Until 7 January, when the two remaining challengers and the America's Cup defender have to unveil their hulls, everything remains speculation. But it is conceivable what will then be visible under the sterns of the yachts.
This week, Alinghi skipper Russell Coutts revealed that his team, along with Team New Zealand and possibly Oracle BMW Racing, are developing new underwater appendages. But it's not the kind of appendages you'd expect. Work is underway on a second hull shell that will be attached under the stern section of the boats and will be considered an appendage rather than a hull section. Nobody except a few designers, team members and surveyors know for sure at the moment. But it is obvious that a way has been found to extend the effective waterline.
For boats that do not planing, the longer their waterline, the faster they are for the same displacement. Although it is not precisely defined, the waterline length of an America's Cupper is one of the three points that are included in the measurement formula and determine the appearance of the boat. For example, a longer waterline is offset by less sail area. To avoid this penalty, it is important to lengthen the waterline in such a way that it does not affect the measurement.
This is made possible by the fact that there is a difference between the measured waterline and the actual effective waterline when sailing. Long overhangs at the bow and stern lengthen the effective waterline of any boat, especially when it is heeled. However, the overhangs are not measured, so the aim of the designers is to make them ever longer and more effective. One result of this endeavour is the so-called kink bow, which all teams have used.
Now there also seems to be a revolution under the sterns. The shape of the hull from the trailing edge of the keel to the waterline determines what the overhang looks like, because there must be no bulges or hollow areas in the hull, it must be streamlined, i.e. run out to the stern in a harmonious line. The greater the angle of this line to the waterline, the shorter and less effective the overhang will be. The flatter the angle, the closer the overhang is to the waterline and the longer and at the same time more effective it can be. However, this costs volume in the entire stern area in front of it.
For the America's Cupper, two main rules apply to this area, which influence the angle of the underwater profile. There is a point near the stern where the so-called girth measurement is applied (there is also one at the bow). It is measured around the hull, has a predetermined length and thus determines the aft taper of the stern. It is used to keep the shapes of the different hulls approximately the same. The first rule states that this girth measurement is applied at the point where the hull intersects a line 200 millimetres above the waterline (in survey trim). The second rule determines the profile of the hull in the area between the girth measurement and the point where the hull intersects the waterline. The profile must not exceed 12.5 degrees there.
The smaller the angle of the underwater profile, the longer the overhang, which is desirable. The problem is that this also reduces the volume of the stern area in front of this measuring point.
However, the great art of cup design is to distribute the volume of the 25-tonne boats as effectively as possible along their length. The need to place a certain amount of this volume in the stern section prevents the designers from going below the 12.5 degree profile angle, which in turn results in a shorter overhang. So if it were possible to keep the profile as flat as possible and somehow compensate for the missing volume, the boat would have decisive advantages on the water thanks to the longer effective overhang.
The so-called "clip-on", the click-on appendix that the New Zealanders are said to have, appears to be a solution to this problem. The lack of volume due to the flat fuselage profile is simply compensated for by the volume of the appendix.
The trick is to make it appear to the rule that it is an appendage and not the hull itself. This is because the number and shape of appendages is only regulated to the extent that only two movable parts may be present under water (normally the rudder blade and the trim tab on the trailing edge of the keel). Immovable appendages are not limited. However, the water flowing past this appendage should believe that it is a fixed form of the hull. To achieve this, the appendix must not be firmly attached to the hull, except at some attachment points (the area of which is limited). There is therefore water between the hull and the appendix. However, as this water is virtually dammed up, this type of skin is only surrounded by volume on the outside.
It will be interesting to see on 7 January how the teams have mastered the problem.
