34th America's Cup: multihulls!

34th AC

I disagree about the twist. I think it is an excellent design but unfortunately I don't think the crew was ready to handle the boat in those conditions. Seems to me the "pilot" with his hand on the joystick for the foil may have experienced a failure because if lift had been increased on that foil on an "emergency" basis I'd bet they would have made it thru. Another emergency action might be to lower the windward foil with max positive incidence-and in a hurry. If you have a boat with the characteristics of this one it will take innovative handling and an emergency plan to act fast.

 

I also agree on the flex of the platform and low reverse bows Gary, but why the pod?
And there's more IMO, like groper says; a lot of things coming together.
This is what I make of it in seconds.
0-3th second: ww hull skimming above the water, boat start turning, platform start canting to ww, see mast
4th second: ww hull hits a wave. Mast still tiny bit to ww
In the 5th second of the vid you see the ww hull just above the water and lee hull starting to climb the foil, ww. hull stays on same hight, mast more to ww.
6th second: ww. hull hits the water, lw. hull climbs even more, platform cants more, see mast.
7th second: lw hull is even higher on the foil, ww hull hits the water, mast still canting more to ww.
8th second: boat can't accelerate because of the ww. hull in the water;
9th second: lw. hull starts coming down.
12th second: lw. hull also hits the water and also starts submarining.
Etc...

You also don't see any changes made to the mainsheet or anything else done to depower the wing until capsizing, only when the boat is already toppling the sheet of the headsail is released.

Steve, can you, or anyone else, explain the allegation:
"Deploying an emergency chute / drogue off the back would only load up the rig faster and accelerate the pitchpole.

 

Interesting interview at new http://www.ocean-racing.net page 50. Kermarec of Oracle makes it clear low freeboard is for windage reduction. Verdier of TNZ says (in so many words) his team saw that as a potentially dangerous choice.

In the early history of the America's Cup the British "plank-on-edge" types had a reputation for "sailing under". It seems the more things change the more they remain the same!

Paul Lindenberg revealed a trick to me he used in catamaran stern design to keep the bows up, so one thing I ask myself watching the video is whether that trick would have kept the bows from diving in the first place. It would only have had a moment to do so before the sterns lifted clear. The exact angle of the rudder foils could have been a factor on the same principle. I haven't been following the rules carefully - am I correct in saying adjustable rudder flaps are not allowed? Perhaps this incident argues they should be.

Two other things I'd like to note:
Deceleration is a factor. It looks like the Oracle designers were aware of this. As has been noted, the boat does not exactly slam to a stop. But that should have given the crew time to release sheet / depower (at least the upper portion of the wing). They were not quick enough to do so. The founders of http://www.harborwingtech.com have argued that fly-by-wire is the key to making rapid adjustments to avert catastrophe at speed. Should the America's Cup allow fly-by-wire?

 

HASYB, your second-by-second analysis sort of confirms my theory : As the boat is sailing normally and flying the windward hull, it is in displacement mode with all of weight on the leeward hull. As it starts to foil, drag is suddenly reduced allowing the leeward hull to accelerate rapidly, during this transition, if the wing is not sheeted in rapidly or another means of compensating for this acceleration, the windward hull would drop in the water as the pressure in the wing changes during acceleration. We've seen this on previous videos of OR17 and even with TNZ. TNZ seems to have figured this issue out.
Looking at the video I believe that they went from sailing slowly to trying to bear away too rapidly, which could be the cause of the pitchpole. If they had sailed the boat up onto the foils, settled down and then tried the manouvre at sufficient speed, they may not have had the accident. What I see is the boat sailing in displacement mode, and during the bearaway it accelerates (as it would), hops up onto the leeward foils and drops the weather hull in the water, just as the wing loads up.
In answer to your comment on my theory on the drogue off the back making it worse, pitchpoles are caused when you have too much power too high up in the rig on a platform that is sailing too slowly for the amount of power. So hanging a chute or drogue off the stern would increase the wind-loading on the wing, which would then power up exponentially as the boat slows down. It might prevent pitchpole, but the wing would probably then just explode.
If they were past a beam reach with the bows going down, releasing the wing would have the opposite of the desired effect, as the wing would power up radically before it depowers, so the pitchpole would happen faster if they released the mainsheet.
When faced with this situation the only thing you can do is sheet in hard and bear away,
and hope the rudders stay in the water. Once they are out, you are at the mercy of the gust. I reckon the L-foils just compounded the problem, as once it's pointing down, they are pulling downwards and although some think that could save the day, it does the opposite for the reasons I've mentioned - it slows the boat down in an already bow-down attitude, allowing the top of the wing to overtake the boat.
Perhaps they need to have sidestays with load sensors that have a let-go mechanism built in when they reach a certain load, the wing would fall down rather than the catastrophic events we've seen here, but then, someone's head is likely to get cut off by the released stay.

 

A load sensor on the windward shroud is something I've advocated in response to Tom Speer with reference to the windward foil deployment rule. Perhaps the sensor could release something other than the shroud ...?

Steve, you're suggesting the crew should have powered up the wing to lift the windward hull, while I'm suggesting they should have dumped pressure from the upper section. Do we need to talk this out, or do we just agree that the way these boats are currently set up "transition" is a sensitive time? Is a design change called for to make it less sensitive?

[note: If a RULE change is called for and it's too late to change the rule for America's Cup 34, the discussion can be had at http://www.boatdesign.net/forums/sailboats/next-2016-so-americas-cup-ac-35-a-44996.html.]

 

You have a force (forward vector of the total aerodynamic effort) acting at the center of effort + the momentum acting at the center of gravity, you have the total hydrodynamic drag, and you have the vertical lever(s) between. The moment couple related to the center of effort is pushing the bows down whenever there's force on the rig with a forward component, and the one related to the center of gravity comes into play anytime the boat is slowing down. A sudden increase in hydrodynamic drag therefore increases the moment(s) depressing the bows. You might say it "trips" the boat, causing the boat to fall forward.

