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SteamboatsThe stability of ferries
Another ferry sinking in Indonesia and this highlights a problem I’ve been going on about for a long time – unsafe designs and exceeding designed passenger carrying capacity.
Obviously, we can’t do anything about Indonesia but we can do about our own ferries – we’re an island nation [for now]. Look again please at the photo below.
Courting disaster
In the article in which that pic was posted, I argued that there are designs which on paper and under ideal conditions might work but that this is unrealistic in the real ocean and under real wind conditions. Commercial considerations are why a ferry runs in the first place but shifting that to the hull design and cutting corners with seaworthiness is plain criminal.
All it would have taken were two amas [outriggers], for very little overall cost and they would have avoided much of the risk:
Even on large ferries, these are possible to fit. They may look a bit less beautiful than the westernized “stabilizers” but they don’t fail when the chips are down.
As for the overcrowding, well, nothing can compensate for that.
Filed under: Leisure & travel, Tech & ideas
Why they use top heavy designs for ferries when there are much more stable catamaran designs available is a mystery. I assume the use of ferries started with the conversion of standard passenger designs and the designers are working on the assumption of ‘it works let’s keep using it’. At the moment it appears the cats are sold as high speed units only when they should be sold on stability.
The idea of retro fitting outriggers seems to make sense but will not be adopted because of looks.
The stabilty of floating structures is governed by calculations of
some complexity.The visual appearance is not a factor in these which
are cosiderations of the relative positios of the centres of gravity
and buoyancy.The CG is fixed unless weights are moved on board,the CB
moves as the ship rolls and pitches,but provided the CG is below CB
the ship is stable.Positive stability requires a positive metacentric
height.Dynamic forces have to be considered,ie moving passengers,wind
pressure,and free surface in ballast and fuel tanks etc.However these
cannot be assessed merely by looking at a picture.
“CB moves as the ship rolls and pitches,but provided the CG is below CB
the ship is stable.”
As mentioned Mr. [?] Greengrass, the calculations are conceded and the stabilizers are computer designed and controlled. That much is fine.
Agreed also that a photo doesn’t a theory make. However, look at the dynamic weight high up, i.e. the swimming pools and the unknown of where passengers are likely to be at any given time plus varying passenger numbers on each voyage.
Combine that with broaching seas and high winds slap into the side – look at the surface area – because, unlike a cruising yacht, this liner is following a straight line where it’s scheduled to go, then by definition, roll is not able to be either predicted or controlled and that is not designed in.
One only needs the CG to be above the CB for a short time, together with a rogue wave and a roll is set in motion. Rolls up to 50 degrees have been known. See the other post for the video. Admittedly there are the other factors such as length of the boat [anti-yaw], speed through the water and entry section at the bow, all of which make a difference and yet it does not seem right, even then.
Frankly, it is relying on something inherently unstable to be compensated for by the technology – fine while the technology compensates. Given the total weight of materials in there, why could they not have put the top two stories at least beamwise and the pools at the rear?
The answer is that everyone on a cruise such as that demands a porthole looking over the water and the only way to achieve that is upwards. The pool up top saves on heating costs and energy usage.
In the end, as a designer of sailing yachts, I can see there are principles not being observed here and I’d love to see the margins of CG under CB at different angles of heel.
I think this is the ferry disaster that James is referring to: http://news.smh.com.au/breaking-news-world/hundreds-rescued-as-26-die-in-indonesia-ferry-sinking-20091122-isql.html
In that article in the Sydney Morning Herald, there is some information on what might have gone wrong, including very bad weather, possibly overloading, possibly unwise seagoing in known bad weather.
The hard information, if any newspaper article can be considered hard, is: “Strong waves hit the ferry and caused the front part to crack. Water got in and within half an hour it sank”, together with Navy spokesman Iskandar Sitompul said the vessel sank after being hit by waves as high as three metres (10 feet), but other officials said the waves were towering up to six metres.. So, this disaster could be down to a design fault (though very likely other than stability: that is too high a centre of gravity relative to centre of buoyancy). It could also be down to (any combination of) it being a very old ship, poor maintenance, being at sea in conditions known to be outside the appropriate operating envelope of the ship, or just bad luck in meeting weather far worse than expected.
It’s a nice picture of the Oasis of the Seas, but I’m not quite sure of the validity of any connection.
