Generally abbreviated to TM, a formula for the measurement of the tonnage of yachts. It was introduced in the mid-1850s by a London-based yacht club, the Royal Thames, to produce a fairer method of handicapping yachts for racing. Until that year the tonnage of yachts had been calculated by the Builders Old Measurement formula, but some astute yacht designers had found a means of reducing a yacht's tonnage measurement, and thus increasing its handicap allowance, by shortening the keel as much as possible. In an attempt to prevent freak and unseaworthy yachts—such as skimming dishes—being built to beat the racing rules, and to introduce a more equitable means of handicapping yachts of widely differing sizes, the Thames Measurement rule was introduced. The new formula, still based on the Builders Old Measurement, was where L equals the length in feet taken from the forward side of the stem under the bowsprit and measured at deck level to the after side of the sternpost, and B equals the beam in feet measured to the outside of the hull planking. After its foundation, what is now the Royal Yachting Association decreed that L should be measured on the waterline not the deck.
This measurement system was adopted generally by British yachtsmen and long after it had been superseded as a rating rule it was still used to describe a yacht in terms of tonnage, though the formula bore little relation to its displacement or weight. So universal did Thames Measurement become—and it is sometimes still applied to describe a classic yacht—that it was one of the details listed by a yacht's name in Lloyd's Register of Yachts right up until it ceased publication in the 1980s.Johnson, P., Yacht Rating (1997).