On a stationary sail boat, air is blown from a fan attached to the boat. The boat

In this case air blown by the fan will try to push the sail boat in a certain direction. But at the same time due to Newton's third law of motion the fan will experience a force due to the air blown in a direction opposite to the direction in which the sail boat will experience the force due to the blown air. So, as a whole the system will be at rest and it will not move.