ORACLE Team USA's quest for instability
At the America’s Cup World Series in New York earlier this year, ORACLE Team USA Director of Performance, Ian Burns, told a group of journalists that the defenders’ chief design goal was to create the most unstable boat they felt they could get away with.
We cornered Burns immediately after his presentation and got him to explain why the American syndicate was banking on intability to bring them a record third consecutive America’s Cup victory.
SRM: Why is an unstable boat a better bet than a stable one?
IAN BURNS: We have evolved a long way since the first boats started flying. In the last Cup it was interesting the way the Kiwis approached the stability problem by making a very strong and stable boat that could sail in all conditions and did a lot of flying.
At OTUSA we approached it from the other direction by building a very unstable boat and did very little flying prior to the Cup. In fact, I remember were still developing our techniques around how to gybe just a few weeks before the Cup.
Because of the range of stabilities that they had designed into their boat, it was probably a little slower than ours when they were both flying. That is pretty much the game of stability versus speed that’s played all the time. We sort of converged to very similar foils for the actual Cup - that's often the case and probably will be in 2017 - but the answer to the stability question is part man and part machine and how you trade those two things off is one of the big uncertainties of this America's Cup.
A big challenge is how to project/predict where your sailing crew's abilities are going to be by the Cup. You have to be saying to yourself, well our guys can control this level of stability now, but in a year's time they will probably be better and able to handle a less stable boat. So you have to be working on a final foil design that is something that, if we built it right now, the guys probably wouldn’t be able to sail.
We see in our testing quite often that we come in from a day's sailing feeling like we were the fastest boat, but having lost the race because we were less stable and we were doing crash downs and having the windward hull hit the water a lot.
It's common place in the sailing we are doing in Bermuda - and I wouldn't be surprised to see it in the America's Cup - if someone goes for the wrong foil, or something in the system is not quite working, or even if a crew just has a bad day, you see costly touchdowns and a faster boat could easily be beaten by a more stable boat.
SRM: What do you actually you mean by instability?
IAN BURNS: There are three different modes of instability you can get into. You can have a board that flies the boat absolutely dead level but flies too high and loses its sideways stability. Then there is pitching instability which can happen when you are tacking or gybing with both boards in the water and they cause the boat to rear up by the bow or dive bow first into the water. With that problem you could be really fast in a straight line but you just can't tack or gybe the thing.
Then there's the stability that comes from adjusting the cant of the boards – which you are allowed to do under the rules. Generally, when you cant the boards so they are more under the boat, that makes the boat more stable, but that's not the fastest configuration because you lose overall righting moment.
So, all of these factors are traded off in how you configure the boat factoring in how you want to be able to sail the boat and how stable or fast you want to make it. You can see it's not a simple problem and there is no straightforward way of analysing it and coming up with a single answer. The ultimate solution is a complex combination of crew, systems and board design.
Another big factor in Bermuda will be the conditions on the day. For example, a northerly wind where it is blowing down the cut to get into the Great Sound is very different to a southerly where it is blowing straight offshore. Differences in sea state from different wind directions has a big impact.
In certain situations, sailing downwind on one gybe could require having a very different daggerboard design and I wouldn’t totally rule out the possibility of boats sailing with an asymmetric daggerboard configuration.
[MAIN IMAGE: SAM GREENFIELD/OTUSA]