Blogging about Sails

Now there's a thing I know about sails, that they can sail against the wind.
Physics test today got me stumped.
When the apple or whatever fruit hit Newton on his noggin I hope he cried. Very loudly.

THE UTTER RELEVATION OF SAILS
(How boats go upwind, from the University of New South Wales)



This is the part I understand. Wind flows around sail smoothly and deviates in direction, so that the sails exert a force on the wind, with an acceleration so and so and a reaction force of wind on sails in the opposite direction.



This is the part that I don't. No wait, I would have, if I hadn't left the bloody keel out! The University of New South Wales will take on from here...

Why doesn't the boat drift sideways? Well it does a little, but when it does, the keel, a large nearly flat area under the boat, has to push a lot of water sideways. The water resists this, and exerts the sideways force F(k) on the keel.

This cancels the sideways component of Fw. As to the forwards component: it accelerates the boat until the drag force Fd holding it back is big enough so that

Fw = -{Fk + Fd}.

So a boat can sail close to the wind: typically 45 degrees, although many high performance boats go closer than that.

A BOAT SAILING UPWIND CAN GO FASTER THAN ONE SAILING DOWNWIND!!!!!!!

Any idiot mathematics can make the physical laws of the universe, but only a sentient being can make them go mental.

HERE IS THE SITE. CLICK IT.