Sails work by “catching the wind” only when the boat is sailing directly downwind. The rest of the time, a sail is much like an airplane wing standing on end, and it works the same way:

1. When the wind blows on a correctly trimmed sail the air flow separates and passes on either side of the sail.
   

2. Because of the curve of the sail, the air flowing on the side facing away from the wind (known as “leeward” side) must travel a greater distance, so it speeds up. This creates an area of low-pressure on the back of the sail which has a suction effect.
   

3. On the side of the sail facing the wind” (known as “windward” side) an area of high-pressure is created which generates a push force.
   

4. This difference in air pressure on either side of the sail pulls the sail into the wind as the sail “lifts,” or moves toward the lower-pressure zone, pulling the boat along with it. This is exactly the same force that lifts an aircraft wing.
   

5. The force across the sail acts in two directions: a side force pushing the boat sideways; and a forward thrust.  The shape of the hulls helps resist the sideways force, and the boat moves ahead. 
   

6. Sailing boats typically have keels sticking down from the main hull to help resist the sideways force.   The waka has only small planks as keels and rounded hulls so it tends to slip sideways more easily than a conventional boat.  In the early days we tried “dagger boards” which (like a centerboard on sailing dinghy) could be lowered down below the hull line.   (Dagger boards lift up and down vertically while centerboards pivot).  But they did not work well and we now just accept the side-slip as a fact-of-life on Te Aurere and take it into account when working out how far we have traveled and where we are.

Dipping the steering paddle in the water when Te Aurere is under sail has the same effect as a centre-board - it adds to the resistance of the hulls to the sideways force of the wind and makes the waka move to leeward - away from the direction from which the wind is coming.  Keeping a straight course line is a matter of dipping the paddle in and out of the water to balance out the forces and optimise the forward thrust.  Using a jib at the front of the waka also helps as it moves the centre of effort of the sails forward and reduces the turning effect (in boating parlance, the jib reduces the “weather helm”).