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.
this post was submitted on 14 Oct 2026
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For those unfamiliar with swamp boats. They are designed with no under water propeller to get stuck on/murder swamp stuff. They can go pretty fast.
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.
This is the correct answer.
In my mind there is no difference in these setups
Imagine standing on the boat and pushing on the sail.
Which is the same as the fan pushing on the sail.
The force is all contained in the boat.
But the air is not contained on the boat, it is pushed away from the boat after hitting the sail. Since that air has mass it will produce and equal force pushing on the sail, aka thrust.
A jet engine with deployed reverses is just a fan mounted on the plane pushing against the plane but it works just fine.
A sail is just a very inefficient deflector of air, but the principle is the same.
For what it's worth, a thrust reverser is also a very inefficient deflector of air. It's better than nothing, and jet engines produce a lot of thrust, so even inefficiently reversed it still provides a significant amount of reverse thrust which happens to be quite useful for slowing down an aircraft when there's minimal weight-on-wheels or on slippery runways, but it's still only a tiny fraction of what the engines produce in normal operation for forward flight.
A fan blasting air at a sail is also quite inefficient, but as other comments note, it does technically work. It just works so poorly for the amount of energy you're putting into the fan that you'd probably be better off rowing or just using the sail properly.
What about this?
This guy screws
You need a magnet thrust reverser
Put another magnet on the front of the truck and the magnets reverse thrust one another for maximum results
And both work, the sail is just very inefficient as a trust reverser. Just remove the sail and use the fan directly. You can rotate the entire fan if you want to reverse.
They tested the fan on a sailboat on MythBusters, and were shocked that it actually worked. https://mythresults.com/blow-your-own-sail
It didn't work well, but it did work.
"The idea works if you just remove this half and reverse the other half."
Well yeah, because the sail is not needed at all, you just need the fan.
A modern commercial jet engine is just a very powerful fan anyway. You move air backward, you move forwards. Works just as well on boat as on a plane.
The sail would just be a very inefficient reverser.
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I prefer Lemmy to Reddit but the userbase here is way more insufferable.
It's a venn diagram of people banned from reddit and people who think they're too good for reddit, what do you expect.
Haha, good point.
Just wait, what does that make us?
Insufferable.
Checks out.
It works by redirecting the flow of outgoing gas forwards.
This may shock you to learn, but typing your question into a search engine and reading any Wikipedia articles in the results is something you can do yourself; it's much faster and more accurate than asking random strangers on the internet.
But it isn't as fun
You know that everyone knows that Wikipedia and search engines exist, right? People don't post questions on forums because information doesn't exist elsewhere. It's because of the social factor.
Besides, Wikipedia isn't perfect, and search engines have gone to shit
Hey dipshit, I think you’re lost, the community is called ‘nostupidquestions’ not ‘I want to gargle on CIA cock’
How is the sail in this pic any different?
It is redirecting the flow of air from the fan
The reverse thrusters redirect the air coming out in the opposite direction, so it is like turning the fan in the opposite direction.
A sail catches the air to pull the boat along and some of it bleeds off, but it does not direct any air backwards to propel the ship forward.
I mean sure, maybe there is no difference. But then, like the plane you're better off not using the sail and just pointing the fan the opposite way so it pushes the boat fwd.
The engine on a plane pushes the airplane forward. The reversers redirect a portion of the thrust forward slowing the aircraft down. The best thrust reversers get MAYBE 50% efficiency. On the boat there is no reason to throw out half the fans thrust.
That's combined with the fact that the sail isn't an effective thrust reverser and that the worst case efficiency is negative. If you don't redirect enough air backwards you won't even overcome the fans thrust let alone produce a net reverse thrust.
Can you push your car while sitting inside it? That's essentially what your boat picture depicts. Could you turn around the fan and push the other way? Sure, then you end up with an airboat. Or better yet you turn the fan around and stick it in the water.
Although this could just be another of your physics trolling questions, where you just want to argue in the comments. Apologies if that's a different user and I've mixed you up.
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.
That is true for basic jet engine, but most modern high-bypass engines are basically big fans powered by a jet engine. The air moved by the outer blades provides up to 10x the thrust of the hot air exiting the core.
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.
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.
But... A fan on a sailboat DOES work. Just ask the MythBusters (Episode 165).
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.
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.
It depends on the actual situation or maneuver during sailing. It's a mix between the wing effect and the ancient principle "wind from behind is pushing the boat forward".
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.
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.
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