Best regards
The connection is that the very dynamic factors you mention, Nigel, are the ones which affect stability, particularly primary stability and if that is suffering, it can flow onto secondary stability. In that, the O of the S is not exempt.
What also makes me uneasy is the concept of the ship, reflected in the name. The history of concept craft, for example the variable geometry of the French trimarans, is one littered with strife. There are design concepts and when one relies too much on dynamics and probabilities, it’s getting a bit iffy.
In response to James’ comment at 16:00 today, concerning the connection between the Oasis of the Seas and the very recent Indonesian ferry tragedy, I have the following to add.
First, James and I discussed earlier, at some length, the issue of stability of the Oasis of the Seas.
Irrespective of any conclusions of that discussion, there is little to nothing (given our current knowledge) to connect the two. This is because the Oasis of the Seas discussion was primarily concerned with stability. The recent Indonesian ferry tragedy (if we have any knowledge of it at all) was primarily caused by structural failure: “Strong waves hit the ferry and caused the front part to crack. Water got in and within half an hour it sank.”
Now, further knowledge may well come our way concerning the ferry tragedy. If that knowledge includes loss of stability as a major contributing cause (without illegal overloading) to the structural failure, or structural failure turns out to be an inappropriately premature diagnosis of causation, so be it. However, at this time, surely the balance of analysis is that we have no reason to rate a design fault on stability more than any design fault on structural integrity, and no reason to favour any design fault over bad operational decision, or over Act of God.
Thus, I do not see how our knowledge at 16:00 today gives any rational cause for James to link this Indonesian ferry tragedy with his concerns for the stability of the Oasis of the Seas and other similar big pleasure cruisers.
Moving on, James writes: “There are design concepts and when one relies too much on dynamics and probabilities, it’s getting a bit iffy.”
With only humorous intent, concerning linguistic expression, I observe that ‘iffyness’ is surely the very domain of probability.
Best regards
Nigel, what is the SI unit of Iffyness? I confess I have forgotten.
Iffyness is unit free; it’s just a ratio, as in ‘if’ to ‘if not’.
Best regards
Thanks Nigel, I knew Cambridge Algorithmica would have the answer.
OK boys – you can’t see the connection at 16:00. I might point out that there have been hundreds of ferry disasters in Indonesia prior to this one and that the authorities, from whom comment on the story was sought, did not add any new possible causes, did not speculate on them.
Ipso facto, this was yet another similar disaster. What was the cause of this? Why, overcrowding of course. And what did the overcrowding do? It put a lot of people above the CB and altered the overall CG.
Clearly, in good conditions, they would survive and usually did so. However, they were close to the limits of the boat’s capability, in other words, with lower margins than is safe.
Bring in the X factor – wind and waves and voila – disaster.
Now let’s look at the Oasis of the Seas, with which there is zero connection, according to the boys. What is the cause for worry? Why, putting a lot of weight above the CB and altering the overall CG and in fact, introducing dynamic variables which can’t be factored in.
Clearly, in good conditions, they would survive and usually will do so. However, they are close to the limits of the boat’s capability and as Nigel said, they only need have the CG even slightly below the CB and all is well in good conditions; in other words, with lower margins than is safe in this blogger/designer’s eyes.
Bring in the X factor – wind and waves and voila – disaster, remembering that that boat only needs 52 degrees of oscillating heel to set up a motion in which big seas would send her over.
The only factors militating against that are boat speed and LWL and true, the longer boat is more stable. However, in big seas, that is negated to a large degree and the boat is turned into a wallowing tub with swimming pools sloshing about on the upper deck and passengers where? Cowering in the engine room below? Not a bit of it – they’re in their cabins on the upper decks.
The key factor in both scenarios is the introduction of the unpredictability of wind and waves, which follow a normal pattern in good weather but in changeable weather, anything can happen.
If you’d like confirmation of that, please go to the Royal Yacht Club or any of the bluewater clubs and ask the old salts there about the unpredictability of ships and weather.
On the unpredictability of gear breakage in unpredictable conditions, ask the crew of Skandia. Ask David Pascoe, author of rough water seamanship or the crew of the Hatteras or of the battleship New Jersey about how the sea and wind alters designed parameters.
The bottom line is that in sea disasters [oops - there isn't one as of now with the OotS], any designer knows that CB and CG are variables*, not fixed.
