5.5 Metre (keelboat)

The International 5.5 Metre Class was created to yield a racing keel boat giving a sailing experience similar to that of the International 6 Metre Class, but at a lower cost.

The main class regulation is a restriction on a single quantity output from a formula involving the boat's rating length "L", weight (expressed as a displacement "D") and sail area "S"; the regulation states that the output of this formula must not exceed 5.500 metres. There is considerable scope for variations in design while still meeting this restriction, and as a result each 5.5 metre boat is unique.

If the design parameters of a proposed new boat result in a formula output exceeding 5.5 metres, then one or more of the parameters must be suitably adjusted. Performance data gained from testing models towed in a long water tank (referred to in yacht design as 'tank testing') can suggest optimal combinations of parameters.

The Formula

The measurement formula is given in the [http://www.5.5class.org/technique/55_rating_rules_06.pdf 2006 International Five Point Five Metre Rating Rules] :

:$5.500\; mbox\{\; metres\}\; ge\; 0.9\; cdot\; left(\; frac\{L\; cdot\; sqrt\; [2]\; \{S\; \{12\; cdot\; sqrt\; [3]\; \{D\; +\; frac\{L\; +\; sqrt\; [2]\; \{S\; \{4\}\; ight)$

where
* $L$ = length for rating
* $S$ = measured sail area
* $D$ = displacement in cubic metres

History & Olympic Career

The first boats conforming to the 5.5 metre rule were built in 1949. The class quickly became popular and was raced in Olympics for first time in 1952 Summer Olympics. Scandinavian Gold Cup has also been competed with 5.5m boats since 1953. 5.5 metre boats replaced the International 6 Metre at the 1956 Olympic Games held in Melbourne, Australia. The 5.5 metre participation in the Olympic sailing events continued at the 1960 Olympic Games and 1964 Olympic Games. During 1960s it however began to draw similar criticism as preceding Six-metre class - namely, increasing costs - and the boat lost Olympic status after 1968 Olympic Games, due to excessive design and building costs of one off boats, marking the end of development class keel boats in Olympic regattas. However, the class remained active thereafter and 5.5 metre boats are still very actively raced in 2007 due to an organisational structure which encourages boats of all ages to compete: the class has been divided to Modern, Evolution and Classic subclasses.

External links

* [http://www.5.5class.org/ Official International 5.5 Metre Class website]
* [http://boats-storerdesign.blogspot.com/2007/03/olympic-55-metre-yacht-kirribilli.html A 5.5m about to be restored in Australia]