Can someone explain how the AC75 sails so fast?

[jtsailjt]

I was amazed by the speed of both boats while watching NZ win the Auld Mug ( again) but thought it odd that they were getting towed to get up on the foils and once they fell off how long it took to get back to foiling. Then today I watched the video of Patriots crash due to not handling a gust up to 23 knots. So it seems that these boats don’t have much of a wind speed range they can be safely sailed in. I do like the speed but would also like to see them able to race in anything from about 5 knots up to about 40 knots, which a 75’ sailboat should be able to handle.

[CassidyNZ]

Originally Posted by jtsailjt

I was amazed by the speed of both boats while watching NZ win the Auld Mug ( again) but thought it odd that they were getting towed to get up on the foils and once they fell off how long it took to get back to foiling. Then today I watched the video of Patriots crash due to not handling a gust up to 23 knots. So it seems that these boats don’t have much of a wind speed range they can be safely sailed in. I do like the speed but would also like to see them able to race in anything from about 5 knots up to about 40 knots, which a 75’ sailboat should be able to handle.

You do remember, having watched the “Auld Mug” racing that in 10kn these boats were doing 45kn of boat speed. 40kn of wind?! Seriously!?? The boats that you recall being able to sail in 40kn of wind had a full wardrobe of sails to change down 5 levels and they were going slooooow enough to do that.

[double u]

"I ...would also like to see them able to race in anything from about 5 knots up to about 40 knots, which a 75’ sailboat should be able to handle"

Sure, & a F1 car should be able to take part in the Dakar Rally...

[rslifkin]

Originally Posted by CassidyNZ

You do remember, having watched the “Auld Mug” racing that in 10kn these boats were doing 45kn of boat speed. 40kn of wind?! Seriously!?? The boats that you recall being able to sail in 40kn of wind had a full wardrobe of sails to change down 5 levels and they were going slooooow enough to do that.

I'm just seeing the need for some extremely fancy reefing systems. With the level of tech that goes into those boats, I'm sure someone could come up with a way to make it work.

[jtsailjt]

Originally Posted by double u

"I ...would also like to see them able to race in anything from about 5 knots up to about 40 knots, which a 75’ sailboat should be able to handle"

Sure, & a F1 car should be able to take part in the Dakar Rally...

.....just about as well as all the other F1 cars. The point being that I wouldnt expect F1 cars to compete against dune buggies or for America’s Cup boats to compete against round the world racers. Like I said, I loved the excitement the speed brings but just think these races would be more relatable to more recreational sailors if the multi million $$$ boats sailed by professional sailors could at least withstand conditions that a Sunfish or Hobie Cat or Laser could sail in.

[Dave_S]

Originally Posted by AiniA

OK, I have a different take. The boats from this challenge are now obsolete for the next one. If I buy one of them, what sort of PHRF rating will I get for the Wednesday night beer can races?

They might not change the format for the next one, maybe they will keep the AC75 and give everyone a break on costs and just develop these boats. It would be interesting to see how much better they might be.

[jtsailjt]

Has anyone heard a reason why team Prada fell off their foils in race 8? Winds borderline light but they started the tack with adequate speed. All they had to do was reach around the course and the race was theirs and they onvioufly knew that

[jtsailjt]

....obviously knew that and were being careful. So I can’t understand how it happened with thrift bigger cooks and bigger jib. Has anyone heard anything?

[cal40john]

Originally Posted by jtsailjt

.....just about as well as all the other F1 cars. The point being that I wouldnt expect F1 cars to compete against dune buggies or for America’s Cup boats to compete against round the world racers. Like I said, I loved the excitement the speed brings but just think these races would be more relatable to more recreational sailors if the multi million $$$ boats sailed by professional sailors could at least withstand conditions that a Sunfish or Hobie Cat or Laser could sail in.

Why is the America's Cup required to change? You don't tune the TV to an F1 race and complain why aren't they racing more normal cars, you tune into the local speedway race and watch that race.

The America's Cup isn't the only sailboat race, search sailboat race on youtube and pick the kind of boat and kind of race you want to watch.

[fxykty]

Originally Posted by jtsailjt

....obviously knew that and were being careful. So I can’t understand how it happened with thrift bigger cooks and bigger jib. Has anyone heard anything?

In the press conference they said both a hole in the wind and a mistake through the middle of the tack - new foil lowered too early - caused them to lose more speed than expected. They also started the tack just a bit slower than what they thought they needed - 27.x knots rather than 28 knots. That top right corner of the course was not a happy place - likely the most disturbed area due to spectators.

[fxykty]

Originally Posted by Dave_S

They might not change the format for the next one, maybe they will keep the AC75 and give everyone a break on costs and just develop these boats. It would be interesting to see how much better they might be.

They’ve confirmed keeping the AC75s for the next two cup cycles and limiting new builds to just one boat.

[Issywa]

If you are interested in a reasonably simple explanation of the physics of apparent wind, check out this link:

https://newt.phys.unsw.edu.au/~jw/sailing.html

[Kerry1]

I think most of the answers I've seen here are on the wrong tack. If you think in terms of energy harnessed in order to achieve a certain amount of work with things like friction in the air and water as absorbing some of that energy reducing the effectiveness of the energy available I think it becomes easier to understand the AC75 achieving the amazing speeds they do. The true windspeed is obviously not the limiting factor to an extent.

