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I remember seeing somewhere that she was only two or three degrees from her AVS....The roll on this one seems quite extreme (I think the port prop cavitates at one point) and yet she bobbed back up:
Big ship stability.
- Thread starter Appleyard
- Start date
boomerangben
Well-known member
Not withstanding that AVS is a static situation and the video clearly a dynamic one, I would say looking at the acceleration in roll that the ship is a long way from its AVS.
Appleyard
Well-known member
I'm aware that the upthrust has nothing to do with stability...what I was asking was how depth could affect stability...the upthrust is the only thing which would be constant,so what other factors depend on depth?
Avocet
Well-known member
Aye, but that one isn't quite one of those awful "floating blocks of flats" that the Concordia was! Obviously, I've no way of knowing where the centre of gravity is on these modern cruise ships, but compared to their relatively modest draughts it has always been something of a worry to me! When you look at the silhouette of the great liners of the 1900s = 1960s compared to the current crop, it's the height that seems to have increased out of all proportion to length, beam or draught.
chewi
Active member
I have just seen the 1800 hrs BBC news with sketches of the hull, pointing out the depth.
Bizarrely the cross-ection they sketch is that of a fin keel yacht under a 14 deck ship!
all these pictures show V section topsides narrowing down towards a narrow keel, entirely contrary to all the photos.
Where do they get these people from?!
Bizarrely the cross-ection they sketch is that of a fin keel yacht under a 14 deck ship!
all these pictures show V section topsides narrowing down towards a narrow keel, entirely contrary to all the photos.
Where do they get these people from?!
Malo37
Well-known member
Not withstanding that AVS is a static situation and the video clearly a dynamic one, I would say looking at the acceleration in roll that the ship is a long way from its AVS
Yes but it did eventually sink
Yes but it did eventually sink
Malo37
Well-known member
All these ships have to comply with Flag and Class stability requirements. Non are inherently unstable.
In this case the damage was well beyond that considered by the designers as survivable and was more akin to the damage caused by a torpedo. You can see that penetration is right through the outer tanks, which is survivable, but also into the main compartments. From reports of loss of power it may be that initial penetration went right through into machinery spaces.
In this case the damage was well beyond that considered by the designers as survivable and was more akin to the damage caused by a torpedo. You can see that penetration is right through the outer tanks, which is survivable, but also into the main compartments. From reports of loss of power it may be that initial penetration went right through into machinery spaces.
maxi77
Active member
The depth of water itself does not affect stability though canal/shallow water effect where the draft of a ship is increased because effectively the water cannot flow back in under the hull as fast as the screws pump it out, Mr Bernouli has sometning to do with it. This causes the draft ti increase and thus the centre of bouyancy which will change the metacentric height and thus stability.
alant
Active member
boatmike
Well-known member
As usual there is a lot of rubbish in the press. The sensible comments by knowledgable experts is related to designing a ship to be able to go into shallow water with the top hamper of these floating blocks of flats called cruise liners. With less draught and more top hamper a vessel has to be more unstable than a deeper draught vessel with less top hamper. All OK and within guidelines when all is well, especially when stabilisers add to the dynamic stability, but stabilisers do nothing for static stability and when holed and stationary that's all that matters. Having been involved I can tell you that naval architects work under instruction from owners to take maximum advantage of regs. That means that factors of safety are just within spec and if an owner can add another deck and get away with it he will.
aquaplane
Active member
On one of their container ship trips (as passengers) my Dad was asking about trim and all that, somone told him that the COG was 8" below the centre of buoyancy and they were happy with that. I got the impression that this was while loading and a puter was working it out as the containers were coming and going but I may be wrong.
