|MadSci Network: Physics|
The speed of an ice boat or indeed any vehicle is determined by the balance between the forces making it go faster (thrust) and those trying to slow it down (drag). For a sailing vessel, energy is harnessed by the sail as it forces the wind to change velocity and thrust is delivered through the mast. Now, what determines the magnitude of this thrust force? If the boat is travelling downwind, it can never travel faster than the wind because as the boat speeds up, the relative velocity of the wind and the boat gets less and less and so the thrust force also gets less and less. If however the boat travels at 90 degrees to the wind, there will always be a difference in velocity between the wind and the sail no matter how fast the boat is going. So there can always be a thrust force (depending on the trim of the sail). The only thing that limits the speed of the boat is the drag force against the ice (or water) and the air resistance of the boat, and the possibility of the boat tipping over due to the lateral component of the wind force on the mast being too high.
Try the links in the MadSci Library for more information on Physics.