Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Leigh Knight on 05/03/2010 22:30:03

Title: How does a sailing boat sail into the wind?
Post by: Leigh Knight on 05/03/2010 22:30:03

Leigh Knight  asked the Naked Scientists:
   
How does a sailing boat sail into the wind?

What do you think?

Title: How does a sailing boat sail into the wind?
Post by: graham.d on 06/03/2010 09:26:17

Well it can't sail directly into wind but it can sail at an angle of (say) 45 degrees to the oncoming wind. This is because the sail, when "filled" with the wind has an aerofoil shape, like an aeroplane wing. Like a wing, the sail has an angle of attack to the wind and the wind blows along the length of the sail (at this angle of sailing). There is some drag from the sail, pushing the boat back, but there is a significant force perpendicular to the sail (analogous to lift on a wing). By adjusting the angle of the sail with respect to the boat (and the wind) the net force can be made to be pushing at the boat in a direction just forward of being side on. This is where the keel comes in to play. Beneath the yacht there is a large plate of heavy material fixed to the bottom of the boat and extending deep into the water. This serves as a ballast to keep the boat as upright as possible and, importently, constrains the movement of the yacht to move forward (or back), resisting sideways movement. So even though there is a large side force and only a relatively small forward component, the keel tends to allow only the forward motion.

A better description with pictures is on this site:

http://web.mit.edu/2.972/www/reports/sail_boat/sail_boat.html

Or just Google "sailing into the wind".

Title: How does a sailing boat sail into the wind?
Post by: graham.d on 06/03/2010 13:42:13

You must have a very special yacht, Dave. +/-40 degrees is about the best I've ever had :-)

Title: How does a sailing boat sail into the wind?
Post by: syhprum on 06/03/2010 17:14:19

One obvious development would seem to be the use of a vertical or horizontal wind turbine driving an underwater propeller.
Many prototypes have been built over the last 150 years with varying degrees of success but allthough they have the advantage that tacking is not required gyroscopic effects are a big problem and none have become common.

Title: How does a sailing boat sail into the wind?
Post by: LeeE on 06/03/2010 18:14:35

I think you can sail closer to the wind with a high aspect ratio rigid wing sail, but only because they're more efficient i.e. produce less drag.

Title: How does a sailing boat sail into the wind?
Post by: Geezer on 06/03/2010 23:03:15

Quote from: syhprum on 06/03/2010 17:14:19

One obvious development would seem to be the use of a vertical or horizontal wind turbine driving an underwater propeller.
Many prototypes have been built over the last 150 years with varying degrees of success but allthough they have the advantage that tacking is not required gyroscopic effects are a big problem and none have become common.

Syhprum,

I found myself asking the same question. I wonder if a wind turbine powered boat would get from point A to point B more quickly than a sailboat (assuming it can be made reasonably efficient). Would it be barred from competing in the America's Cup?