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sailquik said..

cald said..
Shame it is now being delayed due to covid...

I think my point was somewhat missed with the course layout, i just want to see what the boats could do, i seem to remember with the previous cats they had a short reach to the finish line?

They basically have the ability to define what they want, they could have kept the upwind/downwind legs and added a 500m reach at the top or bottom - why not let them line up for the 500m speed records haha... and as far as danger, breaking boats etc, pls give me a break! They are constantly battling that on all points of sail now.

I think you missed the point entirely.

This is a RACE. It is about upwind, downwind skills and tactics, and the ability to win the start. Just like every other normal sailing race. Your wish to see them push their speed limits on a reaching course will have to be satified with the the brief time they transition though that as they round the upwind mark where we have already seen some major dramas.

There really is a speed limit for these craft and it is the cavitation limit. They are getting very close to that now in some of the mark roundings, and arguably, this has been reached and caused crash downs already. They are not stupid. They know the danger of ignoring those limits. If they were building a craft for maximum speed on a reach, it would be completely different and almost certainly, the foils would be as well.

Whatever, traditionally its an upwind/downwind race, traditionally it was sailed in displacement hulls. Previous cups did have a reach, this one could have too. A reach could have been used to actually mitigate the dangerous transition you speak of to a degree

Just because something always was, doesnt mean it always needs to be - or they wouldnt be foiling.

I dont give a **** about the cavitation limit, just like nz didnt gave a **** about gravity and forces working against them when they made the first cat foil. If the designers had a reason to push the cavitation issue then perhaps more advancements could have been made.

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cald said..
I dont give a **** about the cavitation limit, just like nz didnt gave a **** about gravity and forces working against them when they made the first cat foil. If the designers had a reason to push the cavitation issue then perhaps more advancements could have been made.

That just proves you dont understand the science of it.

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sailquik said..

cald said..
I dont give a **** about the cavitation limit, just like nz didnt gave a **** about gravity and forces working against them when they made the first cat foil. If the designers had a reason to push the cavitation issue then perhaps more advancements could have been made.

That just proves you dont understand the science of it.

I don't doubt I don't know the science of it as well as yourself... but you are being narrow minded, you seem to think the science is insurmountable and we should probably have never foiled anything, broken the sound barrier or even taken flight. Give them a reason and enough money & I do not believe the issues you raised couldnt be dealt with one way or another...

Why could they have a reach with the cats but not now?

And don't you think a reach at the top mark could both be entertaining, add another dimension, point of sail thay they have to be fast and in control and at the same time reduce the risk that has been seen there...

Oh no. The science of cavitation has already been surmounted: Sailrocket 2. World speed sailing record of over 65 knots. Supercavitating foils.

Hydroptere managed to push the limits of conventional foils quite a few years ago and still holds a World record. They are coincidentally, about the same speeds as these boats are going. Just over 50 knots.

It can and has been done, but not on conventional, non cavitating foils that these boats use. They all know that. Thats one reason why there is no reaching leg in the course. It would be pointless.

But you can read the reports how fast they have pushed them in their training. Not sure if those high speeds they got to were deliberate? I don't think they would be risking a bad crash-down and possible severe damage if they could avoid it.

Whoever wins the start wins the race, almost all the time. It has been that way in sailboat match racing for decades. They should just race the starts, and forget the rest of the course. It was like that for windsurfing formula racing, too.

When I watch the AC I am hoping for lead changes and covering and roll-overs. You know, real racing. We used to get those things in about 1/3 to 1/2 of the displacement mono-hull races in the old days. Now, I have yet to see single one of those things. Tacking duels are nice, but at their speeds, covering is not all that effective.

In windsurfing formula racing, covering mattered most at the start. If you got buried behind the leaders at the start you might as well just go fishing since there was no wind leftover for you to even cross the starting line.

Well you might as well add up all the money they spent and award the prize on that.
Or just throw dice, or flip a coin.

I was certain I had seen multiple changes of lead in the Prada Cup Races.

There was one post race interview where the loser, Terry Hutchinson, was congratulated on American Magic recording the fastest speed of the day. 53 knots passing through the power zone on a top mark bearaway. He just just shrugged his shoulders .. whatever "We've been faster than that".

So hypothetically Sailquick, if they'd let you loose on setting up a retired AC75 for a speed run, (American Magic? only one ding, professionally repaired) what do you think it'd do? You'd only have to put your super cavitating foils on one arm. Gybe into the speed run at 45 knots on the regular foils, no problems with the big drag humps that these speed foils have. Don't have to get towed back up to the top of the course either, you'd get plenty of runs in.

