Submitted 2 weeks ago by Karmanopoly@lemmy.world to [deleted]
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.
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.
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.
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.
And both work, the sail is just a very inefficient trust reverser so you hardly get any thrust from it. Just remove the sail and use the fan directly.
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.
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.
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.
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 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), rather than redirecting it backward.
But… A fan on a sailboat DOES work. Just ask the MythBusters (Episode 165).
Fan on a boat does work, just not efficiently enough to overcome anything but static air. Put a jet thruster on it and it would work really well.
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.
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 “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.
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.
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 boat it’s mounted to; that’s not how physics works.
That’s pretty much exactly how a propeller works …
So you have never seen an airboat?
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.
remon@ani.social 2 weeks ago
A fan on a sailboat totally works you just have to aim it airflow behind the boat and not at the sail.
I_Fart_Glitter@lemmy.world 2 weeks ago
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.
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