船舶稳性如何减缓横摇

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I'm sure you've all seen the videos of cruise ships rolling around.
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A lot of the time this is down to a failure in one of their systems - the stabilizers.
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So how do these stabilizers actually work?
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Let's consider a standard cruise ship in calm water.
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She's gonna sit happily upright.
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Her stability is determined by her center of gravity and her center of buoyancy.
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As long as they're all lined up in the correct place,
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she's going to remain upright and stable.
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If you then introduced some waves to the sea, you're gonna start to interact a bit.
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Smaller waves are unlikely to have much of an impact on a large ship,
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but as the wave size increases, the wavelength increases,
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and you're gonna start to feel the ship interacting.
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She's gonna roll from side to side,
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Following the rhythm of the wave itself,
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and this is purely down to the fact that as the water level on one side of the ship rises,
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the ship generates more buoyancy on that side and this results in a force that makes her lean over.
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Most of the time a ship will happily just continue riding the waves,
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rolling gently from side to side.
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The only exception of course is "synchronous rolling" and that's when the frequency of the wave
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Matches the natural frequency of the ship's roll.
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This results in rapidly increasing angles of roll until the ship actually capsizes.
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Apart from that, gentle rolling isn't an issue on those ships,
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but on a cruise ship you expect to be able to have a flute of champagne
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stay put on a table when you put it down.
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You don't want the ship to be rolling underneath you, spilling your drink everywhere.
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Instead these ships use stabilizers to minimize that rolling.
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We want the ship to remain upright while the waves run underneath her.
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The goal of a stabilizer is to counter the force
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generated by a wave.
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Unfortunately, that force is in constant
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as the crest of the wave hits one side of the ship,
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the tipping force is at its maximum.
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As the wave crest moves through the center line,
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the force becomes pretty much zero and then as it moves to the other side,
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again the force becomes a maximum, but in the other direction.
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The stabilizer needs to counter all of that.
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It needs to be able to change the amount of force
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it generates, depending on the exact location of the wave,
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which, of course is constantly changing.
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To do that the industry has come up with a few types of stabilizer.
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I'm going to split them into two categories:
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Active and Passive.
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A good example of passive stabilizers are Bilge keels.
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These are protrusions on the bilges of a hull and all they effectively do is dampen any roll.
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They generate turbulence as they move up and down.
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To have any effect, they need to apply a force.
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As they are passive, they can only generate that force when the ship is rolling anyway.
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Both keels are OK at dampening a roll and
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they're very cheap to install so they are a popular design feature on many ships.
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Unfortunately, they don't do enough to produce the required counter-effect that we need on a cruise ship.
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Another method of stabilization is passive anti-roll tanks.
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These are tanks installed in the extremes of the ships breadth.
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The key to these is in the design of the crossover pipes.
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Normally with the ship upright,
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they're going to have the same amount of fluid on each side.
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As the ship leans over one way,
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the fluid in the low side is going to increase.
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If you can control the flow of fluid enough,
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You can have the maximum amount of weight in that side as the wave moves across the ship.
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Theoretically, if you keep the fluid there while the wave crest is there,
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it can help to counter the roll.
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Of course, you need to release it to let it flow to the other side at just the right time
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so that it is ready to counter the next wave crest as that approaches.
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You can add in more control onto these tanks making them semi-active;
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Or, you can even add in a set of pumps controlled by a gyroscopic stabilization system that can pump water across,
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Whenever it's needed.
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This would be a fully active system and these sort of stabilizers are good on ships,
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They would work OK on cruise ships as they can actively counter the force of the roll as it's happening.
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Unfortunately, it does take time to pump water across so you still haven't achieved
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instant stabilization.
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For that, We're left with the fin stabilizer.
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This is actually the most common form of stabilization that I've encountered on cruise ships.
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The system basically consists of a hydraulically operated wing that extends out underwater.
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You'll notice it looks similar to the Bilge keels that we discussed earlier.
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The main difference is that with these active fins you can change the angle of attack.
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Looking at the side profile of a fin, you'll see what happens when it rotates:
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Turn it one way and the water flow will produce a huge force in one direction.
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Turn it the other way and you've instantly swapped the direction of that force.
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With the correct gyroscopic control,
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You just need to rotate the fin in such a way that it counters the force
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generated by the wave that is trying to roll the ship.
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Indeed, you can even use it to counter a roll generated by other forces like wind or even internal movement of people or cargo.
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The main disadvantage of these fins, though, is that they're only going to work while the ship is actually moving.
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They need a water flow across them,
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below a certain speed (often about six knots),
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that water flow is not actually enough to generate a sufficient righting force.
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Luckily, most cruise ships have a minimum speed well above this anyway,
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so it's not actually an issue when they're on passage.
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This is why cargo ships with their generally slower speeds
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don't typically install active fin stabilization.
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Saying that, there are container vessels that do,
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but only ones where their typical cruising speeds are high enough to get the maximum benefit.
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So next time you're sipping your champagne in a high sea,
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Just spare a thought to the technology that's going into keeping your glass from sliding off the table
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Hopefully you've enjoyed today's video.
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Be sure to subscribe right here on the channel.
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Until next time, thank you for watching and goodbye.