America's Cup: AC75 sails away after violent high speed nosedive
by Richard Gladwell/Sail-World.com 9 Sep 2020 23:41 AEST
10 September 2020
Recovery of ETNZ's AC50 took over 40 minutes, and full repair to racing condition took Emirates Team NZ's shore team 48hours of intense work. Bermuda, Challenger Semi-Finals, Day 2 , June 2017 © Richard Gladwell / Sail-World.com
After a couple of capsizes, two sky jumps, a minor and now a serious nosedive, the AC75 design concept has run the catastrophe gauntlet over the past 12 months.
Unlike its catamaran predecessors, the AC75 has sailed away from all of these challenges intact. Had these incidents occurred during the Prada or America's Cup, they would have been able to compete in the next race of the day.
That's quite a different story to the first years of the AC72 wingsailed, catamaran before it was turned into a foiler and raced in the 2013 America's Cup. There were two serious incidents - one boat was written off, and the other was rebuilt over several months.
With the AC72's progeny, the AC50, no-one sailed away from a catastrophic incident.
In Bermuda, after a start-line nose dive, Emirates Team New Zealand's AC50 could have been repaired to race the next day, but would not have been competitive. It was only restored to racing condition thanks to the cancellation of racing the following day.
Last Sunday, a second AC75 capsize incident occurred. As with the first one in mid-December 2019, both Emirates Team New Zealand and American Magic sailed away from the incident and continued with their training session in a fresh breeze.
A potentially catastrophic incident occurred a few weeks earlier when an AC75 went into a violent nosedive at highspeed.
No images are available of the incident. However, it doesn't require much imagination to visualise the situation, which has occurred previously in non-foiling monohulls and occasionally caught on video.
The team commented that the force of the nosedive was such that it would have destroyed one of the catamarans used in previous America's Cups.
American Magic released a video of their test boat "the Mule" doing a nosedive after sudden alteration in rudder rake and violent change in the angle of attack of the rudder wing.
The AC75 incident occurred close to the "sound barrier" for America's Cup foilers used in now three Cup cycles. In 2013 Emirates Team NZ was credited with the top speed of 47.57kts in Race 18 of the 34th Match. Artemis Racing went close to that hitting 47.24kts in the 2017 regatta in Bermuda.
Back in January, INEOS Team UK had a close call at a training camp in Sardinia, when the Brits executed a high-speed bear away, that went awry.
Britannia first had a shot at a nosedive, which created a massive, splash before Britannia recovered and rounded up waiting for her crew to compose themselves.
Helmsman Sir Ben Ainslie described the incident and his reactions to Sail-World as part of a broader interview on April 7.
"We were going for a bear away. In these boats, a bear away is probably the most dangerous manoeuvre we've got in the playbook."
"As the boat heels, it loses its righting moment - and even more as it heels to leeward. Then it starts rotating around the foil arm and begins to lift out of the water - so it is a double whammy effect."
"We've got through the worst part of the bear away, and I either came down too low, or the wind died a little bit, or whatever, and the boat started coming over to windward. When that happens, the righting moment starts coming back on very quickly. And, the more you heel to windward, the more righting moment you get, and also the boat is coming low in the water. "
"In that situation, you've to do a reasonably sharp turn up to power the boat up again. Unfortunately, in that scenario, the moment we did that we got whacked by a big gust and with those factors combined. You can see how quickly the boat powered up!"
"I turned down to try and save the situation, but there was just no way it was coming back - it was major stuff."
"Thankfully the boat held together and the guys held together, and that was the worst of it", the winner of five Olympic medals concluded.
One top 18ft skipper, after viewing INEOS Team UK told Sail-World that any high performance monohull sailed that way would have reacted the way that Britannia did. The take-out is that incident can't be laid at the door of the AC75 design concept, but that the AC75, while spectacular is also very forgiving of crew errors.
We have seen several incidents involving foil/wing ventilation - resulting in a spectacular rooster-tail.
There have been a couple of "sky jumps" as the AC75 spins out and despite a 7500kg all-up weight, jumps near completely clear of the water - the issue being caused by crew error and imbalance between the rudder and foil.
Just over five months after winning the America's Cup on June 26, 2017, the concepts of the AC75 Class were released to stakeholders. Work continued, by a 35 strong team, on the development of the Class Rule which was published at the end of March 2018.
While many were sceptical of the use of a boat-type that had not previously been used at this level of competition, the AC75 has lived up to all the design claims, except the ability to self-right. Instead, it does stay in an easily recoverable capsize position with the mast lying on the surface. The AC75 has not rolled over and inverted, which would make quick recovery very difficult.
The recovery of the capsized Te Aihe in December 2019, took just four and a half minutes, compared to the 40 minutes in Bermuda to recover the Team's AC50 in Bermuda on Day 2 of the Challenger Semi-Final.
In all admitted instances of capsizing, nosedive and spinout, the AC75 has sailed away. Had the incidents occurred in racing, the teams would either have been able to continue in the race or concede the race point and started in the second race of the day.
Based on the incidents seen to date, the AC75 seems to be in the same category as 18ft skiffs, and even the Olympic 49er/FX - where a capsize/nosedive is serious but quite recoverable.
With the AC75 we have not yet seen the crew do any high dives/jumps from the windward AC50 hull, in a capsize situation from 28ft (8.5mtrs). All the AC75 crew seem to have remain protected in the crew trenches.
Challenger of Record Luna Rossa is the only AC75 to have suffered severe damage - dismasting at the end of January and then breaking a bobstay supporting the bowsprit, and ripping away a piece of bow section. (The AC75 class rule has subsequently been amended in regard to the method for measuring the minimum load rating of 8,000kgs). The bobstay is the responsibility of the competitor and is not part of the supplied standing rigging package.
In all catastrophic incidents, the teams have gained valuable performance data on the build-up, which can be fed into their various simulators and used in crew training.
The high-speed bear away seems to be the most vulnerable point of sailing in the AC75, and most of the incidents have occurred during or immediately after this manoeuvre - as happens with 18ft skiffs and similar.
With the AC75 (and also with SailGP's F50) there seems to be a "sound barrier" at around the 50kts boatspeed mark which the AC75 and F50 seem to be able to achieve in 17-18kts of breeze when in rapid acceleration.
Historically the America's Cup teams have been very coy in revealing top boatspeeds.
In the media conference at the end of the Qualifier phase of the Louis Vuitton Trophy in Bermuda, the team skippers were asked if they would reveal their top speeds. All declined to answer except for the just eliminated Groupama Team France's Franck Cammas who tossed in "40kts" as his polite response.