A boat's sail harnesses the power of the wind but also creates a wake of turbulent air downwind of the sail for a distance of 3 to 5 mast heights. The size and shape of this wind shadow varies with the selected head-sail and the point of sail but of course it is always downwind of the sail.

You may have noticed another effect as a boat sails across your bow. This is a localized effect known as a "lift" because it allows you to steer a little closer to the prevailing wind direction than you were. It last only a few seconds and if you do not fall away to your original heading before the prevailing wind direction is restored (fills in) then your sails will stall and your boat lose power.

ACC sail 4 wind shadow

Wind shadow: ACC head-sail #4

If you have a fully-featured version of VSK5 then you can access the race editor where you are able to view and study this wind shadow effect at different points of sail.

  1. Before entering the race editor, from the main menu visit the settings page and view the inputs configuration pages. On page 1/3 note the key binding for ViewWindShadow - the default binding is the 8 key but you may have remapped it or even unbound it altogether since it serves no useful function in normal race modes.
  2. Now return to the main menu; select editors then either edit a race or create a new race to open the race editor.
  3. Click on the params button in the top right just below a compass rose and verify that the boat of interest has been selected.
  4. If you are creating a new race to do this then you must first place a minimum of
    • a start line
    • a finish line
    • spawn points for 4 boats
  5. Only then can you click the play! link at the bottom left of the editor window. Your boat will appear on the water.
  6. Now press the 8 key or whatever key you bound ViewWindShadow to.
Clear air 
Having understood the size, shape, orientation and wind strength within a boat's wind shadow you should better appreciate the benefit of seeking clear air in which to sail your boat for maximum boat speed. If you can see that you are unable to pass ahead of another boat then it can be better strategy to tack sooner so as to enjoy clear air slightly ahead of but to the leeward side of the other boat.
Combining this knowledge with smooth (but not slow) rudder movements and initially heading a few degrees below close hauled until your boat speed has reached it's maximum (see polar diagrams) you may ill surprise other skippers and your self by overtaking the same boat at your next crossing opportunity.