ISMA News - Association Suisse 6mJI

ISMA News International 6mJI Association Bulletin 1/2004
The metric rule is at the origin of beautiful yachts such as the J boat (23m), 19m, 15m, 12m, 10m, 9m, 8m, 7m, 6m and 5m,
but we should admit that it has not been the success planned by the conceivers at the beginning of this century. The
multiplicity of the national series slowly dropped down, and boats as Sonderklasse or 6.5m almost disappear. In term of
naval architecture the metric rule was more a step backward than forward, the meter yachts were heavy and therefore
expensive and relatively underpowered (sail area was small compared to other series). The success of the rule is due to the
selection of the 8m and 6m for the Olympics, than for the One Ton Cup and of course of the J and later the 12m for the
America’s Cup.
The evolution of the hull shape has been slow, the formula being very restrictive, and it essentially the fact of massive
investment for the America’s Cup which allowed new ideas to emerge. Olin Stephens first with Intrepid brought the
principle of the trimmer and the rudder splitted from the keel . In 1983 Ben Lexcen was successful with the wingged keel
of Australia and in 1987 USA from Tom Blackhaller innovates with a “canard” rudder ahead of the keel.
The metric rule is still well alive today, new 8m, 6m are build around the world, even if it has been abandoned for the
America’s Cup.
It is worth to mention that the new America Cup Class , is using a formula pretty close to the metric formula , but with
different limitations and specially no more““d” factor (girth difference at mid-ship), allowing now to create much faster
canoe body hull with a bulb keel .
Factors from the rule influencing the performances of the sail boat.
The factors used in the metric formula have each a specific role in the performance of the yacht. The measured length L
figures a dynamic waterline length, or more accurately the waterline length when the yacht is heeling. L is measured 1.5%
above the real waterline length (120 mm for an 8m yacht) in order to avoid overhang with a minimal slope which would
immediately increase drastically the flotation when the yacht is sailing.
In addition to the L measure girth differences at the bow and at the stern of the hull are added to take into account the
volume of the extremities. This factor is critical to evaluate the dynamic waterline length which is determinant for the
theoretical maximum boat speed
The square root of the sail area, real engine of the yacht, is part of the formula. One may question why the suare root, in fact
this compensate the fact that the displacement increase proportionally to the cube of the waterline length.
The average Freeboard F is included as a deduction (smaller it is less the yacht can carry sail area), its impact on the
yacht performance is to increase the height of the centre of gravity and therefore increase the heel what is unfavourable to
the yacht performance.
The last visible item in the formula is the d factor and is the most complex one to understand. At the origin its goal was to
control the displacement avoiding the creation of canoe body hull with low immersed volume and therefore corresponding
to light vessels.
In its last revision(!(£$) the rule has embedded as a constraint a minimum displacement function of the cube of the
waterline length LWL. From this perspective the factor d today only impede the architects to create flat (light and fast!)
hulls as can be seen in the 5.5m and America’s Cup Class series.
The numerical factor’2.37 at the denominator of the formula has only the merit to obtain results such as 6m, 8m, 12m,
etc...
Guy-Roland Perrin
ISAF measurer
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