But if you don't think they'd match Sailrocket's 65 knots why bother?

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Ian K said..
There was one post race interview where the loser, Terry Hutchinson, was congratulated on American Magic recording the fastest speed of the day. 53 knots passing through the power zone on a top mark bearaway. He just just shrugged his shoulders .. whatever "We've been faster than that".

So hypothetically Sailquick, if they'd let you loose on setting up a retired AC75 for a speed run, (American Magic? only one ding, professionally repaired) what do you think it'd do? You'd only have to put your super cavitating foils on one arm. Gybe into the speed run at 45 knots on the regular foils, no problems with the big drag humps that these speed foils have. Don't have to get towed back up to the top of the course either, you'd get plenty of runs in.

But if you don't think they'd match Sailrocket's 65 knots why bother?

There are a few examples of sailing craft doing peak speeds of 53 'ish knots on conventional foils so clearly that is possible. Sailrocket did similar speeds before they go their SC foil working (and then suddenly jumped into the 60's) i assuming Hydroptere also peaked at a little higher than it's 52.86 knots world record average.

I dont think the ACV75's will go more than a knot or two faster, if even that, on conventional foils. To set them up with supercavitation foils would be highly problematic. i think the whole craft would probably have to have a different design to utilise SC foils and be able to control it's flight. Eg. I am thinking that the existing control mechanism (trim flaps) would not work on a SC foil so the whole foil (and beam arm?) would have to be adjustable for AoA (ah la AC70?). There could be control issues with the steering foil as well if there is enough load on it for cavitation to occur, although I suspect it could go a bit faster if the load is very low.

There is an unknown (to me at least) factor here regarding how much cavitation could be tolerated on a conventional foil in this situation. The science tells us that the drag will increase substantially as cavitation starts to occur. The question is, how much power would be required to overcome this drag, and at what point (speed) does power equalise with the drag. Or how much cavitation can be overcome with the power available and how much speed would that add? It is quite possible that they are working in that zone now at 53 knots. The other related question, is how much cavitation can be sustained before control and/or lift is lost? I would be surprsed if the AC75 designers have not explored those questions and have some data on the answers. Whether we ever hear the answers is another question.

Can't find much on "super cavitating foils with flaps" on the internet, however, as usual, someone's been there before.

eprints.utas.edu.au/11838/2/Pearce-whole.pdf
Ventilated Supercavitating Hydrofoils for Ride Control of High-Speed Craft
Bryce W. Pearce, B.Eng (Hons.) National Centre for Maritime Engineering and Hydrodynamics Australian Maritime College

We're going top have to find you a healthy R&D budget but I'd reckon there's another 10 knots in the AC75 format if top speed was the aim.

That paper is a great find Ian!
Andthere are quite a few references listed within that might be interesting to follow up on.

Interesting that in the introduction, a study referenced says:

"Optimum sub-cavitating foil sections have an upper speed limit of about 45 knots (Conolly, 1975)."

I assume this is where incipient cavitation is predicted (or observed) to start. Mal Wright studied all this 15 years ago and designed his windsurfing foil shape to resist cavitation up to certain speeds. He initially released his TM 45 series. With the numbers based on the best assumptions and data at the time, he was confident these fins would be cavitation free until at least 45 knots. Later he designed the TM48 and TM50 foils.

Interestingly, the TM45 fins went well beyond 45 knots quite a few times, so it seems the numbers predicting cavitation speed were somewhat conservative and the later foil designs may well be also capable of faster speeds than their nominal design limits. Of course, there are many variables involved in the initiation of cavitation. The big ones are load and angle of attack. Obviously, the lower the load, the higher the point of initiation of cavitation. Ie. When on a very broad reach, at high speed the load on the fin and the AoA may be reduced enough to prevent cavitation until slightly higher speeds. And if the initial cavitation is very mild, there may not be enough extra drag produced to overcome the power available, and so allow some further increase in speed.

It is my speculation that this is what we are seeing here with all the craft using conventional foils in the low 50 speed range.

One of the effects of incipient cavitation described in the papers is vibration. I was interested to see there was quite obvious vibration present in the helm wheel of the AC75's (from the onboard video) when they were pushing their highest speeds. Is this a telltale sign of cavitation?

