America's Cup: Stats and video reveal the high price paid for falling off the foils - 750 metres
by Richard Gladwell Sail-World NZ 3 Sep 2024 01:57 PDT
3 September 2024
Alinghi Red Bull Racing chases INEOS Britannia - Race 9 - Day 3 - Round Robin 1 - Louis Vuitton Cup - August 31, 2024 - Barcelona © Ian Roman / America's Cup
Falling off the foils is a hazard of life in the current America's Cup.
Analysis of the onboard "black box" data in conjunction with video shot during the pre-start and racing reveals the interesting effects on race results, particularly given the punishment for such a transgression could be an early exit from the Round Robin phase of the Louis Vuitton Cup in just over a week.
The "Black Box" data is a stream of data of the basic performance gathered on the boat, collected during the sailing and made available on the America's Cup website and available to all teams, media and fans.
We're going to look at Race 9 between Alinghi Red Bull Racing and INEOs Britannia. There are two incident, the first in the last seconds of the pre start, in the lower starboard corner of the course, and the second as INEOS Britannia tries to gybe in the same area, on their final approach and escapes with a lesser punishment.
Interesting enough, in between these two points the performance between the two boats is not dissimilar, indicating that the outcome of this part of the regatta is largely determined by the teams which teams make the most/worst mistakes - remembering unlike other high performance that all mistakes are basically equal but the price paid varies enormously.
First, run through the video below. Stop at 1:00 on the start countdown or 2:33 on the video time at the bottom left. At 00:50 of the start clock top left hand corner you should be onboard Alinghi Red Bull Racing, with INEOS Britannia. Just completing a tack on Alinghi's starboard bow. At 00:39secs commentator Glen Ashby makes the comment "I think they (Alinghi) might be off their foils".
Now review the plotted data below:
At 13:15hrs on the Speed Over Ground (SOG) vs Time graph, we can see Alinghi Red Bull Racing's speed start to plunge - it stays low as they try to get across the start line. Meanwhile INEOS dues a nice speed build and tacks (reflected in a sharp dip in speed at about 13hr:18mins. Note on the video what when Alinghi does get above foiling take-off speed, they are effectively reaching across the wind, and even though their speed is up to 20.5kts on the video their VMG (Velocity Made Good) or progress to the next mark is just 0.7kts, while INEOS is sailing at just under 14kts VMG, and has already clocked up a lead of 750metres, which extends to over 1000metres after Mark 1.
The Altitude vs Time data is a measure of time above a base height (-1) is probably the indicative benchmark that the hull is clear of the surface (verified on video). The solid blue splurge indicates the time that Alinghi Red Bull racing were off their foils - which can be checked in the SOG vs Time graph above. Looking along the graph we can see that at 13:40 INEOS also comes off their foils - this occurs as they approach the finish line and out in the middle of the course. We can see the same relationship with speed and foiling height as happened with the Swiss. But it must also be remembered that while the boat can be up to speed, it doesn't mean their VMG is up to the target.
In the True Wind Speed (TWS) vs Time graph we can see that in the prestart incident the British boat is still sailing in clear air, their turbulent air spewing back onto the British - who are sailing in what is recorded as 4kts - well below foiling take-off speed and equally relevant it is a sharp drop in windstrength. When it does start to reform/recover the British are sailing in a lesser wind strength, and it not until about four minutes after they fell off their foils do the Swiss escape the turbulence and chopped up breeze.
It is also worth remembering that in the body of the race 13:20 to 13:37 the SOG vs Time data (below) has a lot of overlap for both teams so maybe there is not much of a speed difference between the two teams. However not shown but the critical end number is VMG (Velocity in the direction of the next rounding mark).
From the Video: