[ASC1951]

There was an article in my paper today about the Americas Cup, which said that one of the two contenders could sail three times as fast as the wind.

How does that work?

[threebikesmcginty]

Sounds like Colemanballs!

[numbnuts]

Beans...just feed the crew on beans

[Mister Paul]

if you've got the wind behind you, and discounting all the other resistance factors, you can only go as fast as the wind. But if you sail at an angle across the wind, it keeps blowing on the sails, as you don't equal its direct speed. speed.

[ASC1951]

threebikesmcginty said:

Sounds like Colemanballs!

I don't think so. It was in the FT and was quite specific - "USA easily sails at 75 kph in a 40 knot wind and can go up to three times the speed of the wind".

[Tim Bennet.]

It's easy. The only direction where the boat speed can't exceed the true windspeed is directly downwind (ie 180° to the wind).

But at every other angle from there to about 30° true wind direction, it is theoretically possible to exceed the true wind speed (TWS). However the closer the angle to the wind, the less thrust you can extract from the wind and the more drag there is on the boat and rig.

The other problem is that as boat speed increases, the apparent wind will always appear to more from ahead. (Even on really windy days cycling, when you go down hill the wind is always in your face!). Therefore maximum speed are normally seen when around 135° from the true wind although the people on deck (and the sails) will think the wind is more like a 50° apparent windspeed.

[ASC1951]

Tim Bennet. said:

It's easy.

So you say, but I only know about Roman galleys and the Argo.

Anyway, this thing had a sail the height of a 20 storey block of flats. I expect you can generate a lot of power like that.

[Yellow Fang]

If this is true then I think it might have something to do with lift. In wind turbines, the lift force is much stronger than the drag force. When the wind blows across the sail, the sail acts as a sort of aerofoil. Wind blowing over the curved front of the sail would have to move at a faster speed than the wind blowing behind the sail. This causes a pressure difference which moves the boat forward. If the wind was blowing straight behind the boat, you'd just get drag force.

[threebikesmcginty]

Is it going downhill?

[Tollers]

Currents play a part. I expect it can only do 3x the speed of the wind when there is little wind and currents become the dominant force.