"The pessimist complains about the wind;
      the optimist expects it to change;
          the realist adjusts the sails."
                     -William Arthur Ward

The sails of a boat moving perpendicular to the wind will feel not feel a
decreasing wind force as the boat accelerates (unlike downwind sailing). When
the forces acting on the boat come to equilibrium, the boats speed is formed from
the counteraction between the now constant force of the wind and the drag
force created by the hull's motion through water. If we can find a way to
significantly reduce the drag force, then we can really get the boat moving.

Wait a minute. Why isn’t the boat pushed sideways by the wind?

 It is. However, the keel or centerboard counteracts the sideways push of the
wind force, and helps keep the general motion of the boat in a forward direction.

What is the keel really doing?

 The keel has two jobs, one is to keep the direction of motion toward the direction
 that the boat is pointed, and the other is to keep the boat upright. To accomplish
 both of these jobs the most proficient keels are thin from front to back and long;
this makes sailing in shallow waters dangerous, but helps to reduce several
 resistance factors.