There are several reasons for being concerned with estimating dimensions and doing tonnage calculations for sailing warships. One is for analytical reasons. Tonnage seems to be a reasonable metric for comparing fleets. Alternatively, you might estimate displacement. Tonnages (burden or burthen) is easier, as displacement requires estimating drafts and block coefficients. Tonnages only require estimating beam and keel length. Usually, we know either the keel length, length on the gun deck, or length from stem to sternpost. We are then reduced to comparing with the few known examples to estimate the values for a new ship. Even harder is to do estimates for Dutch ships, which were measured on a totally different basis (length from stem to sternpost and beam inside the planking).