# Adding an additional fan to a fan powered sail according to Newton's 3rd Law?

So we've got the classic physics problem with the fan powered sail. If we have a boat with a sail, and we attach a fan onto it, facing the sail, the boat will not move due to the balance of forces:

The fan pushes air forward and the air pushes the fan backward, thus pushing the boat backward. But air also pushes forward on the sail as the sail pushes backward on the air, so since the sail is ALSO attached to the boat, the boat is being pushed both forward and backward by air. Thus, no motion. (does this explanation seems accurate?)

But I was wondering. What happens if you add a second fan? Does that mean you have a greater force pushing backward on the boat compared to the force pushing forward on the boat? So adding a second fan causes the boat to move backward? Or does it not matter what strength the fan is? I think I'm getting confused because something about my above explanation is off, and I'm comparing the wrong forces.

• However, a curved sail changes it all. Commented Jul 25, 2018 at 21:14
• Why do you think adding a fan would change the situation? Commented Jul 27, 2018 at 20:42