Eighteenth-Century Shipping Tonnage Measurements

During the eighteenth century various methods existed for obtaining shipping tonnages; Most official records of shipping throughout this period record a vessel's “registered tonnage” which was the figure which appeared in the vessel's certificate of registration. This registered tonnage was less than a vessel's “measured tonnage.” The latter was the basis for building, buying, and selling vessels and was computed by means of a given formula. Registered tonnage was then estimated at roughly two thirds of measured tonnage and the figure rounded down to the nearest large whole number. This practice was encouraged by ships' captains and ship owners who wanted to minimize lighthouse and port duties that were based on tonnages. Thus only when the registration of measured tonnages became compulsory did shipping tonnages in the records become reasonably accurate.