Can someone explain how the AC75 sails so fast?

[rspeiser]

I'm not talking about the foils; I get the way they can produce positive or negative lift, that this force is used to fly the hull and keep the boat flat so the rudder+foils keeps working even while flying, and that obviously drag is greatly reduced on foils.



What I'm wondering about how the sail plan(s) used can achieve four-five times the speed of the wind. (Saw them achieve 44kts with 7-8kts of wind in America's Cup). Even with the reduced drag while flying this just seems impossible.



Can anyone here explain this?



Thanks.

[MicHughV]

pretty simple....if you travel in a car doing 10 mph...and the wind is blowing at 10 mph from ahead, the relative wind speed is 20 knots.

These AC boats sail so quick, that even going downwind, the relative wind is from ahead

[double u]

some decades ago they clocked ice-yachts here in Austria & were surprised, that they made 160km/h (100mph) without trouble...

[Rucksta]

When a sail generates lift it will accelerate the boat until drag prevents further acceleration

[Muaddib1116]

I can understand how they can sail faster than windspeed going up wind, but I don't really understand how they were doing that downwind. I assume it has something to do with the fact they they were basically tacking up wind and downwind symmetrically, but I can't figure it out.

[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.

[Muaddib1116]

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.

I had that thought, but I still can't get my head around it. At that point, it starts to sound like perpetual motion.

[MicHughV]

simple....

say there is no wind at all....
but you are travelling in a car going 60 mph...
you stick your hand out of the window....what do feel.....wind pressure..that's " relative" wind.....ie, " relative" to the car moving at 60 mph...the wind " feels" like it's blowing at 60 mph, even though there is no wind at all.

These boats don't go dead downwind, they sail downwind at an angle to the wind...and create their own " wind" just like a car would..

it's really not a difficult concept to understand at all....a little bit of trigonometry knowledge is all that is required..

in essence the AC 75 boats are always beating into the wind...whether they go upwind or downwind...

I don't know if I can explain it any better than that...wind is not a " thing"....yes, you can't see it, unless its blowing smoke or something, but wind is air pressure....and the shape of the sail, much like an airplane wing, creates "high pressure" and " low pressure" due to its curvature shape, this creates " lift" which enables a plane to fly and a sailboat to sail against the wind.

There's a bit more to it, because the lift (or drive" of a sail is dependent on it's location, more or less to one side or the other of the centerline, and if you want to move forward, you need a counterpoise...ie, the keel...beyond that, I can't explain without drawing diagrams...but plenty of these available online.

[MicHughV]

sadly, I have to agree, that these new AC75 foiling boats, do little for me.

to add to the above explanation, an AC75 boat actually has a " double" mainsail, this is to more accurately copy a plane wing...

for my money, bring back the old 12M boats....and longer races....20-30 minutes on a race course is not much bang for your $100 mil investment...

[CassidyNZ]

Originally Posted by MicHughV

These boats don't go dead downwind, they sail downwind at an angle to the wind...and create their own " wind" just like a car would.

To me that’s not an accurate comparison. The car is using gasoline to create their “own wind”. The AC75 is using wind to create more wind. Wouldn’t it be great if my car could use gasoline to generate more gasoline

Whilst I think I understand the concept of generating apparent wind (I see it on my own boat to a far lesser degree), the ability to multiply the true wind speed by 4 is still ever so slightly mysterious to me.

I guess it’s all about resistance. My boat will generate extra speed in the same wind on some points of sail but the ability to do that is severely limited by the dragging of a great big belly through the water. On foils this drag is almost eliminated. It’s probably similar to the cyclists who ride behind a big wind deflector on a car and then pedal their bikes to nearly 200 miles/hr. Not only zero drag but a “vacuum” dragging them along.

https://www.npr.org/2018/09/18/64922...w-world-record

The AC75 also has near zero drag (relatively speaking) and a “vacuum” is formed on the back of the sails. An AC75 cannot foil DDW because on that point of sail, it is generating no apparent wind.

[MicHughV]

true, most people don't think of wind as having resistance...or pressure
but in simple terms, the force (or pressure) of wind) goes up as the square of wind speed (velocity)

for this reason very few cars can exceed 200 mph...it takes too much horsepower to that, ie, to overcome the wind pressure...generally in the 600 hp plus range...

for the same reason, sticking your hand out of a car window at 100 mph, could likely break your arm..

it should also be noted, that most sailboats are of the displacement variety, which mean a whole other set of parameters come into play, whilst the AC75"s are " foiling" boats, this allow them to escape the limitations a regular displacement hull, as besides the immersed foil (and rudder) there is no hull in the water...unless, off course, you are not foiling,

it's interesting to note, that these AC 75's, while incorporating the latest of everything, are slower than molasses when dropping of their foils..

[Dave_S]

They can't sail DDW at, or greater than wind speed, they need to gybe - sail at an angle to create lift by bending the airflow, the faster you accelerate the air when you bend it around the sail the greater the potential for speed.

Drag is the killer, drag increases at a faster rate than lift so you will always reach a limit where drag matches lift and you can't go any faster. If you could eliminate all drag then you could sail at infinite speed.

The fastest boat will have the least drag.

[Flagman101]

two words
Apparent wind.

[Chotu]

It’s very simple.

1) Lots of sail area.

2) Very little wetted surface area.

Look for these features in ALL fast sailing vessels. They all share these traits.

Given the same size rig, the boat that has less boat underwater is faster.

Note: the high aspect ratio rig also makes the sails more efficient, as if you had more sail area

[taxwizz]

Simple....
There is a 200 hp diesel under the cockpit.