The high aspect ratio rig I like to think of in the same terms as a magnifying glass. In ordinary sunlight there is radiation energy. At the lower level of this in normal sunlight, which might be measured in terms of so much energy per square meter (or sq ft), it will get things warm or relatively hot depending on the ability of the surface to absorb it or harness it. But a magnifying glass will take all that energy (the limitation is determined by the area of the glass lens involved) and concentrate all that collected radiation energy into a much smaller spot which "magnifies" the energy to the point where much higher temperatures are evident. It hasn't broken the rules of thermodynamics and made more energy from less, it's just leveraged the energy from a large area into a smaller focal point. The large sail area of the AC75's do the same thing, focus wind energy from a very large area, into a much more focused area with a very low resistance compared to normal sailing vessels.

Another way of looking at it might be by relating it to the principle of a lever and fulcrum. Power or energy is applied at the long end of a cantilever in order to achieve work at the shorter end on the other side of the fulcrum. If we apply power measured in terms of weight - a 1kg weight over a lever four times longer from the fulcrum to the outer end than the other side of the fulcrum to where the load is, then that 1kg of potential energy will achieve a lift power of 4kg, or four times it's own "power". This is clumsy and no doubt I've trashed both the laws of physics and grammar but this is the idea that we have to get our heads around with these amazing boats. It's not so much about wind "speed" but energy harnessed over a wide area and focused over a much smaller "area of resistance" to achieve a certain amount of work. It's about leveraging energy, not breaking the law of entropy, or creating more energy from less.

"In physics and chemistry, the law of conservation of energy states that the total energy of an isolated system remains constant; it is said to be conserved over time.[1] This law, first proposed and tested by Émilie du Châtelet[citation needed], means that energy can neither be created nor destroyed; rather, it can only be transformed or transferred from one form to another."

[LargeMarge]

Commercial pilot here.
Any satisfactory explanation requires a foundation in fluid dynamics.

Ready?

The rules:
* Air is a fluid.
* Air is sticky, fickle, and occasionally cranky.
* Air can be stagnant, or move in any direction (aka 'wind') including up, down, sideways, or all the preceding within a few paces.
* Moving air (aka 'wind') can ricochet -- unpredictably -- off any stationary object such as a mountain or water.

Similar to the vertical 'vacuum' 'lift' created by an aircraft wing as moving air (aka 'wind') flows past, the AC sails are 'lifted' horizontally by creating a 'vacuum' in front of the boat.

Of course, in the aircraft example, relative 'wind' is created by the aircraft propeller to cause the aircraft to move through the air... creating a 'wind'.
(None of the propeller-motivated theory applies to gliders or helicopters, neither of which have any business leaving the earth as anybody with a lick of sense can plainly see.)
As if the aircraft was not present, the speed of the air is irrelevant; the air could be stagnant (aka 'dead air') or moving in the aircraft direction (aka 'tail-wind').

In the case of a tail-wind, a small General Aviation aircraft such as a Cessna trainer designed to Vmax (maximum air-speed... but that has nothing to do with ground-speed as we shall soon discover) at about 100kn air-speed can often achieve a ground-speed of double that in a tail-wind of about 60kn or so.
Looking out a window, you may notice 'we are flying right along!'.

Of course, landing in such conditions invokes the 'any landing you can walk away from' undertaker-humor required of anybody involved in the lunacy of flying in the first place.

Of course, landing into a head-wind is preferable because it reduces the ground-speed.
An example is the above Cessna trainer on final into a 60kn head-wind.
Getting to the ground safely probably requires a light hand with a full-throttle approach.
Of course, at the moment the wheels touch the earth, the transition from flying machine to ground-based vehicle is instantaneous.
At that moment, the air-speed drops to near-zero.

Why 'near-zero' instead of dead-stop?
Any air moving across any airfoil generates lift irregardless of the wishes and hopes of the pilot and crew.
An example is an aircraft designed to operate from a body of water -- a sea-plane.
The first thing I teach my students -- point the aircraft in the direction you want to go prior to starting the engine(s), because as soon as the propeller swings, horizontal lift from the propeller air-foils will be tugging you along in that direction irregardless of how many times you stomp the brakes.

All in a death-defying day's work.
'But this was supposed to be fun!' I can hear somebody protesting.
And it is.
Mostly.

So, how about some strangeness in all this befuddlement, flying or its water-based equivalent, sailing.

In extreme short-landing contests, some specialized aircraft can land while traveling backward over the ground.
The air-speed can be 30kn with a ground-speed of zero/less than zero (aka 'reverse').
Two different activities, occuring simultaneous... and neither has any influence on the other.
The vacuum-generated lift on the wings allows those specialized aircraft to barely yet adequately manoeuvre in the air at an insanely slow and utterly irrelevant ground-speed... during the time the air-speed is in command.
'Sticking a three-point' is one adventure I highly recommend.
Loads of fun!

Show of hands:
* how many can say 'lift is generated anytime air moves across an airfoil such as an aircraft wing or sail to create a vacuum'?

Of course, in these examples, the word 'vacuum' indicates a relatively-lesser air pressure, and has nothing to do with that sucking sound of cash leaving bank accounts of the AC investors...

[Mike Burch]

Originally Posted by Woodland Hills

If you sail downwind at 40 kts in a 10 kt wind, you will have a 30kt head wind. This is common in aviation and these craft are a lot more like air vehicles than boats.
My feeling is that the farther the AC departs from traditional sailing craft, the less the public is interested. This is proven by the complete ignoring of the series that just ended by the media. Not too long ago the Cup was headline or at least above the fold news, now it’s buried on page 10.

Only because the American boat fell apart in the preliminaries. The interest in other countries has been huge.


The boats essentially generate their own apparent wind. The faster a boat sails, the closer to the bow the apparent wind becomes. They don't sail directly downwind—indeed, the mainsail stays close-hauled at almost the same angle as when they are on the upwind leg.


Spectacular they may be, but they are ridiculous boats. If you can't go below and make a cup of tea, I don't want to sail on it.