For a seaman's rank see rate (2). Otherwise, it is a calculation of a yacht's expected performance relative to another yacht during a race, based on the physical measurements, and other definable characteristics, of the yacht. Two early methods in Britain of physical measuring were the Builders Old Measurement and Thames Measurement, but a common variant nowadays is performance handicap where observed performance, rather than physical measurement, is the determining factor. These rules are best suited to local racing because of the difficulty of assessing comparative performance across wide areas. In North America by far the most popular rule of this type is the well-established Performance Handicap Racing Fleet (PHRF). This has been in use for many years and a version of it is used in other countries around the world. The Portsmouth Yardstick is the British equivalent. With this, the results of handicap racing are sent by clubs to the Royal Yachting Association, which calculates a national figure for each class of boat. The result is a Portsmouth number and the corrected time (CT) is obtained with the formula CT = (Elapsed Time × 1000)/Portsmouth number.
However, the most widely used rating rules are still based on measurement, the principle underlying them being that a yacht's ultimate speed is related to her length. This speed is only reached in stronger winds, and the amount of sail area that a yacht carries, and also her displacement, will affect her performance. Most rules therefore specify a measurement of length, which is usually some combination of the waterline length and the bow and stern overhangs, if any.
There are also rules for the measurement of sail areas in the different parts of the rig. The displacement of the yacht will be included in some form, either by weighing or by full measurement of the hull shape and then flotation depth. Corrections are also included for items such as beam, draught, freeboard, engines and propellers, centreboards, and keels, all of which have some effect on a yacht's performance. Some measurement of stability or ballast ratio is also necessary.
The most common methods for combining these individual measurements to produce rating are either formula rules, where a series of linked formulae combine to produce the rating, or velocity prediction programme (VPP) rules. In the latter the performance of the yacht is mathematically predicted under a range of conditions using a computer program, and the rating derived from the outcome. Historically, ratings were expressed in linear units of feet or metres with race organizers then applying time allowances to them. More recently, ratings have been directly expressed as time allowances.
There have been many rating rules since yachting became a popular sport and yacht designers have always been ingenious in evading them. The earliest rating rules were intended to restrict the variations of proportions of yachts within classes that were to race without handicap. Examples of these rules were the rater classes of the late 19th century and the International Metre Classes where the actual rules differed in their values and limits in determining the various metre class yachts which raced from 1907. More properly, these variants on rating rules are known as ‘box’ rules because they simply impose dimensional restrictions and do not generally incorporate any form of time allowance. Some of these classes are, as classic yachts, still competing today, perhaps the best known being the 12-metre which was used in the America's Cup races from 1958–1987. In the USA the Universal Rule was used the same way to rate the various classes, including the J-class, until the International Rule which governed the metre classes was gradually adopted.
Subjects: Maritime History.