 

OK. Thanks a lot, both of you, for elaborating on the chute issue. Still a bit hard to imagine all the forces working in such a moment, but it kind of makes sense now.

So its more important keeping the speed & bow up than the stern down.
How about small wings in the front on deck level with adjustable progressing angle according how deep the bow submarines?
I've seen such wings before on multhulls to prevent digging in.

On the "wing sheeting/trimming dispute": I can imagine both ways of handling in different stadia (wind-angles) of bearing off, but please have a go at each other on this matter.

 

No one has mentioned Operator error yet? From the press releases it seems they just gave it a go to see what happens? Any cat sailor doing the same thing with nearly any racing cat has a fine line to sail when bearing off in 25nts. Then chuck in a wave or two and its a lottery to pull it off. They knew the answer before they did it. Peter S

 

The T rudders, for some of the time, would have been holding the sterns down - but when rig toppling power became so great, along with the completely submarined bows, and the main beam, and the silly low pod, all slowing the boat, then out popped the rudders ... endover.
Small bow wings were tried three decades ago by the 72 foot French Elf Aquitaine catamaran (with very large chord wing mast and balestron rig) but when they got into a bow dive situation, by the time the gunwhale foils (at high AoA) began to do their job, it was too late (they too slowed the front of the platform) ... the result was the same endover.

 

AC 34 on Foils

===============
Peter-See post 706. I think they needed a better "Oh ****" plan......

 

Hi Stephen,
What I said was that the transition from not sailing at full speed to the bearaway may have contributed to the pitchpole. Transition from sailing upwind to bearing off around a mark on catamarans is always a sensitive time. If they had taken enough time to get up to speed, the boat should have been fully foiling before the bearaway began. On a catamaran the faster you are sailing when approaching a bearaway the safer it becomes, as you are then sailing into apparent wind and the tendency for the bows to go down decreases. When you power up from a standstill and try to do a bearaway immediately you haven't given the boat enough time to develop the apparent wind which gives you the safety margin. What I see in the video is that the boat only starts foiling mid-way into the bearaway, as the boat accelerates the windward hull falls in the water adding drag, with the rig powered up. So I think they made the error of trying a bearaway in rough conditions without setting themselves up to succeed at the manouvre first. So I think that part of the accident can be attributed to operator error, although I have huge respect for the sailors involved.
I do believe that the boat design was flawed in that if they had designed it to twist on purpose as some believe, it sounded like a really good concept that had too many variables, and they may have produced a boat more suited to flat water moderate wind sailing than what they sent it out in. In our language we say 'slim vang sy baas' which is not easy to translate, but basically means that a clever person is his own undoing. So if the engineers designed the twist into the platform, it was a great concept that couldn't be properly controlled, and if the twist was not intentionally designed into the platform, then it is as a result of trying to reduce weight to the point where they have sacrificed platform stiffness. Either way, I think they goofed. Most catsailors will agree that platform stiffness is critical, to the point where A-class sailors glue their beams into the hulls as a bolted connection allows some flex.
I don't think any rule changes are needed - we don't know what type of foiling height control TNZ is using, and some are speculating that OR17 were using manual foil AoA control, perhaps that was their undoing. The rules basically made it impossible to design foiling multi's, and TNZ's engineers acchieved the impossible, perhaps OR have to rethink their strategy.
My statement about NOT dumping the mainsail when the bows go down, is that if on a beam reach or deeper, you do this, you will power up the wing, not depower it. The way to stall any wing is to increase it's AoA, ie sheet it in. A stalled wing produces very little lift but still gives a lot of aero drag, so I'm not saying they would have saved it, but I know from experience that when the bows go down you sheet in the spinnaker & main and bear away as hard as you can. We have saved the boat this way many times when the rudders are clear out of the water and the boom-mast gooseneck is underwater ! The time to ease the main would have been while going into the bearaway, which they did not do. This would have been wise especially since they did not have the kite up.
If you are not a catsailor, none of what I just wrote will make any sense as it's completely contradictory to how you would think as a monohull sailor.

 

Actually I think keeping the stern down IS important. If the "joystick" controlled flaps on the rudder like on a Moth I think the "pilot" could have kept the stern down.

So Doug, in the power up or depower the upper wing-sail debate you say neither; drop the windward foil? Or increase lift on the leeward foil? If the boat was in take-off mode wouldn't the leeward main foil have already been angled/cambered near max? Stalling it would have decreased lift and increased drag. Come to think of it, that could be exactly what happened, yes?

 

You guys are hard to please, but I won't give up!

Load & angle sensors connected to stuff the honorable mr Nobel used to deal in, or perhaps a more modern variety, to trigger a controlled blow out of the film in the top part of the wing to depower?

Small rockets hidden in the bow and/or stern, again connected to load & angle sensors to start "a la moment suprème" to level the boat out to desired equilibrium?

A chute on top of the mast to decelerate its speeding relative to the platform?

Peter, Yes please, let this be an operator error!

 

If the boats are so touchy that humans can't correct fast enough, is that the crew's mistake or the designer's?

 

Wow, that's something else. Looking at the vid those bows got pressed down for a long time, (check how far the wing is leaning forward) really makes you wonder if there's enough volume in the bows. Also with the boards that far forward at some point the horizontal element is trying to pull the bow down .
Of course it's pilot error, it always is but the platform has to be such that it can survive pilot error.
Once the platform had been drug out past the 'gate it was rough out there, lots of racking around. Interesting to hear how much structural damage. 9-1/2 hours to drag it back to the dock. Sounds like Mark Turner & Core are going to be busy