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

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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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  • #2
Do you mean what the system is? Or do you mean why/how it activated?
 
  • #3
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!"?
 
  • #6
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.
 
  • #7
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?
 
  • #8
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?
 
  • #9
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)
 
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  • #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.
 
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