ISMA News - Association Suisse 6mJI

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58 ISMA News International 6mJI Association The linear rule was considering the major players defining the performances of a yacht and in addition a defined scantling ensuring that yacht were build to last and to guarantee a minimum of security. The rule also mention several constraint such as a minimum displacement (not anymore in the formula), and a girth difference at mid-ship “d” to avoid light displacement and canoe body . Jauge linéaire = L + B + 0.75 G + 4d +0.5 SQR(SA) 2.1 d= différence de chaîne à "mid-ship" = différence entre la longueur G mesurée le long de la carêne en suivant la coque dans la section centrale et la longueur G' d'une chaîne tendue dans la même section d'un bord à l'autre. G G' In the meantime America was not inactive and Nat. Herreshof proposed a simplified formula embedding key items in relation with the performances of a sail boat , That formula favorising to much heavy displacement has been rapidly abandoned. Jauge Herreshof = D = Déplacement L x SQR(SA) 5 x (D) 1/3 Finally the English linear rating has been modified to become the meter rule in 1906. In addition to a strict scantling, this new rating rule could be modified, a technical commission meeting regularly. Jauge métrique = F = Franc bord moyen L + B + 1/2 G + 3 d + 1/3 SQR(SA) - F 2 In1919 the magic formula is updated: the maximum beam is not anymore penalised, and factors at denominator and numerator are changed. Jauge métrique = L + 1/2 G + 2 d + SQR(SA) - F 2.5 In 1934 the metric formula adopted it’s final (sofar!) version, getting rid of the G measure (perimeter at mid-ship), penalising too much the draught. To limit nevertheless the draught a maximum has been defined as a function of thewaterlinelength LWL but not included in the formula.’ Jauge métrique = L= Flottaison dynamique S= Surface de voile d= Différence de chaîne a "mid-ship" F= Franc Bord moyen L + SQR(S) + 2d - F 2.37
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 59
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