America's cup

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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.

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.

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.

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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.

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."

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.

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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?

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.

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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'

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!

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.

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.

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...

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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

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.