No Stupid Questions

A fan on a sailboat totally works. You just have to aim the airflow behind the boat and not at the sail. Like those swamp boats in Florida.

People have already covered that the jet doesn't just block air but redirects it (not unlike a sailboat moving across the wind), and really that's most of it. One differing factor that I don't see mentioned in here that I think is worth mentioning is that unlike the fan, the jet engine doesn't merely move air, it heats it enough for pressure to increase before it leaves the engine. So in terms of energy, that added heat has nowhere to go but wherever it's directed. That added heat energy lets the engine's "push" be greater than it's "pull", so when the "push" is redirected backwards it can overpower the "pull"

Also sorry about some of the commenters being dicks about answering your question, it's a good question and people need to remember what this community is for.

The reverse thruster is force originating from the plane pushing against the air.

The fan on the sailboat is force originating from the boat pushing against the boat.

The fan is pushing the boat backwards. The sail is pushing the boat forwards, but with slightly less efficiency since the air is losing speed between the fan and the sail. So you'd go slightly backwards.

So.. it would work, just so poorly there's no reason to bother.

A sail absorbs the wind’s energy, transferring it to the boat (which is the same place the fan borrowed it from). There’s no net airflow backward, as there is with reverse thrusters.

If you replaced the thrust reverser on the aircraft with a cloth sail, much the same thing would happen.

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.

But... A fan on a sailboat DOES work. Just ask the MythBusters (Episode 165).

A fan on a boat would work. There are these fan boats people ride through the Everglades. There are propeller sea planes. The difference to a sailboat is the profile of the vessel under water. A sea plane has two or three smaller areas that are submerged. The everglade ones are just breaking the surface of the water and are very flat so they can glide over water and plants. A sailboat has a big, often deep profile under water, the whole length of the boat, possibly a keel underneath. And that's moving through water, which is thicker than air. So you need possibly more than one propeller and the engine may need to give you more horsepower to move the boat through water with the force of the air above deck. That's inefficient so boats tend to move their propellers under water.

A few container ships have a spinnaker sail they can deploy. If the wind is favourable they can reduce the engine output and save diesel. Just in case you're wondering about the reverse situation.

A sail boat isn't really pushed by the wind. The sail acts more like a wing and the difference in air pressure going around it is what actually moves the boat.

Adding a fan to a sailboat might help you stop said sailboat. Adding a fan to a swamp boat is not stupid and does work.

https://www.airliners.net/forum/viewtopic.php?t=738011

I tried to find a figure for what percentage of the normal operating thrust a thrust reverser configuration can provide and couldn't readily find that info. However, according to that forum, thrust reversers only really reduce stopping distance by 10% or so and are deployed primarily to help maintain control in case there is uneven braking due to poor runway conditions.

Also, multiple sources say thrust reversers only work effectively at high speeds, and lose effectiveness at low speeds. I didn't find an analysis on why that is the case, but it has something to do with the flow rate of air through a jey turbine being much lower at low speeds.

A sailboat will always be at low speeds relative to the wind. A thrust reverser configuration might provide some usable thrust, but you would be much better off just using the fan directly for propulsion and skip the sail altogether. Even that is very inefficient compared to propelling against water.

Fan on a boat does work, just not efficiently enough to overcome anything but static air if you're talking about facing the fan into a sail and not something like an airboat. Put a jet thruster on it and it would work really well. But an airboat with opposing fans would also work.

Not exactly an answer, but related. Sailing Faster Than the Wind

Your confusion is justified. It shouldn’t work, all things equal.

I think it’s because the intake of a plane is more diffuse; it’s pulling air in from different directions. Meanwhile, the “reverse exhaust” is ejected from a smaller nozzle at higher temperature; it has a higher velocity, therefore more momentum to “push” the plane than the intake “pulling” it.

For the record, there’s a similar scheme for sailboats. Hook a fan in the air to a propeller in the water, and the wind imparts less momentum than the prop “pushing” on the water. Such a “rotary sailboat” can sail straight into the wind.

ITT: people who would rather argue than read.