How Ships Stop If They Don’t Have Brakes?

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Have you ever wondered why ship accidents take place and in such situations why can’t ships just hit the brakes to stop as you would do with a car? Well, of course, because ships don’t have brakes! But why?

Well, the first thing is that ship moves on water! How do you even brake in water? But then you must be aware that even airplanes have something similar to brakes, and they work to slow down a plane in the air. It is not that simple. Let’s understand how ships stop in the water.

Every kind of brakes out there works because of friction. When you hit the brakes in a car, it stops because brake pads tightly grip wheels’ rotors and cause a lot of friction, restricting them from moving at all, and if wheels stop, they cause friction with the road, and more friction means less speed until the car eventually stops. Now, you can easily tell that something like this won’t possibly work in water.

Motorised ships are mostly driven either by the motion of propellers or by a jet stream of water shot from the nozzle. Simply stopping those won’t produce nearly enough friction with water to effectively stop a ship. Ships still have brakes, though, they just need a lot more to properly work.

To produce friction, the first thing a ship needs to do is to reverse her thrust. This is much more efficient with jet engines with controlled nozzles and less so with propellers. If we are talking about a big modern ship, you can almost be certain she uses propellers. Reversing the rotation of propellers will start to slow the ship down, but the heavier the ship, the harder it is to stop. They are slow to muster speed, but inertia doesn’t let them decelerate fast enough.

It seems the water itself is the main reason why it’s impossible to use brakes on a ship, but in reality, the clever use of water’s properties is the key to stopping it. The friction with water depends on three main factors: the surface of the ship, her velocity, and the viscosity of water.

Now, you can’t do much with the water’s viscosity though, it will stay same, no matter what. The ship’s velocity, though, is another matter. Imagine a ship with a velocity so huge that it almost reaches infinity, Funny thing is that she won’t go anywhere fast. You must be wondering why so? Well, the conflict here is that the bigger the velocity, the more power with which water will drag the ship back because the friction and the water resistance will also almost reach infinity, meaning it’s not really useful in stopping the ship.

There is one thing you can still control- the wetted area of the ship. Some ships have so called stabilizer fins, submerging them underwater will help the ship to slow down. These are actually the closest ships get to having brakes because there is one kind of brakes that operates similarly- air brakes.

Stabilizer fins

Most jet planes have a special kind of spoilers that allow them to slow the plane down significantly by increasing the drag of air around. Stabilizer fins don’t stand against the mass of water coming into them, but they expand the wetted surface and slow ship down this way. Imagine a spoiler coming from the bottom of the ship – it will meet an incredible pressure from the water. So, a mass of water pressing on the brake would be so huge there is simply no way it won’t break off eventually. Even if we’re able to make an unbreakable water-brake spoiler, we’ll meet another problem. Water pressing on the spoiler will simply force a ship down, maybe even submerging her nose underwater. Also, the maintenance of these things would be too expensive, and you would need a whole team of scuba-divers for that at all times.

A much better decision is to start turning the ship, while propellers provide a reverse thrust, the ship’s inertia is still moving it forward. All ships are made in such a way that they cut water in front of them, and their streamlined design lowers the friction. When the ship is turning, she reduces this advantage and the speed goes down.

So, in general, ships don’t even need emergency brakes. There is usually enough space to make a maneuver even without reversing thrust, and to stay on the spot ship deploy anchors. Now, you might think that anchor is the brake stop we are looking for! But this looks good only in action movies. Should an anchor get a grip on the seabed, it won’t move an inch, and this would result in a huge dunk for the ship. This can quickly turn into a full blown disaster.

The only way to implement that strategy is to combine it with all previous steps- reverse engines, start a sharp turn, and when the speed goes down, the anchor is released on the side of the ship’s turning. It will only work if the cable of the anchor or the chain is not stressed too much, and is more suitable for smaller vessels in general. If everything is done right, the ship will spin around the point of the anchor drop, but won’t go any further.

Still, this way of stopping a ship is awfully extreme, and if a mistake is made it can do more harm than good. Most of the time, ships will use a cycling method to lose all the speed. Just laying rudder as far as you can sounds better than risking your ship sinking because of an anchor.

So, overall ships can’t stop in place, because water won’t let them do that, and ship will start drifting with the water.

Here is a detailed video explaining this interesting subject:

So now you know.

Source.

Rgds, Sailor

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