What is a cruise ship's "propulsion safety system" and why can engaging it injure passengers?

[berkeman]

Which, I dunno, seems to suggest it may not by diven by azipods after all.

You need the deisel engines and their generators to power the electric azipods...

 

[gleem]

Some user account. Apparently a wave struck from the side. This means the orientation of the ship at that time was indeed with the wind and waves coming from the side. This is not an ok orientation for such a ship to be in...

The captain warned of a severe condition and told passengers to lie down or hold on to something so he saw something coming. The ship came to an "abrupt stop". So a rogue wave off the starboard bow?

Image is not ot the Discovery. If you are on one dof the top decks there is a huge motion and can cause sea sickness in even relatively calm conditions. I do not see the captain heading in any direction but close to into the waves

 

[DaveC426913]

You need the deisel engines and their generators to power the electric azipods...

I thought we weren't allowed to discuss perpetual motion devices here...

 

[m1ke]

So, I'm pretty sure that a ship of this size can not be stopped in such a way that it causes people to be knocked over.

I’m surprised to have to bring this up considering this is a “physics forum”, but have you not heard of “Newton’s Third Law of Motion”?

This is really all easily answered by these principles. The very size and weight of that cruise ship and the force at work to propel it through the water would then be felt through the entire ship itself and that would easily knock people over or send them tumbling.

 

[berkeman]

Welcome to PF, @m1ke

have you not heard of “Newton’s Third Law of Motion”?

Of course we all have, but the issue is how much thrust can those propeller pods deliver versus the mass of the ship. If they are monster engines compared to the ship's mass, then you can get more of an impulse. But it's hard to believe that the propeller pods would be sized so big and powerful to generate such large accelerations for such a massive ship. After all, when would a cruise ship need to take super-quick evasive maneuvers?

 

[sophiecentaur]

“Newton’s Third Law of Motion”?

Of course but turning fast can tilt a ship a long way.

 

[DaveC426913]

I’m surprised to have to bring this up considering this is a “physics forum”, but have you not heard of “Newton’s Third Law of Motion”?

This is really all easily answered by these principles. The very size and weight of that cruise ship and the force at work to propel it through the water would then be felt through the entire ship itself and that would easily knock people over or send them tumbling.

A ship traveling at 20 knots will not knock anyone over, nor will it cause their uteruses to fall out.

The question is how fast can it stop (or turn).

The Spirit of Discovery weighs in at 58,000 tonnes.
How big do the engines need to be to decelerate it (in the viscous medium of water) to knock people over?

 

[sophiecentaur]

We are looking for a mechanism that can actually achieve what happened on record.

but the issue is how much thrust can those propeller pods deliver versus the mass of the ship

It doesn't have to be a matter of thrust and mass, it's a matter of torque and moment of inertia which will make a ship roll / list. If a large wave turns up and you take the wrong action, you can end up with a worse situation than if you didn't bother to steer at all.

How big do the engines need to be to decelerate it (in the viscous medium of water) to knock people over

Have you ever been down below on a big ship in heavy weather - or even just a moderate sea? A very slight list is enough to make you fall over if you are not expecting it. The two sensory inputs - change of where "down" is and the visible clues due to vertical lines and walls need only to disagree a bit to make you unsteady.

The engines on that ship use steerable propellors (like many power boats with stern drives) can push the stern hard one way and steer the ship faster than a simple rudder. Turning fast will make a ship roll a long way. Add to that, the effect of a large wave arriving abeam. The 'g forces' are just from the apparent direction that g acts. Experienced crew soon learn to deal with that environment.

In a sailing yacht, you can throw people off the deck and into the water if you turn sharply without warning. It's not merely being polite when the skipper warns "ready to go about?".

 

[m1ke]

A ship traveling at 20 knots will not knock anyone over, nor will it cause their uteruses to fall out.

The question is how fast can it stop (or turn).

The Spirit of Discovery weighs in at 58,000 tonnes.
How big do the engines need to be to decelerate it (in the viscous medium of water) to knock people over?

A cruise ship travelling at 20 knots and suddenly experiencing a full propulsion shutdown causing the props to essentially seize in place would create massive, but only momentary amounts of great friction when you consider aerodynamic principles. The stopped props on the ship, once you consider the sheer size of these propellers which are designed to move as much water with a very low rpm making it nessecary to increase the friction generated under propulsion with big thick blades that when halted, work just like the air brakes.

Also you should consider how much of the ship is actually under water compared to top side and it makes more sense that the ship tilted in one way or another simply because the transfer of weight under the forces stopping it. Think aerodynamics and how a plane on flight can spin towards the earth uncontrollably simply because one elevator get jammed or a jackscrew comes loose like with a flight decades ago, I think it was a dc8.

Water and air are both considered fluids so aerodynamic properties or Bernoulli’s principle apply both in the air and on the water.

