Airplanes deploy revers thrusters when landing which are basically just shaped metal shields that move into place behind the engines to provide reverse thrust and slow the plane.
But adding a fan on a sailboat is seen as stupid and doesn’t work.
Airplanes deploy revers thrusters when landing which are basically just shaped metal shields that move into place behind the engines to provide reverse thrust and slow the plane.
But adding a fan on a sailboat is seen as stupid and doesn’t work.
Newton’s third law works well in explaining this. Think of where the momentum is going in each case: in the jet engine, normally the exhaust is directed rearwards, which creates an equal and opposite force forwards. When the reversers are deployed, the exhaust is directed partially forward, which creates an opposite force rearward, thus slowing the plane.
In a sailboat with a fan pointed at the sail, the fan initially pushes air forward, which would push the boat backwards. But the air immediately hits the sail and either stops or is directed out the sides, which creates a forward force. The forward and reverse forces roughly cancel each other, so there’s little or no net motion of the boat. This assumes the fan and sail are in the configuration shown in your picture in another comment. It is possible to arrange the fan and sail in such a way that you actually get forward motion, but at that point it’s just an air boat with more steps.