Originally Posted by pigslo
Any forward movement is from the fan or ballon blowing against the air around the boat ( the same as an everglades boat moves forward). Any low presure (that is what moves a sailboat upwind) created around a sail by an object on the boat and therefore moving with the boat will be subject to an IDENTICLE counteracting low presure and there by canceling it out. That is just physics. File your drawings of this contraption at the patent office along with a perpetual motion machine.
Any explanation that does not include the force of the water on the keel is incomplete for explaining tacking into the wind.
This page provides a good explanation of it:
The physics of sailing.
Note especially the section that begins:
"Sailing close to the wind
uses the shape of the sails to generate lift. To flow around the sails, the wind has to deviate in direction, as shown by the arrows for initial velocity vi and final velocity vf, which are given with respect to the boat. The change of velocity dv is in the direction shown. The acceleration aa of the air is dv/dt, so the force that sails exert on the air is in the same direction."
As you can see from the images accompanying this section, the lift produced only results in a force on the boat that has a rearward
component, not forward. More precisely, the lift acts to move the boat sideways
It is only the action of the water on the keel that provides a forwards
force component that allows the boat to move forwards.
Here's another way to look at the scenario. Suppose you had your giant fan on a raft with no sail or keel. This raft is connected to the sailboat by a rope. Suppose this rope initially is slack. You turn on the fan directed towards the sailboat. The wind produced by the fan allows the sailboat to move forwards by tacking into the wind. The raft and fan also move forwards separately due to the momentum thrust of the fan. The instant the rope is about to become taut you turn off the fan. When the rope tightens, the sailboat will get a higher speed because of the forward momentum of the raft. The raft will also be pulled backward but it won't affect the forward speed of the boat as long as it does not contact the boat. You make the rope long enough so this doesn't happen. Once the rope is slack again. You turn on the fan again. The process repeats.
You see the result will be that the sailboat will wind up having a speed due to both
the wind produced by the fan acting on the sails and from the momentum thrust produced by the fan.