Can someone explain how the AC75 sails so fast?

[Chotu]

Originally Posted by taxwizz

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

OK, everybody can stop posting. You just won the Internet for today. LOL
OK, everybody can stop posting. You just won the Internet for today. LOL

This was hilarious. I’m imagining a prop down on one of the foils. 200 hp diesel. Everything is just for show. Ha ha

[zstine]

Originally Posted by Woodland Hills

.
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 think it was ignored in the USA b/c NBC bought the copywrite and expected $175 to purchase the America's Cup channel. The vast majority of the world 150+ countries were able to stream it live for free. Greedy capitalism ruins the advancement of wind technology for America.

I personally love the AC75 class. Crazy exciting racing! Just a matter of time before foiling is mainstream... there's already a lot of small foiling race boats available for similar costs to conventional racing dinghies (ex. UFO < $8k). Beneteau makes a foiling 32ft production boat at ~$200k (Figoro), And there are large performance cruising yachts getting up on foils, albeit at high cost. It won't be long before foiling is mainstream... Why wouldn't you want the smooth and fast ride over the top of the waves, vice plowing through them with a day-sail range of 100+miles compared to 60mi?

[bmz]

The speed of the wind bears no direct relationship to the ultimate speed of the boat. A sail simply captures a portion of the energy in the wind to maximize the forward force vector. It is simple physics that a sailboat will accelerate until the water resistance, i.e. rearward force vector, equals the forward force vector. The speed of the wind is basically irrelevant. The only relevance is the energy that can be captured from that wind; and as the wind speed goes up, so does the amount of captured energy, but the ultimate speed of the boat has little to do with the speed of the wind, but more so with the resistance of the water.

This is why ice boats can so easily exceed the wind velocity--the resistance of the ice on their skates is so small.

[meirriba]

Originally Posted by 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.

True wind is behind the beam, but at high speed the apparent wind is moving so far forward that the sails feel an upwind direction of the AW.

[CassidyNZ]

True wind is everywhere, so is apparent wind. Only if the boat is stationary is true wind and apparent wind the same.

[MicHughV]

that is apparently a very true statement...

[mawtty]

You will get the picture if you analyse the velocities in VECTOR Mechanics.

[jtsailjt]

The difference in drag became pretty apparent when they both fell off their foils during race 8.

My Raymarine instruments allow me to toggle between true and apparent wind and I like to try to guess what the true wind will be before I switch from apparent wind at various points of sail. Of course my top speed is only about 8.5 knots but it’s still pretty interesting to see the difference when I’m on a beam reach.

[huhns]

A proper explanation does require vector mechanics, but I believe none of the above posts has the complete explanation. The key concept is the wind (via the sails) pushing the foils (keel) against the water. A good analogy is you can shoot a watermelon seed out of your fingers if you squeeze it, with a speed much faster than your squeezing. Water is basically incompressible, so most of the wind energy "squeezes" the boat ahead, as long as the hull is not DDW. You might also think of a kite, which requires a kite string to fly -- if the string breaks, the kite falls to the ground. (Another, more difficult analogy, is the cutting point of a scissors, which moves much faster than the blades of the scissors when you cut something.)

[OBT]

How do they sail so fast you ask? Simple. They are made out of “Youcantafordium”.

[Rowglide]

https://www.youtube.com/watch?v=5CcgmpBGSCI

Check out the blackbiord landyacht!! 50MPH dead downwind in 20mph wind???

[Dave_S]

Originally Posted by Rowglide

Check out the blackbiord landyacht!! 50MPH dead downwind in 20mph wind???

Re: Blackbird Land Yacht.

This is very intriguing. My head is screaming BS. But I've spent a while checking this out and smarter people seem to think it works although thier arguments haven't done it for me yet. I'll spend some more time on it but it seems like you are doing more work than the energy you put in.

I ended up going down the rabbit hole that is surfing the internet and found interesting subjects such as wind shadow gliders which also seem to defy physics and can fly an oval course continuously at up to over 800kph (current record) in less than 80kph winds.

[Dave_S]

Originally Posted by Dave_S

Re: Blackbird Land Yacht.

This is very intriguing. My head is screaming BS. But I've spent a while checking this out and smarter people seem to think it works although thier arguments haven't done it for me yet. I'll spend some more time on it but it seems like you are doing more work than the energy you put in.

I ended up going down the rabbit hole that is surfing the internet and found interesting subjects such as wind shadow gliders which also seem to defy physics and can fly an oval course continuously at up to over 800kph (current record) in less than 80kph winds.

Ok, I think I have it.

My stumbling block was when the propellers pitch velocity equalled the air speed I hadn't considered there would be any motive force generated by the propeller which is not right. The angle of attack needed so the cart can continue to acceleate is in the propeller blades. They are still rotating at 90° to the air flow (relative to the ground) and as such even though the pitch of the blades matches the air flow, the blades are still forcing the air to go around the assymetrical foil shape of the blade so still creating lift. Providing the air speed at which the foil shape is neutralized and the angle of attack is neutralized occur at different air speeds it will work.

Because it can always maintain an angle of attack the carts velocity is only limited by drag. No different to the AC75's or any other sailboat except the boats don't have a differential to allow an angle of attack when sailing at 0° or 180° to true.

Voila.... What a good boy am I.

Give me a little while to enjoy this before you destroy it.

[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?

[RaymondR]

Newton done it, F=ma they are not displacing any water which almost entirely gets rid of the form drag and the low surface area of the foils generate low skin drag.