Why Did Activating the Propulsion Safety System Injure Cruise Ship Passengers?

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The discussion centers on the injuries sustained by passengers aboard a UK cruise ship after its propulsion safety system activated during a storm in the Bay of Biscay. The activation caused the ship to lurch abruptly, injuring about 10% of those onboard, with five seriously hurt. Participants speculate that the sudden engine shutdown, possibly triggered by adverse weather conditions, led to a loss of control and the ship's abrupt movement. The conversation also touches on the mechanics of propulsion systems, including the potential effects of stopping one engine and the role of stabilization systems. Overall, the incident raises questions about the safety measures and operational decisions made during extreme weather.
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So this almost qualifies for the Weird News thread in the GD forum, but I think it's more of a technical question. You've probably seen the story in the news about the unfortunate UK cruise ship that got caught in a storm while trying to get back to port, and a number of passengers got injured.

I don't understand what they mean by this:
Catching the brunt of the storm in the Bay of Biscay, the ship’s propulsion safety system was activated – causing the ship to lurch to the left, bringing it to an abrupt halt, and injuring around 10% of the passengers onboard. Five are understood to have been seriously injured.

The ship’s crew then decided to stay in position, rather than attempt to continue the journey to the UK.

The Bay of Biscay is notorious for its treacherous weather and high waves.
https://www.cnn.com/travel/article/saga-cruise-ship-storm/index.html

I tried using Google to figure out what a cruise ship's "propulsion safety system" could be, and why engaging it would cause such a large force transient to knock people down like this. The closest thing I could find is this (which really doesn't explain the force transient as far as I can tell):

https://www.marineinsight.com/main-engine/how-marine-propulsion-engine-of-the-ship-is-protected/

Does anybody have any insight into what happened? Thanks.
 
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Do you mean what the system is? Or do you mean why/how it activated?
 
Well, what it is, and I guess how it is activated. I mean, what can cause such a large force transient other than "Reverse all engines captain!"?
 
berkeman said:
That's the same link I found with Google and posted. Can you point to where in the article you are finding whatever that word is?
Which word? Shutdown or lurch?
Lurch is my word; my hypothesis.

But item 3 is Shut down: When there is a higher fluctuation in the engine parameters which can harm other systems of the engine, the shut down protective devices cut off the fuel supply and the engine stops.

Oh, you mean propsafesys. That was just lazy typing on my part. Sorry.
 
DaveC426913 said:
Which word?
DaveC426913 said:
propafesys
I've never seen that word before.

DaveC426913 said:
But item 3 is Shut down: When there is a higher fluctuation in the engine parameters which can harm other systems of the engine, the shut down protective devices cut off the fuel supply and the engine stops.
So when the engine stops the associated propeller stops turning? From the turbine engine picture in the article (or a different article I found), I would have thought that an engine stop would have just free-wheeled the associated propeller...?

And they mention a lurch to the left -- does that mean that just one of the propellers stopped immediately?
 
berkeman said:
So when the engine stops the associated propeller stops turning? From the turbine engine picture in the article (or a different article I found), I would have thought that an engine stop would have just free-wheeled the associated propeller...?
Actually, looking at the linked article shows piston engines instead of a turbine, so I guess an engine stop would put the engine into compression braking, which could maybe stop the propeller fairly quickly?
 
But if you are dealing with rough seas, why would you kill the engines? I would have thought that you need full power to keep steering into the waves...? (But I'm relying on my TV and movie watching experience on that...) :wink:
 
  • #10
berkeman said:
I've never seen that word before.
Sorry, it was just lazy typing. I didn't feel like typing the whole thing up. That backfired...

berkeman said:
And they mention a lurch to the left --
Then a step to the right...
1699479862394.png
Sorry, I guess I don't have any more information on this than you (as yet).
 
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  • #11
LOL, Dave. Still helpful though. If their "safety measure" was to push the big red button for "all stop", that might explain the injuries. I still have no idea why that would be the "safety" procedure...
 
  • #12
The article suggests it is an automatic cutoff to protect the engine and related parts.

"the shut down protective devices cut off the fuel supply and the engine stops."

They can probably fire up the engines for safety (because having no propulsion is kind of at the top of "unsafe situations"), but that wouldn't happen until after the engine had shut down.

