MadSci Network: Physics

Re: Why does an ice boat go faster tan the wind that propells it?

Date: Sat Nov 6 22:31:27 1999
Posted By: Graeme Bushell, Faculty, Chemical Engineering, The University of New South Wales
Area of science: Physics
ID: 939661832.Ph

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.

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