Now, Nigel points out that “as long as one is kept below the other”. Yes, Nigel – as long as.
Human factor. Human error. Human error, compounding naturally atypical conditions with lowered design margins relying on computers.
Changing topics
I was on a Qantas aeroplane flying to Australia from London and we dropped suddenly. I was taken to the cockpit [I couldn't see such a thing happening now] and the Flight Engineer pointed out that yes, as I’d thought, this was the plane which had dropped 15000 feet not two weeks before.
Sine wave. The computer had interpreted new and unusual data and had overcompensated, whereas it hadn’t needed to. Thus they dropped until finding a new ordinary situation which the receptors could cope with.
That was the thrust of the post, using two seemingly different examples but coming down to the same problem in the end.
Thank you, kind sirs.
Notes
* The variability of CB. I thought you might p;ick up on this and it is referring to gear breakage, buoyancy sections stove in and the like, obviously a lesser factor than altered CG but still present.
** To the question of why it has not sunk yet … jolly good luck and a design which caters for upwards of, say, 90% of situations.
James quotes me: Now, Nigel points out that “as long as one is kept below the other”.
Oh dear, did I really write that; it looks terribly imprecise to/for me. I hope I didn’t, and apologise if I did. I just cannot find it (by computer search) in either of the 2 posts on Nourishing Obscurity mentioning ship stability (November and October), nor anything similar in my archive of blog comments.
I think a lot of the disagreements that James and I have do centre around imprecision.
He references something as evidence for his viewpoint. I challenge that it is not really precise enough to be evidence. James agrees (partially or totally) and says somewhat similar things have happened and are much better evidence. But he provides no references to them; I then become unhappy and write, probably, far to much expressing that view.
On ship stability WRT The Oasis of the Seas and other large pleasure cruisers, we had first the ‘evidence’ of the Herald of Free Enterprise; now we have the ‘evidence’ of this tragedy in Indonesia. The disaster of the Herald of Free Enterprise was down to gross human error (leaving harbour with the bow doors open). James writes that the most recent Indonesian ferry disaster was down to overloading; I don’t know, except for newspaper reports I read yesterday, which mentioned that as a possibility, and mentioned structural damage as the immediate cause.
Overloading is not a stability design fault, it is an operational fault. I accept it is, very likely, a common problem with ferries in Indonesia; that still does not make it a stability design fault. Also, doubtless ferries travel more regularly in bad weather and sea conditions than do pleasure cruisers: they very likely have a higher risk per trip because of this. Ferries, also doubtless, are more numerous and travel more: they very likely suffer a greater absolute number of disasters because of this.
Best regards
Nigel, you are choosing to ignoe the main thrust of the argument which I have reiterated more than once now and have chosen to run with this “imprecision” thing, on the implied basis that establishing imprecision in an explanation [which you have not actually established in any way] is the same as imprecision in fact and therefore impinges on the veracity of the argument.
In the last comment, the connection was shown. One can lead an erudite man to water but one cannot …..
The only major ferry disasters I’m aware of in Europe are the Herald of Free Enterprise (which left shelter with its bow door still open) and the one in the Baltic a few years ago, cause unknown (unknown to me, anyway).
Like Nigel, I suspect the fact that there have been “hundreds” of disasters in Indonesia is probably due to (1) old, poorly-maintained (or unmaintained) ships – note that there’s an eyewitness account of structural failure in this case, (2) Gross overloading and cavalier operation outside of the ship’s specs and standards, (3) Inadequately-trained and qualified (or untrained and unqualified) officers and crew, (4) disregard for weather forecasts (or non-availability thereof), and other things of a similar nature which we can all imagine.
Modern ships do look top-heavy, I agree. Look at any cruise liner: horrible ugly things, it’s a wonder they stay upright at all. But they do, when competently run according to their design specs by people who know what they’re doing and pay attention to the requirements and conditions. Have you heard of any First-World liners capsizing recently? It’s a bit silly of us chatterers here to start second-guessing professional naval architects, imho. The aesthetics we can all debate, but the functional design is a technical matter which I hesitate even to comment on.
Outriggers on a ferry? I fear, James, that you’re allowing your enthusiasm for multihulls to get the better of you. Can you imagine the impact such a thing would have on the harbour and berthing arrangements? The cost of doubling or trebling the width of every ferry berth? The resulting reduced capacity? I think it’s pretty much a non-starter.