Why can some ships stop immediately?

[BlueBell]

"

"Why can some ships stop immediately?"​

Usually because they run aground, sadly.

Or, on a happier note, because they are jet boats with huge reverse buckets.

 

[jehardiman]

Why can some ships stop immediately?

The simple answer is that no ship, of any propulsion method, can stop "immediately" at any speed.. It is physically impossible and to even consider it means the person failed to grasp the basic concept of the 2nd Law of Motion....F = ma. Since a = del v/ del t and "immediately" means del t =0, that implies infinite deceleration and therefor infinite force regardless of velocity or mass. Infinite force just ain't gonna happen.

And slowing down too quickly, in any scenario, is fraught with issues. The 2nd law is trying to kill you. For example, let us say you are in powerboat going 19 knots and you stop in 1 second. That means that "a" is now 9.8 m/sec^2 ...i.e. you are being thrown towards the forward bulkhead at 1 G. If this is a high speed ferry, you probably have a mass casualty incident. Certain military vessels, when above certain speeds, require everyone to be strapped in.

So in reality "immediately" is not that fast... the maximum most vessels should slow down is somewhere about 4 knots/sec. And then we also have to consider what physically has to happen aboard and/or in engineering to make an "immediate" astern bell happen. Like opening the astern steam throttle wheel while closing the ahead steam throttle wheel.... or waiting for the ship to slow and the prop to slow down before reversing a LSD to prevent snapping the shaft or a blade.... lots to consider, mostly where the engineering watch officer protects the OOD.

 

[jehardiman]

Found from Google and Gemini:

Here's a video on some techniques for stopping a ship.

It discusses high frequency rudder cycling and low frequency rudder cycling; about 5 to 7 ship lengths.

Crash stopping on two anchors usually rips the winlass out of the deck, but 1.5 ship lengths.

-Will

Can't stop on two anchors....not enough people for the necessary coordination...one..then the other windlass will be ripped off the deck and will then be attempting to dive down the hawse pipe. In my experience, the brakes fail and the anchor chain just smokes out...though the deck may buckle around the windlass foundation. This is based upon witnessing an anchor test where, in 5 shots of water (i.e. 450 feet), 4 shots were let run (360 feet) and then the brake was applied. Not a good place to be....

 

[TheBoatVeteran]

On a Serious note. You can BACK the Main sail Boom on your Sailboat to stop it from moving forward and the wind will then cause it to move Backwards. You may have to loosen the Boom Vang.
if you wished to stop completely just hold the sail long enough before luffing and Bamm, you come all ahead stop.

Practice before you go crashing into amazed onlooker. No a Sailboat with the Motor running is NOT SAILING.

 

[montero]

Since I am not a sailor:

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[montero]

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[montero]

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[sun]

Can reversing the ship's propeller help it stop?

 

[montero]

Yes.

 

[montero]

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Bit offtopic :
We have a modern propulsion system here.
Can someone knowledgeable explain the rationale behind such complexity?
We have a good engine driving a generator, which powers an electric motor that requires a gearbox (for what?) on top of that, there’s a variable-pitch propeller—which is essentially another gearbox.

I’d prefer a longtail drivetrain instead.

 

[gonzo]

We have a modern propulsion system here.
Can someone knowledgeable explain the rationale behind such complexity?
We have a good engine driving a generator, which powers an electric motor that requires a gearbox (for what?) on top of that, there’s a variable-pitch propeller—which is essentially another gearbox.

I’d prefer a longtail drivetrain instead.

Can you propose a longtail system to operate an engine that weighs over 2200 tons attached to a shaft of 2 feet diameter?

 

[bajansailor]

In my experience, the brakes fail and the anchor chain just smokes out...though the deck may buckle around the windlass foundation.

Here is a good example of this happening.

 

[portacruise]

A backup quick stop system might be needed to help with a potential disaster, should there be a loss of power and steering control. Wonder if anything would have helped with the Dali disaster? I'm thinking maybe something using a drag chain principle, deployed in such a way that it safely turns the ship, so it runs aground instead of something of more significant value straight ahead?

https://www.cnn.com/2025/11/18/us/ntsb-dali-bridge-collapse

Sorry about the ads, maybe someone else can find a better link..

 

[gonzo]

If you were to drop anchor in 75 feet of water with a 8:1 scope, the anchor would start setting after about 400 feet. It would drag for a distance depending on the bottom. Dragging is good because it would not rip the windlass of the deck. So in an ideal situation the ship could stop in maybe 2000 feet? I doubt that the ground tackle or deck hardware could hold though.

 

[Ike]

Sir Isaac Newton's three laws of motion.

1. A body remains at rest, or in motion at a constant speed in a straight line, unless it is acted upon by a force.
2. At any instant of time, the net force on a body is equal to the body's acceleration multiplied by its mass or, equivalently, the rate at which the body's momentum is changing with time.
3. If two bodies exert forces on each other, these forces have the same magnitude but opposite directions.[1][2]

So to stop a moving ship or boat you need to act on it with a force. Reversing the propellors, or backing the sail, or dropping the anchor. But it remains in motion until the backward force acting on it equals or exceeds the mass times acceleration of the object going forward . F=(Mass X acceleration). The mass stays the same but the acceleration varies until +(M x A) - (M x - A) = 0. at this point it stops, but not instantly/