Malo37
Well-known member
As usual there is a lot of rubbish in the press. The sensible comments by knowledgable experts is related to designing a ship to be able to go into shallow water with the top hamper of these floating blocks of flats called cruise liners. With less draught and more top hamper a vessel has to be more unstable than a deeper draught vessel with less top hamper. All OK and within guidelines when all is well, especially when stabilisers add to the dynamic stability, but stabilisers do nothing for static stability and when holed and stationary that's all that matters. Having been involved I can tell you that naval architects work under instruction from owners to take maximum advantage of regs. That means that factors of safety are just within spec and if an owner can add another deck and get away with it he will
If you have the time and inclination (sic) try floating a shoe box in the bath. You will no doubt be suprised by the stability when say, only 5% of the total height of the vessel is under water. Additionally these skyscraper passenger vessels take advantage of lightweight materials and construction especially in the upper decks. By saying that naval architects, of which I am one, take advantage of the rules you are misrepresenting the design process. What they do in fact is to ensure compliance with the rules which in themselves incorporate substantial safety factors. There is nothing that can effectively design out the risk of someone driving their ship onto the rocks. The focus should rather be on navigation and hazard identification systems and the ability/authority of individuals to override them.
If you have the time and inclination (sic) try floating a shoe box in the bath. You will no doubt be suprised by the stability when say, only 5% of the total height of the vessel is under water. Additionally these skyscraper passenger vessels take advantage of lightweight materials and construction especially in the upper decks. By saying that naval architects, of which I am one, take advantage of the rules you are misrepresenting the design process. What they do in fact is to ensure compliance with the rules which in themselves incorporate substantial safety factors. There is nothing that can effectively design out the risk of someone driving their ship onto the rocks. The focus should rather be on navigation and hazard identification systems and the ability/authority of individuals to override them.
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maxi77
Active member
Humans will always cock it up from time to time so systems need to cope with problems, whay I still find some what worrying is not they clipped a rock, but that the ship rolled over quite so quickly.
Malo37
Well-known member
Humans will always cock it up from time to time so systems need to cope with problems, whay I still find some what worrying is not they clipped a rock, but that the ship rolled over quite so quickly.
How big a hole in the hull do you think should be allowed before the vessel becomes unseaworthy? Investigations have to focus primarily on the root cause, not the effect - with a view to eliminating the cause in the future.
How big a hole in the hull do you think should be allowed before the vessel becomes unseaworthy? Investigations have to focus primarily on the root cause, not the effect - with a view to eliminating the cause in the future.
chewi
Active member
On vessels like this the COB will prob be many metres BELOW the COG.
Read up on "metacentric height" and it will explain how the tilting ships underwater profile moves its COB sideways to produce a couple with the Higher COG to correct the list.
Float an empty orange carton on its edge, its COG is its centre, but its COB is the centre of the tiny displacement well below it. Tip it and it will return to upright, within reason.
But it has limits, and filling the bilges with rocks and water goes beyond them.
binch
New member
Squat
This subject is little understood, and it is possible to have "lateral squat".
If a ship passes an underwater obstruction close to at a significant speed, then there is interaction between the obstruction and the ship, drawing the ship laterally towards the obstruction. This is most obviously seen as canal effect, but that leaves the ship short of water on both sides..
From my personal knowledge of Giglio, there is a 20m rock some 200 to 300 m offthe shore of Giglio and bearing about ESE from the lighthouse on the pier.
If the CC was taken within 100 metres of this rock at cruising speed, she would have been drawn towards this rock in a way that would make her strike it amidships or just abaft amidships. And hence the tin-opener effect.
The act of striking on the port side would have slewed her head to port thus starting the "about turn", which may not have been done on purpose at all, but might have been involuntary.
When I navigated a large vessel (albeit smaller than CC) from Napoli to Genoa, I passed to the west of Giglio and Elba in clear unobstructed waters. That channel off the tip of Monte Argentario is not congenial for big ship navigation.
A further complication might be that the ship was at "seaspeed", which is a way of linking out the diesels so as to increase power and lower fuel consumption. It can take 15 mins to get out of seaspeed once in it, during which time the ship would be locked in her forward motion.
My provisional assessment is that the captain of CC hazarded his ship unnecessarily by passing to close to danger.
Another factor is that this strait's chart is based on old surveys. It is a seismically active area, and one should not allow for depths being dead accurate. Also the soundings were probably made by lead and line. The surveyor might have sounded a hole!
Prudence. There are navigators who s**t when they cannot see land, and others who s**t when they can. Safer to be the latter.
