The rating system changed officially six times through 1817, while Admiralty establishments may have altered practice on additional occasions. That distinction explains why reference works give different totals for the number of revisions. Changing guns and service needs pushed the categories without automatically moving every ship into a new rate. Tons burthen must also be separated from modern displacement: the formula estimated internal carrying volume from length and breadth rather than weighing the completed hull. Both rate and tonnage were planning measurements governed by historical rules rather than modern precision data.
Notable Point · 48:17-52:04
Rating revisions and tons burthen
Official revisions and the tons-burthen formula show why rating tables and historical tonnage figures must be read as administrative approximations.