From: nospam@de-ster.demon.nl   
      
   Tom Roberts  wrote:   
   [reposting, because of black cyberhole in the way, enlarged a bit]   
      
   > On 8/15/17 8/15/17   1:14 PM, Richard D. Saam wrote:   
   > > Basic momentum transfer indicates faster than wind is not possible   
   > > assuming uniform wind gradient from surface to sail or propeller height.   
   >   
   > It is quite common for iceboats and modern catamarans to sail faster than   
   > the wind, sometimes much faster (3X - 5X). They do this on a reach (i.e.   
   > traveling cross-wind), not a run (downwind), because then the sails can   
   > create aerodynamic lift to propel the boat forward.   
      
   In fact ice sailers will also cruise when going downwind,   
   because that is faster than just going before the wind.   
   The extra speed more than makes up for the extra travel.   
      
   > For most modern sailboats a close reach is the fastest point of sailing,   
   > unless waves interfere.   
      
   Just the same for power generating windmills.   
   Tip speed over wind speed is a measure of design quality,   
   and it should be much larger than one,   
   (typically about 10)   
   Most of the power comes from near the tips,   
   precisely because they move very fast.   
   (power is force times velocity)   
   You can hear it for yourself, because the fast tips   
   are responsible for the disagreeable windmill noise,   
      
   Jan   
      
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