This subject is little understood, and it is possible to have "lateral squat".
If a ship passes an underwater obstruction close to at a significant speed, then there is interaction between the obstruction and the ship, drawing the ship laterally towards the obstruction. This is most obviously seen as canal effect, but that leaves the ship short of water on both sides..
From my personal knowledge of Giglio, there is a 20m rock some 200 to 300 m offthe shore of Giglio and bearing about ESE from the lighthouse on the pier.
If the CC was taken within 100 metres of this rock at cruising speed, she would have been drawn towards this rock in a way that would make her strike it amidships or just abaft amidships. And hence the tin-opener effect.
The act of striking on the port side would have slewed her head to port thus starting the "about turn", which may not have been done on purpose at all, but might have been involuntary.
When I navigated a large vessel (albeit smaller than CC) from Napoli to Genoa, I passed to the west of Giglio and Elba in clear unobstructed waters. That channel off the tip of Monte Argentario is not congenial for big ship navigation.
A further complication might be that the ship was at "seaspeed", which is a way of linking out the diesels so as to increase power and lower fuel consumption. It can take 15 mins to get out of seaspeed once in it, during which time the ship would be locked in her forward motion.
My provisional assessment is that the captain of CC hazarded his ship unnecessarily by passing to close to danger.
Another factor is that this strait's chart is based on old surveys. It is a seismically active area, and one should not allow for depths being dead accurate. Also the soundings were probably made by lead and line. The surveyor might have sounded a hole!
Prudence. There are navigators who s**t when they cannot see land, and others who s**t when they can. Safer to be the latter.
bignick
Active member
On most ships the CofG is above the centre of buoyancy. Its the transverse movement of the centre of buoyancy as the vessel heels which generates a righting moment.
For more info have a look at GZ curves -
http://books.google.co.uk/books?id=...&ved=0CFMQ6AEwCQ#v=onepage&q=gz curve&f=false
Oops, just saw what chewi put.
Nick
Latestarter1
New member
You are quite correct, depth of water has no impact.
However when I did my ship stability in the 1960's there were established rules on metacentric height (GM) which is is a measurement of the initial static stability of a floating body on passenger ships. A larger metacentric height implies greater initial stability against overturning, however too large the metacentric height affects a ships period of roll. A very stable ship has a very short period of roll making life very uncomfortable for pasengers.
The design of passenger ships today is totally different, with far greater beam length ratio than the old days. Downside is that these ships are far more sensitive to 'free surface effect. As the ship floods, the loss of stability is caused by the increase in hight of the centre of buoyancy, and the loss of waterplane and a loss of the waterplane moment of inertia which in turn decreases the vessels metacentric height.
Designing a floating Fawty Towers giving almost everybody a room with a view assumes that hull integrity is never compromised.
The report into the loss of the Spirit of Free Enterprise covered this subject in great detail.
The Captain had a difficult call, but made the wrong one, he had little time to make a damage assessment and his call was to run for a safe shore wasting valuble time and preventing him from deploying his lifeboats as they cannot be launched underway. The Captains gamble did not pay off, within a few cables of safety the free surface effect caused the vessel to loose any self righting moment and take on a list, by now making launching of the lifeboats close to impossible.
Anyway inherent safety is impossible on ANY ship unless crew share a common language.
Kukri
Well-known member
I think the reference might have been to the change in stability once the ship is aground.
If she grounds on a flat bottom, fine. if she grounds on a ledge of rock, and continues to flood, as of course she will with a big hole in her side, she is going to want to fall over.
(fwiw I agree with binch's post.)
If she grounds on a flat bottom, fine. if she grounds on a ledge of rock, and continues to flood, as of course she will with a big hole in her side, she is going to want to fall over.
(fwiw I agree with binch's post.)
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toad_oftoadhall
New member
Jesus, I didn't know this:
The court also described the attitude of boatswain Terence Ayling, believed to have been the last person on G deck, as most unfortunate.[5] Asked why he did not close the doors given there was no one else there to do it, he said it was not his duty.[5] However the court praised his work in the rescue.[5]