My question for you Ian, is how do you calculate that "I'd reckon there's another 10 knots in the AC75 format if top speed was the aim."

Just a wild guess Sailquick. But you've got wonder where the next challenge to Sailrocket's record will come from. F50s have a simpler foil system, do 50 knots out of the box, and only cost $5m a year to run. Maybe you could hot one of them up with super cavitators?
Maybe Sailrocket's record is safe for a while yet. It's almost 10 years now.

These fellows have dug up some interesting lab tests. At 1.00 check the ventilation coming from a metre or more back in the wake down the centre of the tip vortex! They make an interesting proposition that cavitation is delayed on larger foils, less pressure drop in negotiating wider curves. Of course you need more power to drive a larger foil through the water. But the AC75s seem to have heaps of that to spare, even in 20 knots.

Another great find Ian!

I referenced those guys some time ago in an earlier discussion on what Supercavitation is.

But this video discussion is directly to the point of the current discussion.

Well we have already seen that the AC75's are peaking at just over 50 knots so that suggests their foils are large enough and lightly loaded enough to avoid damaging, or loss of lift cavitation up to those speeds in at least some situations.

One things that puzzles me is that, in the broadcast video, you can see that the outboard tip of the outboard foil is quite often just breaking the water surface. This surprised me as I thought this would need to be avoided to prevent unwanted ventilation. Maybe there is some other trick going on there?

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sailquik said..
One things that puzzles me is that, in the broadcast video, you can see that the outboard tip of the outboard foil is quite often just breaking the water surface. This surprised me as I thought this would need to be avoided to prevent unwanted ventilation. Maybe there is some other trick going on there?

Puzzles me too. Maybe they rely on the inboard wing to do all the heavy lifting? But then they'd lose righting moment? There's a bulb in between housing the 800 or whatever kilograms of lead to stop ventilation spreading inboard. It's only a design rule that requires the foils to be symmetric. Adding to the confusion is the Kiwis having a straight wing and all the others with anhedral wings.

I don't find them, my search engine has me sussed. It's just thrown up this one. They're aiming for 80 knots. Using Sailrocket's foil vs. foil tug of war format. sp80.ch/

I recall when the preliminary design of the AC foiling monohull first came out and an interviewer didn't think it looked as fast as the previous AC cats. The designer had no worries, words to the effect of "It may not appear to be but its righting moment is actually greater" . Is it that simple? Does a big enough gravity balanced boat have enough power to get up on super cavitating foils?

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Ian K said..Does a big enough gravity balanced boat have enough power to get up on super cavitating foils?

That my friend, is the 64,000 Dollar question.

Come to think of it, that saying is badly outdated. It should be the 64 MILLION Dollar question!

I'm just discovering there's quite a lot going on with this SP80 project. They might pull it off, they are French after all.

Luna Rossa seems to think they can hit 60 knots so who knows what the limit is. www.stuff.co.nz/sport/americas-cup/124296278/americas-cup-luna-rossa-skipper-believes-monohull-boats-can-reach-60-knots

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Roo said..
Luna Rossa seems to think they can hit 60 knots so who knows what the limit is. www.stuff.co.nz/sport/americas-cup/124296278/americas-cup-luna-rossa-skipper-believes-monohull-boats-can-reach-60-knots

Like some famous bloke onece said: 'Tell him he's dreaming'

Anyone catch the races today? It really is win the start or die hey. These two boats are fairly close overall - with luna being slightly better one would say, but even when they were only slightly behind they still couldn't get the job done! its enthralling stuff but some lead changes would be good...

I still say give them some reaches... haha

Yes. 2 great races today, especially the second where Luna Rossa made some significant gains from behind which made it very exciting.

Clearly, if the wind was more gusty and shifty there would be more losses and gains, but this is always the case in Match Racing. There would zero chance of overtaking on a reach. This is why sailing races are Upwind/Downwind

Bring on the next race!

Yes, and team NZ is clearly doing tests nearby..

They would have been crazy not to take advantage of the less busy water (due to covid), they would have got a lot of data to compare to the race data for sure...

Can't wait to see what that kiwi boat can do.

It;'s strange to me that the way the rules were written seems to severly handicap the Kiwi boat. While the challengers are racing each other and tuning, learning, developing etc, the Kiwi's have no sparring/tuning partner. Thats got to be a disadvantage.

Actually in the rules Team NZ can practice against their other boat.
But either it's too expensive to have two crewed boats in the water or the second boat is so much more advanced than the first.