Lastly one poster mentioned how a ship without propulsion is not affected by the rudders which is not correct as oceans have currents which can easily move a large object like a ship without propulsion in a modestly predictable manner.

Further thoughts?

 

[Arjan82]

Essentially it is about the ratio between weight and thrust. The DC8 weights 125tonnes (quick google, don't kill me on this) and the total thrust it can deliver is roughly 60tonnes force. So that's roughly a ratio of 0.5 of thrust to weight. This means that thrust suddenly going to zero generates massive accelerations.

The Cruise vessel weighs 58000tonnes, and both propellers together will generate a thrust somewhere in the order of 100 tonnes (rough estimation...), that is a ratio of 0.002... That was my point, the thrust these units can make is just not large enough to generate significant accelerations on the ship.

When you decide at 20 knots (actually much less in this kind of weather, more like 10 or so) to throw the rudder around, and if you decide you do not want the azipod units to break off your ship because of all the force needed to rotate your ship (limiting the amount of rudder you can give), then you will indeed get a bit of list, a couple of degrees I think, maybe even 10. Very inconvenient but certainly not something the ship couldn't handle. Also, the captain is fired if he would do that...

Oceans indeed have currents. But the ship moves relative to the body of water, not relative to the ground. So currents have no effect on the effect of the rudder...

Any comparison with a ship of 10 meters or less is completely missing the point...

If you decide to turn around your ship, there will however inevitably be a point at which the waves come from the side. This is a serious problem since in this kind of weather the length of the waves can be in the order of the width (breadth / beam) of the ship. This means the ship can start to roll severely. That is really bad. Also, if you loose power the ship will naturally want to orient itself with the waves coming from the side also... That is again, really really bad....

 

[Arjan82]

Have you ever been down below on a big ship in heavy weather - or even just a moderate sea? A very slight list is enough to make you fall over if you are not expecting it. The two sensory inputs - change of where "down" is and the visible clues due to vertical lines and walls need only to disagree a bit to make you unsteady.

How big is big here? This ship is 236 meters and has stabilizers. Did you look at the video? That is not during the accident. It hardly moves, even in this weather (that is, up until the point the engines shut down...). Also, list itself is not that important (up to a certain extent), the accelerations are the problem, that is what makes you topple over. Try to imagine (or compute...) the forces needed to get significant accelerations on a mass of 58000 tonnes. The waves can do that however, if they come from the side. But the engine or steering simply does not.

 

[DaveC426913]

A cruise ship travelling at 20 knots and suddenly experiencing a full propulsion shutdown causing the props to essentially seize in place would create massive, but only momentary amounts of great friction when you consider aerodynamic principles. The stopped props on the ship, once you consider the sheer size of these propellers which are designed to move as much water with a very low rpm making it nessecary to increase the friction generated under propulsion with big thick blades that when halted, work just like the air brakes.

Yes. All of which is a much more applicable than "...but Newton's third Law!"

 

[sophiecentaur]

Also, list itself is not that important

I'd disagree with that. A temporary change in the local perceived vertical is extremely confusing and even dangerous. A list of just over 5 degrees can give you the impression of a sideways force of 10% of your body weight. That can throw you over easily if you are not ready for it and even if it takes several seconds to develop. We've already agreed that the 0 to 60 acceleration of a ship with even the most powerful engines is far too long to cause the effects described. I'm trying to present an alternative which is actually common experience.

If you take a fairground ride, the forces acting on you are usually quite trivial but the experience can be 'impactful'. Go below deck in a close hauled sailing boat and try to brew a cup of tea. You are fighting against gravity which is acting in totally the wrong direction (easily 30 degrees from vertical) - until you are familar with the effect and then your brain edits it out.

The tables and chairs that were 'thrown about' will have toppled over and slid from side to side; very dangerous but also very Newtonian.

 

[DaveC426913]

Have you ever been down below on a big ship in heavy weather - or even just a moderate sea? A very slight list is enough to make you fall over if you are not expecting it. The two sensory inputs - change of where "down" is and the visible clues due to vertical lines and walls need only to disagree a bit to make you unsteady.

Yeah, now that you menton it, you're right. When you're on a ship so large that you might as well be on land, you don't walk or stand with caution, and there's no horizon to orient you. It wouldn't take much.

I was in a cabin on the lower far forward deck in moderately rolly seas once. I set up a pendulum graph in my room to see what kind of designs I could get.

 

[sophiecentaur]

Lastly one poster mentioned how a ship without propulsion is not affected by the rudders which is not correct as oceans have currents which can easily move a large object like a ship without propulsion in a modestly predictable manner.

You only have steerage way if there is relative motion between ship and current. After an hour or so after engines off (or sooner than that), you can consider the ship as stationary. The rudder can have no effect