International rule (sailing)

The International rule, also known as the Metre rule, was created for the measuring and rating of yachts to allow different designs of yacht to race together under a handicap system. Prior to the ratification of the International rule in 1907, countries raced yachts under their own national rules and international competition was always subject to various forms of subjective handicapping.

The word 'ton' and hence 'tonnage' originates from the word 'tun' which is a measure of volume equivalent to the size of a barrel which could contain 252 gallons of wine. In the UK, merchant sailing vessels were historically measured using Builder's Old Measurement to measure tonnage and taxed according. Due to the complications with accurately measuring of the internal volume of a sailing vessel (think of the calculus that would be involved to determining the 3 dimensional area (or volume) of a ship's hull and its continually changing curvature from the bow to the stern) a set of distances at points on the hull was defined to be measured and entered into a formula which then defined the tonnage and hence the taxes to be paid to the government of the day. As each ship was measured on the same basis it did not matter that the tonnage measurement was not a 100% accurate measure of volume as all vessels were measured on the same basis which led to an equitable basis of allocating tax. This system of measuring or rating merchant sailing ships has a long, well established history, so tonnage, and the calculus involved was naturally also used to measure or rate a sailing yacht.

During the early part of the 19th century interest in yacht racing had achieved sufficient momentum to need an agreed handicapping system to allow different types of yacht to race on an equitable basis. The method of measuring merchant sailing ships carried over into the world of yacht racing so that a yacht also now had a measured tonnage which allowed size comparisons and hence performance comparisons to be made between yachts on the basis that a properly designed big yacht will sail faster than a properly designed small yacht.

Each yacht fell within a Class based upon its measured tonnage. In the early 19th century four ranges of tonnage and hence four Classes were defined. Based on experience gained from the results of numerous races each Class was allocated a distance allowance that the Class had to give away to the next lower Class. This was equivalent to a Class 1 having to sail more distance than a Class 4 yacht during a race.

In 1834 handicapping by distance was changed to handicapping by time.

As yacht racing in particular became more and more popular designers started to look for and found loopholes in the measurement rules to enable a design to get a better rating. Whilst this encouraged designers it discouraged owners from participating in handicap racing because designs were being outdated almost before the yacht was launched.

...
Wikipedia