34th America's Cup: multihulls!

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Yes there was but it had no effect since the race was called at about the same time-at least as I understood the talking heads. TNZ didn't stay clear of the leeward boat as I understood it.

 

Oracle protested and the protest was denied - no penalty light on TNZ.

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CutOnce

 

34th AC

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I went back and watched the start of the second race-you're right-green flagged-thanks for catching that.

 

MJ, the Kiwis don't need to push for higher winds; they are one crash away from taking the cup.
But looking at the cancelled start race 12, Aotearoa started from less advantageous starboard position but cunningly out thought Spithill and were all over him at the (cancelled) gun start. The wind was up and twice Oracle looked to be in trouble burying bows and the second time, covered in white- so what I'm saying is that Aotearoa can actually handle the heavier stuff better than the O boat. This has been already proven.

 

Great start!

To help my understanding can we come up with an estimate of what the change from 33 to 23 knots had on the loads the boats would see?

My first pass is that the power to drive the boats can be represented by the square of the wind velocity.

33^2 = 1089
23^2 = 529

529/1089 = 48.6%

The change has reduced the available power by 50%

Does that sort of work?

Drag limits boat speed and that also is a square function.

Assume a drag coefficient of 1

Power needed for 50 knots should be 50^2 = 2500
a 20% reduction in top speed (assuming Cd = 1) would be 40 knots so 40^2 = 1600

1089 (wind power 33)/2500 (drag 50) = .436 (power/drag)
.436 * 1600 (drag 40) = 697 (wind power)

697^.5 = 26.4 knot wind limit if goal is to limit boat speed to 40 knots?

Looking from the other direction with a design estimate of 40 knot top speed in 33 knots of breeze I get
1089/1600 = .681 so the 23 knot limit would have limited boat speed to (.681 * 529) = 18.97 knots ...

That is close enough to a 50% reduction in wind power equaling a 50% reduction in speed for the change from 33 to 23 knots.

To solve my puzzle I need an estimate of foiling hydro drag and non foiling hydro drag.

Does anyone have a SWAG or better estimate of the total Cd of the foiling boat compared to the non foiling boat?

A 50% reduction in power bring the boats to the 45-48 knot range for top speed suggests that at those speeds foiling reduced the drag to less than half of the non foiling number.

If you can follow my logic here please point out errors in assumptions or whatever.

 

You need to consider hitting the wall (50 plus knots boat speed) because the foils used are not anti-cavitation (I think). Then there's the problem of high speed wedge foils being useless at lower boat speeds necessary for lighter winds for these boats. So in the wind reduction, there's not going to be much change in top speed because of the present foils crapping out long before maximum (originally proposed) WS limit.

 

It is not so simple, because you are assuming that all the forces scale down with V^2, which implies that all angles and drag coefficients remain constant in the process. Neither is true, however. The angles will change because of the different apparent wind and the coefficients will change (a lots!) because of the different foil height, ventilation and cavitation.

And, besides that, don't forget that words are important too. You have been calculating force, not power. Power is proportional to V^3.

For the same reasons, in the post you have quoted I have changed the wording "they could possibly exceed 55-60 kts" into "they ideally might be able to exceed 55-60 kts" - because there are all the above factors between the theory and the practice.

I would limit the calculations just to the following few simple indicators:
- Aero forces (33 kts) are proportional to 33^2 = 1089
- Aero forces (23 kts) are proportional to 23^2 = 529
The relative decrease of aero forces is (529-1089)/1089 = -0.51 (-51%), with respect to the original wind speed limit of 33 kts. The structural loads are approximately proportional to these forces, so we can say that they have also been reduced by the same amount.

We've seen from the races that these boats are doing around 45 kts at around 20 kts true wind speed (approximately). So Vboat/Vtw = 2.25
Assuming that the same ratio would be maintained up to 23 kts true wind speed, you get that their max speed has been limited by the rules to 2.25*23 = 51.8 kts (which I had rounded to 50 kts in my previous post, because things are not so linear and because these are all approximate numbers anyways).
We cannot assume the same Vboat/Vtw ratio would be valid up to 33 kts true wind speed, because it would give 2.25*33 = 74.2 kts max boat speed, which is way beyond the cavitation limit. We can guesstimate that the max speed would not exceed 55 kts (a generous guess which requires a careful foil design) because of the cavitation limits. So the Vmax ballpark is:
- Vmax (23 kts wind) = 50 kts
- Vmax (33 kts wind) = 55 kts (wild guesstimate)

The hydrodynamic loads would be proportional to:
- Hydro forces (33 kts wind) are proportional to 55^2 = 3025
- Hydro forces (23 kts wind) are proportional to 50^2 = 2500
Should these be confirmed, the relative decrease of hydro forces would be then (2500-3025)/3025 = -0.17 (-17%), with respect to the original wind speed limit of 33 kts. The structural stress is again approximately proportional to these forces, so we can conclude that the structural stress due to hydrodynamic forces has been reduced by 17% with the decrease of wind speed limit.

That's imo all we can approximately calculate with the limited data we have.

Cheers

 

Sounds like the Sail Rocket crew might have a future working on catamarans soon !

 

Nope, sounds like the americas cup boats might have one last huraaaa once the cup goes back to NZ...

With some new foils dedicated to only high speed broad reaching, no upwind performance constraints - and perhaps a crack at the speed record... they should be able to better hydropteres speed surely? Only reason they wouldnt bother is theyve calculated that they cant possibly better sailrockets record... thoughts?

 

Groper;
And who has the runs on the board for hi speed foils ?

 

I'm not saying they go for sail rockets record, I'm saying they have the knowledge to optimise the AC boats.

 

That is a brilliant justification for the wind limits Daiquiri. If you'd provided this to the AC34 race committee back in June they'd have made it the official line!

Artemis, who along with Prada argued the most for lower wind limits, were (and are still) unable to publicly admit their errors which led to the main beam failing. With Simpson's death having shocked everyone, and silence from Artemis on the reasons for the beam failure, the public perception was soon that all the boats were perhaps just as dangerous; So the safety recommendations then had to err on the side of caution.

I don't recall anyone mentioning cavitation back then, I suppose they must have all studied when it might occur.

The two teams now sailing for the Cup have demonstrated quite a few times they can sail in more than 22-23 knots, and when they are on record saying they can do so safely (as both have claimed) then it's regrettable not to see them racing in a few additional knots of wind with fewer postponements. There was a taste of it with how the boats were being thrown around in the Race 12 start (then abandoned).

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Gino Morelli expands a little on the subject of wings:

 

I quite like the sound bite that the new limit is required due to unforeseen design success.

The spin now is that the designs are a failure because they cannot sail safely in slow boat breezes.

The next question becomes will anyone want to pursue highest speeds given the extremely high cost of failure?

The risk/reward for speed records or a longer racing season are quite different to those for the AC.

 

Iain Murray would have found it very convenient when Jobson pestered him on the committee boat about another postponement, that he could reply "Our hands are tied Gary. You can't argue with physics!"



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VSail/Pierre just tweeted:

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Here's the third part of an article about Charlie Barr, his life and times.

 

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New YouTube item from Oracle,

GO ORACLE!!


Let's have two races today,
and the next day,
and the next,
and the next!





... and Spithill says "Somehow it becomes... strangely normal".

Ain't that the truth.