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515 said..
Actually in the rules Team NZ can practice against their other boat.
But either it's too expensive to have two crewed boats in the water or the second boat is so much more advanced than the first.

Ahh, that makes sense. But I guess they knew that all along so have some sort of plan .....

Also I believe with the country situation, it is NZs cup at the moment - not Team NZs and if another yacht club could get backing they can fight for the right to defend it - so there can be a defender series as well as a challenger series - this has happened in the past when USA has been the holder/defender..

There was supposed to be more racing pre xmas as well but covid put pay to that...the kiwis will feel underdone...

With the new class of boat and steep development cycles it will be interesting to see what is more fruitful, shorter development with more racing or longer dev with less racing...

Love the races, even the "boring" ones. Seeing tacks with a minimum speed around 30 knots is pretty amazing.

Another cool thing is the similarity to slingshots in speedsurfing. The boats regularly get their top speed at the windward mark when they switch from an upwind to a downwind course. Just after the course change, they are a few knots faster than further down the course. Which raises the question: are they faster near the mark because they have to go too deep a bit later, or is it because the "loaded up with power" on the upwind course? I have heard the "load up" theory for slingshots several times, but not all theories are correct.

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boardsurfr said..
Love the races, even the "boring" ones. Seeing tacks with a minimum speed around 30 knots is pretty amazing.

Another cool thing is the similarity to slingshots in speedsurfing. The boats regularly get their top speed at the windward mark when they switch from an upwind to a downwind course. Just after the course change, they are a few knots faster than further down the course. Which raises the question: are they faster near the mark because they have to go too deep a bit later, or is it because the "loaded up with power" on the upwind course? I have heard the "load up" theory for slingshots several times, but not all theories are correct.

It's because the angle of attack for the highest boat speed doesn't equal the fastest route to the downwind mark - i.e out and an out boat speed is not the only determining factor in velocity made good (VMG) the distance you sail to the mark plays a massive role. so when they round the top mark, yes they are powered up, but they also sail through faster points of sail than their out and out best downwind VMG.

On a more exciting note, there was a lead change

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boardsurfr said..
Love the races, even the "boring" ones. Seeing tacks with a minimum speed around 30 knots is pretty amazing.

Another cool thing is the similarity to slingshots in speedsurfing. The boats regularly get their top speed at the windward mark when they switch from an upwind to a downwind course. Just after the course change, they are a few knots faster than further down the course. Which raises the question: are they faster near the mark because they have to go too deep a bit later, or is it because the "loaded up with power" on the upwind course? I have heard the "load up" theory for slingshots several times, but not all theories are correct.

Yes. me too! Even the 'processional' racers were really interesting to watch.

'Slingshot': There is no 'magic' extra speed gained by some sort of bearaway manouver than can't be attained in a constant optimum angle of sailing.

The term 'slingshot' is sometimes misunderstood to imply that some how the manoveur, or act of bearing away adds some extra speed somehow. This is not true. Finding, and sailing on the optimum downwind angle for maximum speed in a given set of conditions is the fastest.

I guess the term slingshot really refers to the feeling of acceleration during a bearaway, which is an addictive feeling for sure.

The fastest VMG downwind course, as Calc correctly described, is broader than the course for maximum speed.

It was a great series and personally I reckon the racing has been epic. Not sure I really understand those saying it's boring and want to go back to lead mines, has anyone any watched any of those 3hr races? they are pretty bloody boring If you're into the technology of sailing at the absolute cutting edge and the intricacies of the tactics surrounding high apparent wind sailing it doesn't get much better.

Prada in the conditions of the Prada Cup final certainly had a click of speed on Ineos, particularly apparent in that last race where they hung off the hip of Ineos to the boundary off the start, then just tacked off, went high mode, job done once they came back on Starboard.

If the conditions are similar in the AC final it's going to be very interesting. I doubt ETNZ with their foil size, rumoured to be 25-30% smaller have that high mode available to them which is a very powerful weapon for Prada. Anyone who races knows that having a good high mode, particularly off the start is an absolutely devastating tool to have in the toolbox. The question is, does ETNZ have enough raw speed to be able to get bow forward in that situation with as good or better VMG and hence neutralise it. Rumours are they definitely have a fair click on everyone so who knows. As they always say the fastest boat usually wins the cup and as was their position in the last cup, that speeds allows them to take a more reactive approach....that said they definitely had a super effective high mode in Bermuda.