The article descibes the warnings that would have preceeded a full shut down. (one of which is engine slow down - which would have mitigated an injury-inducing lurch, so... still a mystery)
 
  • #13
"Catching the brunt of the storm in the Bay of Biscay, the ship’s propulsion safety system was activated"

Maybe I'm reading more into this than I should but they seem to be suggesting the storm conditions were involved in triggering the safety system.

I wonder what would happen to an engine that size if the prop came out of the water...
 
  • #15
"I wondered if one engine had automatically shut off, due to the waves setting off low oil alarms, and hence the ship did a sudden turn?

Like the cruise ship off Norway whose engines turned off due to low oil alarms going off - oil.levels were fine, but the oil in the sump.was splashing about with the waves..."
https://forums.ybw.com/threads/ships-propulsion-safety-system.602947/
 
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  • #17
I was wondering about the suggestion of synchronous rolling. If the ship was rolled over to the right when they killed the engines perhaps it righted itself (the "lurch to the left") a bit more enthusiastically than expected.
 
  • #18
Ibix said:
I was wondering about the suggestion of synchronous rolling. If the ship was rolled over to the right when they killed the engines perhaps it righted itself (the "lurch to the left") a bit more enthusiastically than expected.
Good thought. Don't cruise ships have stabilization systems to keep passengers from feeling normal-size waves? I wonder if any shut-downs would affect that system. I'm off to Google...
 
  • #19
A safety system that does that sounds like a mixed blessing at best...
Not a mixed blessing so much as a "not a magic bullet" blessing.

The engines shutting down is a sign that something is already very wrong (since the first two stages of warning went unheeded) - wrong enough that - if allowed to proceed unchecked - worse things could happen than the passengers getting knocked down (such as engine failure, which would leave them sitting ducks in a storm of undetermined duration and ferocity).
 
  • #20
berkeman said:
Good thought. Don't cruise ships have stabilization systems to keep passengers from feeling normal-size waves? I wonder if any shut-downs would affect that system. I'm off to Google...
So it looks like there are several options available. I wonder which one(s) this cruise ship had operating...

https://quantumstabilizers.com/roll-stabilization-systems/
 
  • #21
Here's a thought. Given that they were in rough seas, they could have had some sort of cross-wave action occurring. The rudders would have likely been turned to some degree to compensate for this (something akin to tacking into the wind or aircraft crabbing during landing). When the engines suddenly stopped, the angled rudders (or the cross-wave itself) would then have the effect of throwing the ship to one side.
 
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  • #22
99% of what is on the interwebs is marketing. "Propulsion safety system" could be anything, and it may well have been a term invented by PR people who are now managing a crisis.

This ship uses "pods" - you have two independently rotatable nacelles under the waterline each with an electric motor turning the screw.

You want to avoid certain configurations, such as each impeller trying to push water into the other. I suspect - but do not know for certain - that they are discussing some kind of interlock that keeps this from happening.

And while it is easy for us on shore to be critical of it, it might well have kept a bad situation from becoming worse.
 
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  • #24
berkeman said:
Yeah, interesting. It looks like Diesel-Electric Propulsion is common now, with the electric motors in the pods:

https://www.marineinsight.com/tech/cruise-ships-powered/View attachment 335059
https://www.cruisemapper.com/wiki/752-cruise-ship-engine-propulsion-fuel
Looks like a posh form of bow thruster. As there's one on either side, you could get a lot of twist if only one were to be lowered on its own - or operated at the wrong power and the wrong direction. Don't panic don't panic - klunk!!
Ye gods - what sort of gear box would do that without electric motors?
 
  • #25
sophiecentaur said:
... you could get a lot of twist if only one were to be lowered on its own...
These don't raise/lower as far as I know.
 
  • #26
Looking at the image again, that figures. I couldn't believe the power involved. Maybe they reversed just one drive. Perhaps a bit of panic?
 
  • #27
If the ship had the usual right-handed propellor and put in hard reverse the backwash from the propellor would have caused the stern to lurch to the left if it had a single propellor. A rapid stopping in this situation is unwarranted unless they tried to avoid an obstruction or it was just a mistake.
 
