits from Mayflower by Nathaniel Philbrick online. One of the interesting bits of information is that the Mayflower (of pilgrim fame) was a 180-tun ship -- not "tons". So called because of her ability to carry 180 of the 252 English gallon wine tuns (barrels).

A bit more poking revealed this:

In sixteenth-century England, the size of a vessel was estimated in terms of tunnage --the ships capacity to carry 252 gallon tuns of hogshead barrels of wine. A 50-tun ship could carry fifty hogsheads. The tun was a measure of volume, not weight, and it was hardly uniform. The capacity of a Spanish tun, for example, was considerably less than that of an English tun. Thus a Spanish vessel of 50 tuns was not the same size as a 50-tun English ship.

It was not until much later in the Elizabethan Era that tonnage was used, based on the size and weight of the ship and her displacement in the water. It wasn't a standard measurement for many years.