Top designer analyses new America's Cup class

by Richard Gladwell on 1 Nov 2007
The new AC 90 rule has been published, and reveals a boat which is at the lower end of the displacement range, indicating that the new class will be significantly faster downwind than the boat used in the 2007 America’s Cup, but with similar upwind speed.

Sail area is expected to be significantly greater in the new class, and this will also increase downwind speed.

The imponderable is the effect of the big increase in draft by 2.400 metres which will generate a significant increase in righting moment from the previous type. The new class may be more reliant on form stability than the old – with the trend to narrower and narrower boats being halted.

The new boat is to a 'Box Rule' which means that any hull shape will be tolerated within the overall length and freeboard measurements, compared to the previous ACC rule which attempted to control hull shape through the use of hull girth measurements at certain points along the hull length.

Sail-World spoke to leading designer Brett Bakewell-White and got his view of the new class.

'These new boats aren’t massively bigger over all than the old ACC V5,' Bakewell-White said, 'at 83-84ft in overall length. But the waterline length is a lot longer, going from 65ft to 90ft, as a result of having a plumb bow and stern.

'The old rule wasn’t physically measuring overall and waterline lengths, but girths between measurement points. These new boats are going to be 25ft longer on the waterline.

'They are going to be fast at a tonne lighter. The rigs are going to be bigger and the deeper keel will generate more power. The designers will be racing to see who can generate the most power for the least drag' he adds.

The mast dimensions are significantly bigger at 500mm fore and aft, where the old ACC V5 ones were 300mm, so they have obviously grunted them up'.

'Dimensionally these boats are not dissimilar to the maxis from the period of Rambler (the old 90ft Shockwave) and Zana. If you cut eight feet of overhang off Zana’s stern effectively you are close to the new rule except they are three tonne heavier with less draft. Sail area and everything is all pretty similar.

Zana, designed by Bakewell-White does not have a canting keel, and is waterballasted.

Beam on Zana is 5.2 metres and Shockwave is 5.8metres (the current AC 90 max beam is 5.3 metres). Draft on the old ACC yachts is one metre less at 4.5 metres – so there is significantly more righting moment in the AC90 yacht, says Bakewell-White.

Zana is currently for sale. Rambler is in full racing condition and doing very well on the offshore circuits.

'All Zana needs is eight feet taken off the stern and you basically have an AC boat,' says Bakewell-White. Aside from these two, there is only one other supermaxi that would fit the AC rule.

Purchasing Zana provides a work around for the AC class, which may allow the construction of two new hulls, but prohibits two boat testing of AC boats. Acquiring a boat of similar type and turbo-charging it to equate to AC90 performance would get around the two-boat testing rule. Obviously it would be possible to equalise the supermaxi and an AC90 yacht through computer simulation, and then commencing on-the-water testing with the old supermaxi in AC90 mode

Like most cost saving measures, the ones proposed in the 33rd America’s Cup Protocol will probably have the effect of increasing cost. And, in this case the teams would have to stand the cost of running two crews, plus modifying a supermaxi and moding it to maintain an edge on the AC90 yacht being tested.

The use of waterballast in a supermaxi like Zana, would aid this moding process considerably.

'It is likely that the AC90 class will be more reliant on form stability than the ACC class' says Bakewell-White. 'That will be an area the designers will spend a bit of time looking at, I imagine' he adds.

'There is a lot more horsepower in these boats and a lot more sail area in the new class, but in the end you are always chasing righting moment.'

In terms of on the water performance, Bakewell-White expects a significant improvement with the AC90 class.

'If you look at the performance of Zana and the old Alfa Romeo, they are three tonnes heavier, with less right moment and the same size rigs. We did 32 knots aboard Zana in Auckland Harbour – you wouldn’t do that in an old ACC boat.'

(The ACC record believed to be held by BMW Oracle Racing in one of the Acts which maxed out at 19kts in a rain squall).

'We had a reefed main and fractional gennaker in 35 knots of breeze. But we would regularly achieve 20 knots downwind. The VMG for an ACC boat is about 10kts downwind and 11kts through the water. The ACC yachts were much better performers upwind than they were downwind. That should be reversed with the new AC90 class,' he adds.

'Upwind Zana does 11.3kts, but she doesn’t sail as high as an ACC yacht. I would expect the new class to achieve the same speed as Zana, but sailing higher. Downwind they could do nearly double the speed of the current ACC yachts.

'Whether they are any more spectacular is to yet be seen,' he cautions. 'With the big boats, you are doing 20 knots, but it doesn’t feel like it until you see the speed the water is going past.

Wind pressure will be God in the new AC90 class.

'Something that has stood out from the canting keel 50fters is that you can be in bad tide, and on the wrong side of the shift, but if you have two more knots of wind you will always beat the other boat. It is all about wind pressure.

'The first day we went sailing on Wired (a BW52), we were doing 16 knots and pointing at the mark. The Cookson boats were on the other gybe heading away from the mark, and when they came back they flamed us, because they were doing 22 kts, and we were doing 16kts!'

Due to time zone differences, Sail-World was unable to get comment from BMW Oracle Racing on their view of the rule, along with other potential challengers who had not entered, and were not included in the rule development process.

If Golden Gate YC were to prevail in the New York Supreme Court, it is not known whether they would retain or alter the AC90 rule.

Given the basic parameters of the AC90 rule, and the fact that it is a box rule favouring a lighter boat, the new rule is believed to be more in line with BMW Oracle Racing’s thinking.