  • #28
gleem said:
caused the stern to lurch to the left if it had a single propellor.
Doesn't it say there are two props? I would suppose that they would be counter rotating to avoid 'prop walk'. That was a real killer when trying to go astern on my sailboat; you had to get some finite way on in order for the rudder to steer at all astern. But I thought those big ships had taken care of things like that. Perhaps not but I'd think Operator Error.
 
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  • #29
sophiecentaur said:
But I thought those big ships had taken care of things like that. Perhaps not but I'd think Operator Error.
Yes. on large ships, dual screws are standard and should be opposite handedness. We don't know the status of the starboard engine (neutral?). Don't they have separate controls? Anyway, if you change your mind because of the weather you come about minding sea conditions and even warning passengers of unusual motion.,

sophiecentaur said:
That was a real killer when trying to go astern on my sailboat; you had to get some finite way on in order for the rudder to steer at all astern.

But it can also be of advantage when quickly turning a sailboat around. When moving ahead put the engine in neutral, helm hard to starboard while engaging hard reverse.
 
  • #30
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. If by some hand of God you would force one or both of the PODs to (almost) instantly stop, the units would just shear off.

What I think might happen is the following (I think the BBC article describes it a bit better). Due to the high winds the engines can be overloaded and must shut down to avoid damage (or oil might not go to the correct places, or whatever it is). This leaves the ship essentially out of controls, rudders won't do anything anymore without propulsion. This then causes the ship to orient itself naturally with the wind and waves coming direct from the side (aka beam wind, that is then the stable condition)beam wind), causing massive roll. Ships like that are not meant to roll that heavily and furniture will start to move uncontrollably. This does not look pretty, as a video from another case in which this happens shows:



I can easily imagine this causing 100 injuries...
 
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  • #31
gleem said:
But it can also be of advantage when quickly turning a sailboat around. When moving ahead put the engine in neutral, helm hard to starboard while engaging hard reverse.

If a Cruise ship is cruising at some speed it will never ever do a hard reverse like that or even a hard rudder turn. The forces involved here are enormous, actions like this will simply cause the POD units to come clean off, the propeller blades to bend or things like that. If they want to turn direction they give some rudder of a couple of degrees (think 5 to 10 degrees or so) and just make a huge turn circle, the Bay of Biscay is big enough.
 
  • #33
berkeman said:
out what a cruise ship's "propulsion safety system" could b
Since we're guessing, and since these are electric, could this be a fuse/circuit breaker?

It sounds like the sort of thing a PR team might make up.
 
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  • #34
I went looking for the specs on Discovery's azipods and found this:

an 3D interactive "dollhouse" of the entire public area.

1699568223886.png

Man these showings are getting sophisticated...
https://travel.saga.co.uk/cruises/ocean/our-ships/spirit-of-discovery.aspxAnyway: Wiki says this:

"The ship is powered by four nine-cylinder 32/44 common rail MAN [Diesel] engines equipped with selective catalytic reduction, each producing 5,400 kilowatts (7,200 hp), for a total of 21,600 kilowatts (29,000 hp)."

Which, I dunno, seems to suggest it may not by diven by azipods after all.
 
  • #36
DaveC426913 said:
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... :wink:
 
  • #37
Arjan82 said:
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

1699570978386.png
 
  • #38
berkeman said:
You need the deisel engines and their generators to power the electric azipods... :wink:
I thought we weren't allowed to discuss perpetual motion devices here...
 
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  • #39
Arjan82 said:
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.
 
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  • #40
Welcome to PF, @m1ke

m1ke said:
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? :wink:
 
  • #41
m1ke said:
“Newton’s Third Law of Motion”?
Of course but turning fast can tilt a ship a long way.
 
  • #42
m1ke said:
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”?
🤔
m1ke said:
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?
 
  • #43
We are looking for a mechanism that can actually achieve what happened on record.
berkeman said:
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.
DaveC426913 said:
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?".
 
  • #44
DaveC426913 said:
🤔

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?
 
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  • #45
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....
 
  • #46
sophiecentaur said:
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.
 
  • #47
m1ke said:
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!" :wink:
 
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  • #48
Arjan82 said:
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.
 
  • #49
sophiecentaur said:
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.

1699896961622.png
 
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  • #50
m1ke said:
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
 

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