Sailrocket 65 kts/ 75 mph!

[Dave_5280]

Foil sailboat built for the world record. They say on the site that water boils, called cavitation, when it hits the foils at 50 knots causing drag, so they designed the foils to overcome it. They say the boat went 2.4 times the speed of the wind. This was back in 2012.




http://sailrocket.com/

[Bille]

...
They say on the site that water boils, called cavitation, when it hits the foils at 50 knots causing drag, so they designed the foils to overcome it.

...

I wondered how the AC 75's got around that ; are they operating just
below the speeds, where that boiling happens ?

Bille

[Dave_5280]

...
They say on the site that water boils, called cavitation, when it hits the foils at 50 knots causing drag, so they designed the foils to overcome it.

...

I wondered how the AC 75's got around that ; are they operating just
below the speeds, where that boiling happens ?

Bille

Yes, you are correct. They expect the AC 75’s speed to be 40 to 50 knots. But not sure about cause an effect, might not be the foils, or foils alone with all the design elements that come into play and the rule on maximum wind speed probably for safety.

[dice]

Cavitation is a bad thing?
Myth busters tested that and they had less friction with cavitation.

Unless the use case is entirely different here, it was some case where they wanted to drop a bomb into the water at the highest possible speed I think.

[Mossy 757]

Cavitation is a bad thing?

In the application being discussed, yes; cavitation means "the making of a cavity," where cavity refers to air around a surface that should otherwise be immersed in liquid. In the case of propellers and foils, this cavity offers no resistance against which to do work by displacing water with air.

When something expects to be pushing back on water and all of a sudden it's pushing back on air, that's not good for speed, stability, control, etc.

That's also how torpedoes work, they don't rely on their explosive payload to create an over-pressure situation inside the ship, they lack the velocity to penetrate that way. What they actually do is create a huge airbubble underneath a ship or submarine that the vessel then "falls" into, breaking their keel and usually folding them in half to sink with both ends pointing upwards as the air pockets in the ends slowly escape. With foils and propellers this bubble just happens to form at the leading edge, but the effects can be no less devastating. Some cavitation that occurs on maritime propellers actually pits away the metal because the vacuum explosions are so violent at certain speeds and pressures.

[Dave_5280]

They have a page about cavitation on their web site at http://sailrocket.com/node/288

[bohme]

Try google "supercavitating foils"
Originally invented for high speed propellers. One of the first to use it was "Turbinia" in 1897...

[AndersP]

Here is a more in depth discussion about supercavitating foils.

[Mossy 757]

Here is a more in depth discussion about supercavitating foils.

Very interesting concept, it reminds me of the air intake cones on the SR-71 which are designed to adjust for the transition to supersonic flight. If the AC75's can do something similar inside the limitations of the class rules that would be very cool, but I wonder with course size if they're going to spend much time trying to engineer beyond 50 knots. It seems like a greater ROI to make the boats stiff, durable, and stable versus pushing their design profile outside the normal subcavitation speed range given that they're going to be sailing inside a constrained body of water and won't have much space to go 50+ for very long, if at all. Planning to sail much faster than 50 knots also puts new requirements on structural engineering and safety/stability systems in a way that really pushes the envelope of being able to reliably run dry laps, which ultimately seems to be where these races are won or lost, hence why boats like Sailrocket don't compete in the America's Cup...

[Bille]

...
...
Planning to sail much faster than 50 knots also puts new requirements on structural engineering and safety/stability systems in a way that really pushes the envelope of being able to reliably run dry laps, which ultimately seems to be where these races are won or lost, hence why boats like Sailrocket don't compete in the America's Cup...

Which is something i Don't like about the sailrocket ; yea , it goes
like hell, but only in (1) direction .

Bille