Another one for the physicists

[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

 

[ASC1951]

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]

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.

 

[Coco]

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".

40 knots is 74.08 kph so I'm not surprised it does 75 kph easily

A couple of references:

http://www.animations.physics.unsw.edu.au/jw/sailing.html

This one is easier to read

http://boatsafe.com/kids/bramp1099.htm
'Apparent wind' is what my wife complains of

 

[Davidc]

Vectors

 

[colly]

Vectors

What he said.

The boat will be faster than the speed of the wind because it is travelling across the face of the wind. It only won't exceed the speed of the wind if it's in the same direction as the wind

 

[ASC1951]

40 knots is 74.08 kph so I'm not surprised it does 75 kph easily

Well, there you go! I'm still on cables and shillings.

 

[661-Pete]

Alas, you probably need a yachtsman more than you need a physicist, to answer this one.

But I do know that if you're sailing close-hauled (or is it reaching, or whatever the word is), you can indeed go faster than the wind. I've been in such a yacht, myself.

But I wasn't sailing it . I was hunched over the side of the boat, feeling distinctly queasy and looking green, and wishing the damn thing would stop going up and down...

I'm not the yachtsman you're looking for...

 

[Piemaster]

As above its all about vectors. Think about the wind blowing on the sail, the pressure on one side is greater than the other. Pressure=force /area (of the sail). So what could be a relatively small pressure differential on the sail area can produce a large force which is converted into forward motion by the shape of the hull.
BTW land and ice yachts can achieve 5-6 times the wind speed.