International rule (sailing)

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Whilst this encouraged designers it discouraged owners from participating in handicap racing because designs were being outdated almost before the yacht was launched.

In order to restore the equilibrium, work began on new formulas, which resulted, in the Thames Measurement which was the first formula defined for yachts.

The adoption of the British Thames Measurement by the Yacht Club of France in 1870 may mark the beginning of international rating rules. In 1893, the Germano-Scandinavian Union was formed and it developed its "Union Rule". Starting in 1902, under the leadership of the New York Yacht Club, U.S. yacht clubs agreed to a "Universal Rule" in 1905 which was based on a formula developed by Nathanael Herreshoff.

By the early 20th Century yacht racing had spread across Europe but each country had its own different rating rules but they all typically used similar principles. International competition was always subject to various forms of handicapping which was often subjective and certainly open to protest

What was needed was a common rating or an agreed International rule, which would enable yachts from one country to race competitively in a different country.

The leading yacht racing countries came together and laid down a system that calculated the rating of yachts, measurement rules, construction regulations (scantlings) and rules for racing. This was largely driven by the United Kingdom. The then Secretary of the YRA (now called the Royal Yachting Association), Brooke Heckstall-Smith had been in discussion with Scandinavians German and French yacht clubs and a conference was held at the Langham Hotel in London in July 1906. Representatives from eleven countries attended, together with an observer from the United States, plus several respected nautical engineers and designers and they started to formulate the International rule.

A committee met again in October 1906 in Berlin to elaborate on the application of the Rule. The conference delegates met finally again in Paris in October 1907 and ratified the first International rule which defined not only the rating of yachts and measurement rules but also construction regulations (scantlings) and racing rules. Delegates from this meeting went on to form the International Yacht Racing Union (IYRU); the precursor to the present International Sailing Federation (ISAF).

The USA sent observers to the meetings but did not initially adopt the first International rule and continued with the Universal Rule based on the formula developed by Nathanael Herreshoff.

The International rule eventually became the backbone of yacht racing. The Rule created a formula that took into account most problematic areas that had previously caused dissent among the racing nations. It does not restrict size—many individual classes were created. It allowed designers a degree of latitude—yet controlled unsafe extremes. It laid down construction rules and governed the use of materials—yet understood that the Rule must develop.

The 19mR Octavia racing with the 15mR class (Kiel Week, 1913)

Used from 1907–1920

[1]${\displaystyle R{\mbox{ metres}}={\frac {L+B+1/2G+3d+1/3{\sqrt {S}}-F}{2}}}$

where

• ${\displaystyle L}$ = waterline length (LWL)
• ${\displaystyle B}$ = beam
• ${\displaystyle G}$ = chain girth
• ${\displaystyle d}$ = difference between skin girth and chain girth
• ${\displaystyle S}$ = sail area
• ${\displaystyle F}$ = freeboard

Used from 1920–1933.

${\displaystyle R{\mbox{ metres}}={\frac {L+0.25G+2d+{\sqrt {S}}-F}{2.5}}}$

where

• ${\displaystyle L}$ = waterline length (LWL)
• ${\displaystyle G}$ = chain girth
• ${\displaystyle d}$ = difference between skin girth and chain girth
• ${\displaystyle S}$ = sail area
• ${\displaystyle F}$ = freeboard

Used from 1933–1939.

${\displaystyle R{\mbox{ metres}}={\frac {L+2d+{\sqrt {S}}-F}{2.37}}}$

where

• ${\displaystyle L}$ = waterline length (LWL)
• ${\displaystyle d}$ = difference between skin girth and chain girth
• ${\displaystyle S}$ = sail area
• ${\displaystyle F}$ = freeboard

About 6000 of the larger metre rule boats have been built since 1907. About half of them still exist, which is a remarkable number considering that most of them were built before World War II. Partly this is a result of the fact that the R-metre boats had to adhere to Lloyd's strict scantling rules, that has given them longevity that very few other traditional classes have been blessed with.

^† Numbers without references are estimates.

In addition, over four thousand 2.4mR have built since the classes introduction in the 1980s.

Since 2000, the 2.4mR has been used for the Sailing at Paralympic Games.