If I stood on a sailing ship and had a very strong pair of lungs (possibly motorized) then if I blew the sail would I be able to move the ship forwards?
The answer, despite what several responders think, is unequivocally YES . However, you will get more efficient propulsion just by blowing directly backwards. All a sail does is deflect wind, thus changing the direction of momentum (with the help of your keel or centerboard).
The reason this works is that when you blow, you are expelling air more or less purely in the stern direction, and equal/opposite reaction rules apply. However, when you inhale, you draw air in from more or less all directions, and there is next to no effect on the boat's momentum.
Anyone who continues to doubt this is referred to swamp air-propellor-powered boats, as in agentp's comment. airboat
I've done a lot of sailing (won a couple national championships) and the answer is yes. But you wouldn't want the fan blowing forward from the stern. Instead you would want it blowing towards the sails from the side. Even though the fan would want to pull the boat sideways, sailboats are equipped with centerboards or keels which function like a wing underwater. Since water is much more dense then air, you'll have a hell of a time pulling the boat sideways. Instead, the air will wrap around the sail thus creating a high pressure zone on the "windward" side and a low pressure zone on the "leeward" side and propelling the boat forwards. You'll also get a bit of drive from "deflecting" air backwards (equal and opposite reaction).
Blowing you push the boat backwards. If the sail caught all your wind then it would net to zero.
You would be better off just drop the sail. Face forward to suck and face rear to blow.
Or leave the sail up and rock the boat side to side. On a small boat you can move slow but definitely move.
Bit of lateral (ha!) thinking - set the sail to an angle, stand to one side of it and blow across the beam of the boat - perpendicular to the axis and intended direction of motion.
That way, the reaction of the person blowing is perpendicular to the desired direction of travel and is counteracted by the keel. Forward motion is provided by the forward component of the reaction of the sail deflecting a crosswind, with the keel counteracting other component forces.
Well After some googling I found this!
It does indeed work possibly because the sail deflects the wind backward meaning that you go forward with conservation of momentum.
I thought it wouldn't work but hadn't figured that the wind would bounce off the sail
From my interpretation of the OP the airboats fan is round the wrong way. It's blowing the air out the back which moves the boat forward. You would need to turn the fan around 180 degrees and attach a sail in the middle of the boat for it to be an accurate representation of the OP. Or make a frame and attach a sail behind the air boat.
If you did turn the fan around and attach a sail in the middle from a basic force point of view the air would hit the fan and be transferred down the mast, back through the hull and back up through the fans frame which would cancel itself out.
But when the sails introduced the force dynamics are changed as pointed out by Phil. And if the sail does create twice the reaction force then the boat would move forward.