Friday, November 02, 2007

Yacht designers Schickler-Tagliapietra comment on the new AC90 rule

We welcome the release of the new AC 90 rule. There are not too many surprises in the document, though a general attempt to simplify is clearly present. In fact, many concepts from the latest v5 rule remain, but not the formula trading length, displacement and sail area. This is a true box rule, something we really favor. Displacement is essentially fixed at 23 tonnes. Perhaps this was done in order to limit the design space because of the short cycle. We felt that the AC boats could have been lighter together with a less generous sail area. On a DLR basis, they will be an extrapolation of performance keel boats like GP42, TP52, and STP65, but fortunately this rule does not limit VCG. The new sail plan is quite impressive. Without any stored energy, and only 20 on board, it is going to be a lot of work.

On the boat building side, things such as panel weights and core thicknesses remain, though construction rules have been simplified in general. With added core thickness and heavier panels, even less structure will be utilized within the boats. A particular engineering challenge will be designing the lifting keel structure and its mechanism, to be powered from a source aboard a tender. It would seem the keels will be deeper when raised than originally stated.

There are not a lot of radical ideas in the mast and rigging. In fact, the rule specifies a very similar weight per length as the IACC v5. But with composite rigging being allowed, the mast will be made much stiffer, possibly with fewer spreaders. In the end the spars will need the stiffness, because the RM is going to be huge. The mast will not be rotating, which would have been an interesting development. There is no spinnaker pole – all spinnakers must be tacked on the centerline. This is a big change from previous AC racing, and will perhaps increase the attractiveness of form stability and lower heel angles, hand in hand with higher AWA, i.e. planing.

Twin rudders are accounted for, and the allowable maximum beam may make lifting one blade out upwind possible. Trim tabs are allowed on the keel fin, with only small winglets possible, if any. This lift scenario provides incentive to sail upwind at reduced heel angles, and thus makes a case for form stability as well. Alongside the keel system design, research into transom immersion, and chines, beam will be the main topic of discussion for a while. It is the primary dimension left to the designers.

We are cautiously optimistic that the class will not bunch up on the narrowest beam conceivable for the rigging. There are lot of areas of experimentation we wish were allowed that are not. It would have been nice to work on the twist of the mast, with unlimited asymmetric foils or with innovative new multi surface sails, but time is short and there is a lot to do. The new AC 90 rule is here, and it is a good thing.


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