Airplanes deploy revers thrusters when landing which are basically just shaped metal shields that move into place behind the engines to provide reverse thrust and slow the plane.
But adding a fan on a sailboat is seen as stupid and doesn’t work.
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.
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.
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 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
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.
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?
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
This guy screws
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.”
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.
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.
Yeah, so inefficient as to be completely ineffective. In other words, “it doesn’t work”.
No, it’s not completely ineffective, just very ineffective.
If you blow enough air at a sufficient sturdy sail you can absolutely generate trust from the airflow the sail deflects.
But you’ll be more efficient if you remove the sail and just point the fan in the other direction.
You’d be even more efficient if you put the fan under the boat and push the water instead of the air.
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.
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.
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.
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.
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.
Size matters. Airplane engines create a huge trust. A simple house fan does almost nothing.
A jet engine can blow a car end over end. A house fan just barely causes some paper to move.
No fan of any size will ever push the sail on the boat it’s mounted to; that’s not how physics works.
You might want to check your physics because that’s pretty much exactly how a propeller works …
Show me one airboat with a sail on it, you dunce.
The thing about air boats is that they don’t need sails.
BECAUSE BLOWING A SAIL WITH A FAN DOESN’T WORK.
But it does. It just works even better without the sail.
It can be, as long as the sail is set up in such a way that incoming air is deflected backwards to some degree rather than at a right angle to the fan (such as with a curved sail). Consider a still boat in still air (thus with no net acceleration in any direction to start). You push some air forwards towards the sail, then the sail deflects that stream of air outwards and slightly backwards. This means the combined fan-sail system (the ship taken as a whole) is taking in initially still air, and that air ends up with a net momentum backwards, so there must be push forwards. This is very inefficient compared to doing away with the sail and pointing the fan the other way, so theres no reason to do it in practice, but it doesn’t violate conservation of momentum or anything because the ship isn’t a closed system.
So you have never seen an airboat?
Have you ever seen an airboat with a sail?
