50 knots is when the foils are supposed to cavitate, the wind limits are supposed to prevent that.

I heard that has more to do with design and cavitation, it's not a rule of any kind. I have no idea if that's just speculation or has been tested on the water at full scale.

Keep in mind, I'm not a marine architect, and this is my fuzzy memory of something I read on the web...

Mike

It seems like when they turn that last mark and head for the finish line, that's when they hit the highest speeds. I'm always expecting to see one of them stuff the bows just a little too much, in the transition when coming around that mark, and a pitch-pole. It's been very close a couple times.

What they need to do is put a wake board and tow rope on board, and tow one of the guys across the finish line like this:

https://www.youtube.com/watch?v=N6JiYwQJ4ho

Would the crowd go nutz or what?!

If Richard Branson were in charge of one of these boats, you KNOW he'd do it!

https://www.youtube.com/watch?v=kaGxFaw83FU

Come on Larry, you can do it! Even without the naked chick on your back it would be great.

Ha! Synchronized responses...

What Bacho posted is what I recall reading. There isn't a rule that I'm aware of that stops them from going over 50 (as in, the blue light won't flash).

I think these boats are faster than expected, so I'd be surprised if they're not capable of getting over 50, if the wind is close to the limit and the water is flat enough.

I also don't know why cavitating is something they need a wind rule to prevent. If the foil cavitates, won't the boat just slow down? Seems like a self-correcting problem, or is there a safety issue I'm missing?

Mike

From what I understand, it will lead to a safety issue as the crew will have much less control of the boat.

NZ lost overlap at time mark 0:14 seconds. They were over 20 meters leeward of Oracle. NZ was 3-5 knots slower from the start to mark 1. They never had a chance. Wasn't even close.

Everyone owes it to themselves to watch these races with the live source AND Virtual Eye. Virtual Eye shows everything. Unfortunately, the broadcast, as fantastic as it is, misses so much of the race, and distorts reality because of the camera angle (ie. start/overlap). It doesn't show the overlaps, angles, wind shifts, etc. By looking at both sources you get a REAL picture of what is going on, and it helps to distract from the perpetual psyco-babble spewing form the orifices of the three stooges .

NZ passed the 50 mph barrier in LV series.

But not 50 knots correct?

They touched 54 mph.

My favorite part of the televised coverage of the reaches is that no matter what relative positions the boats are to one another, they always show a water-level shot of the bows appearing to be neck and neck. The driver of that boat has obviously been working on this choreography for quite a while.

Of course, as soon as they switch back to the aerial shot, Fantasy Island starts to sink...

Mike

It's all here. Dig in

http://noticeboard.americascup.com/ac13/

http://noticeboard.americascup.com/lv13-fi/

http://noticeboard.americascup.com/lv13-sf/

http://noticeboard.americascup.com/louis-vuitton-cup-2013-rr/

The wind limits put in place are more general and not specific to any design parameters. It was originally 33 knots. After a couple of incidents and reviewing the boat capabilities and feedback with the teams, it was reduced to what it is now to keep things safe.

The rules create several limitations on the designs that affect the stability of the boats on foils. In particular, the rule about having no foils extend beyond the perimeter of the boat means the teams all have an asymmetric rudder foil that is longer on the inside and shorter on the outside. This rule makes sense...you don't want an overboard crew member to be struck with the foil. In order to maintain stability, the boats need a certain amount of rudder foil surface area but there isn't enough room to put it without making it uneven. Because they are uneven, it induces additional bending stresses on the rudder and rudder mounting systems. The rudder foil is a key element to keep the bows from going under and actually pulls the sterns down to keep the bows up when they are turning downwind. I haven't researched it but I've been wondering if the rules also dictate that the rudders must be on the centerline of the hulls - because they ~could~ otherwise mount the rudders more inboard.

Foil cavitation is where the pressure of the water flowing around the foil drops so low that the water flashes into vapor/steam. It's a technical limit to any submerged foil and these guys are probably pretty close to that at full speed. When a foil cavitates, it suffers an ever quickening loss of lift. On these boats, the rudder foils are much more fine than the daggerboard foils so it's more likely that the daggerboards would suffer the loss of lift before the rudders...we would probably just see the boats settle down toward the water and as their speed drops off from the loss of altitude, the cavitation will stop and they will accelerate again. It's more of a speed limiter than a crash and flip issue.

The reason I stopped watching is the 30sec time difference between Youtube and Virtualeye (spoilers!).

As I understand it (And I'm sure Jake will correct me) when you hit 50 knots you require a totally different kind of foil shape in order for the foil to operate as expected. The problem apparently is cavitation that would lead to potentially the loss of flow attached to the surface of the foil. This would in turn lead to a nosedive like we've seen from ETNZ when their foil lifted them out of the water in the LV cup.

Tony, not sure but I think you can pause/start Virtual Eye, effectively giving you the ability to synchronize the two feeds. And here in North America, if you watch NBCSN, you can enable the audio feed on virtual eye when NBCSN goes to commercial.

Race 8 NZ hit 47.02 knots or 54.11 mph.

Go to 2:50 in this vid. and watch Mischa lose a rudder to cavitation (not sure that's the right term, maybe ventilation)on a C class, and the end result.

http://vimeo.com/75282563

Someone touched on it here before but there are two types of issues that lead to a submerged foil losing lift and they are often confused. Cavitation is a violent action on a small scale and is tiny bits of water flashing into vapor bubbles and collapsing with a tiny explosive force. If something cavitates for a prolonged length of time (like a boat prop) it can damage the material surface. On these boats, cavitation wouldn't be some big dramatic loss of lift but would act like a gradual stall on the foil. The cavitating situation would only be a very intermittent issue and would be limited to small areas of the foil and would result in an drop in speed so it probably wouldn't cause a crash. It would be just like hitting a speed ceiling that you can't exceed.

Ventilation is a different animal that is often confused with cavitation - it's where the foil sucks in air from the surface and it very quickly loses lift. This happens quickly and envelops the entire foil leading to a surprising situation. This probably had some affect on NZ's nose dive as their rudder foil got really close to the surface. Vertical foils (like rudders) can have this issue at high speeds and start to lose lift/stability at the top of the foil if it is configured in such a way that it sucks air down from the top of the foil...our boats don't get near this kind of speed for this to be an issue.

As shown by the design of the Vestas Speed Rocket, foils were a big part of their ability to beat the sailing speed record. Foils (mostly steering foils) are designed like a wedge for higher speed craft to help reduce the amount of cavitation on the areas that are needed for directional control. Where they labeled "air" in this image is a bit mis-leading...it should really say "vapor" as this is the point that the pressure in the flowing water drops so low that it turns to water vapor.



Reducing the number of foils is part of the thing that helped the Vestas Sail Rocket get the record.

I think Mischa experienced ventilation. Ventilation happens when air is sucked down around the foil, and can happen to us on our A's/F16's/F18's/20's in high speed bearaways. Cavitation happens when the water boils because your creating so much lift on the foil sailing at high speed; pressure on the top surface is lowered and the water literally starts boiling (Chem 101).

Nice description: http://lancet.mit.edu/decavitator/Basics.html

Nice cavitation video: http://www.youtube.com/watch?v=DijdU0rmDdc

Airplane wings are designed to stall in particular areas first. I guess these foils are rather small for that.

If the horizontal surface cavitated, would the problem be limited to that area? Would it be possible for that to immediately ventilate the rest of the foil?