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The "12-Meter Designs" collectively refers to a series of racing
yachts designed to compete for the America's Cup. In order to ensure that racing
tested the skill of the sailors more than the boat technology, criteria were
dictated which competing boats had to meet in order to be qualified to race.
From 1958 to 1987, the "12-Meter" Rule was used to ensure that similar designs
competed for the America's Cup, while still encouraging yacht designers to balance
several key design factors in order to produce the fastest boats possible.
The 12M Rule:
- Where:
- L= The length in linear units, measured at a height of 1.5% of the class rating, (12 meters or 39.37 feet), above the flotation line, adjusted for the girth measurements forard and aft.
- d= The difference in linear units between the skin girth and the
chain girth, measured vertically in the transverse section at 55% of LWL (length at waterline),
from the covering board to a point 12.5% of the class rating below the waterline.
- F = freeboard in linear units between the skin girth and the chain girth,
measured vertically in the transverse section at 55% of LWL, from the covering
board to a point 12.5% of the class rating below the waterline.
- S = Sail area in square units, controlled by spar dimensions and
consisting of the sum of the mainsail area plus 85% of the fore triangle.
- 2.37 is the mathematical constant.
When all measurements are taken and divided by the constant, the result should be close to 12 meters. This does not mean that racers classed as "12-Meter" boats were 12 meters long! In fact, the 12-Meter racers ranged between 65 and 75 feet in overall length. Masts were typically about 85' high, and the boats themselves displaced approximately 25 tons.
In practice, the Rule was much more complex and open to interpretation. The 12M rule book contained more than 20 pages of finely printed text. Yacht designers attempted to take advantage of every loophole and omission to create the fastest design possible while still complying with the Rule. By increasing or decreasing the length of the hull, sail area and freeboard, the designer attempts to balance these effects with the shape of hull, keel and rudder while trying to create the least amount of resistance, the greatest amounts of stability and lift.
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