– Crash reconstruction in English follows after intro in Swedish –
Olyckor har alltid flera orsaker. Men ett och samma fenomen från fysiken kan ha varit avgörande för de svåra skadorna både på Titanic 1912 och på Costa Concordia 2012.
Det beskriver jag nedan på engelska, eftersom jag inte har märkt av någon reaktion från farkostdynamisk expertis efter mina tidigare redogörelser på svenska. Se länkar till artiklarna i listan med etiketten Costa Concordia.
Däremot förekommer diverse spekulationer om andra orsaker, som förstås också kan ha bidragit. Men den optiska illusion, som Smithsonian beskriver enligt ett referat i DN, kan knappast ha haft någon betydelse för haveriet med Costa Concordia.
Eftersom både Titanic och Costa Concordia framfördes i hög fart nära is- respektive landmassor borde sugfenomenet uppmärksammas i utbildningen av ansvariga besättningar och trafikledare.
Problemet tycks vara välkänt för kaptener och styrmän på båtar i trånga farleder där hastigheten är låg men avstånden i sidled desto mindre. Men kanske har man hos de stora kryssningsrederierna inte tänkt på att de sugande sidkrafterna ökar kvadratiskt med hastigheten. Frågan är om det beaktas i det säkerhetsarbete, som beskrivs 30mars av DN.
Både Titanic och Costa Concordia uppges ha hållit nästan full fart vid sammanstötningen, trots att åtminstone Costa Concordia girade tvärt under storleksordningen en minut dessförinnan. Se rekonstruktionsbilden och vittnesuppgiften från Jan Mosander i den engelska texten nedan.
Same hazard behind Titanic and Costa Concordia disasters?
Both the 1912 Titanic and the 2012 Costa Concordia disasters may have been triggered by hydrodynamic suction forces. They will at least aggravate the damage and hull penetration in this type of lateral impact.
Nothing has been found, though, on this quite devious phenomenon in recent media interviews of various experts.
In news articles on the January 13 accident in Italy with Costa Concordia, it seems as if the interviewees are unaware of this so called bank effect.
The bank effect may be well-known among many physicists as a consequence of Bernoulli’s pressure reduction in a fluid moving parallell to a surface. Doubling the velocity will change the pressure about four times.
Both the Titanic and the Costa Concordia hulls were penetrated on their side while they were travelling at cruising speed
(15knots for Costa Concordia).
Therefore, it seems reasonable to conclude that – just before impact – considerable suction (attraction) forces acted on their hulls from the resting icebergs (1912) and rocks (2012).
Irrespective of heading and side slip in the pre-impact manoeuvre, the port side of Costa Concordia must have been accelerated towards the rocks by the bank effect – thereby being deeper penetrated than if no bank effect had existed.
It is true that Chris McKesson, professor of naval architecture at the University of New Orleans, pointed out the similarities between the Titanic and Costa Concordia side damages in a CNN article on January 16.
However, professor McKesson tries to explain the lateral damage by referring to a low-speed non-dynamic response of rear wheel steered ground vehicles for on- and off-road transportation: “But just like when you’re driving an RV or something, when you swing the nose to the right, the tail swings little to the left.” (CNN, 25th paragraph)
Even for ground vehicles, that kind of response is limited to low speed conditions. Dynamic effects become decisive at more normal driving speeds (Strandberg et al, 1982). Since the Costa Concordia speed on impact was above 10 knots, dynamic (bank) effects seem more relevant to me.
After a major accident it is often blamed on some present individual (“human error”). Though accidents are unintentional with multiple causes, we have a tendency to disregard devious system properties and conclude that the driver (captain) has “caused” the crash.
Thereby, accidents like this may be repeated in the future when other vessels travel at crusing speed close to the shore and underwater banks. Hundred years ago, the Titanic would hardly have been so close to the icebergs, if the crew had been aware of the bank effect.
People condemning the Costa Concordia captain for the pre-crash manoeuvers, may be more directed to prevention if they consider the fact that Costa Cruises, employer of the crew, wanted the ship to travel close to the small island Giglio.
According to an article by SVT, the Swedish Public Television, Costa Concordia deviated to the small island, Giglio, from her ordinary route more than 50 times before the disaster. Once, in August 2011, the ship was very close to the rocks penetrating the hull on Friday, January 13, 2012. See trajectories from Lloyd’s [PDF] published by the Swedish newspaper, Dagens Nyheter, DN on January 19.
Though it has been recorded by AIS, all this risk taking seem to have been accepted by the authories. Some claim that the deviations even were encouraged and promoted by the captain superiors at Costa Cruises.
RECONSTRUCTION OF THE PRE-CRASH EVENTS
On January 13, 2012 at about 2145 hours the left side of the passenger ship Costa Concordia crashed into an underwater rock near the small island Giglio at the west coast of Italy.
Most of the 4200 occupants escaped without injuries from the ship while it had come to rest on an underwater bank with its left (port) side above the water surface.
A great deal of information on the disaster has been collected and commented in Swedish via the blog bildrullen.se and published by the Newsmill editors at the addresses
One of the passengers, the Swedish journalist Jan Mosander, contributed information via a Newsmill comment form, containing links to professional data and a video reconstruction of the ship motion during the final minutes:
The video is narrated by an experienced captain describing the course of events.
Below you may find transcriptions from his speech and their elapsed time in the video clip (duration: 14 minutes, 30 seconds).
02m26s “The first mistake here was that the vessel should have slowed down, when she approached this island.”
Speed: 15-16 knots
04m30s “It look as if she turned left too late, later than the captain had even anticipated”
Speed: 15-16 knots
04m35s “From his verbal testimony we were told that he was navigating visually … that was a major mistake taken here.”
Speed: 15-16 knots
05m…s Some transcript paragraphs to be developed:
Explanation of side slip, yaw inertia and vehicle dynamics phenomena relevant to the lateral impact and other characteristics of the pre-crash motion. “…”
Speed: >10 knots
06m20s “This is the point of impact.”
Speed: 13 knots
07m15s “Now [s]he is clear of the rock. It is the point of black-out and the point where he [the captain, Francesco Schettino] should have called Mayday, sounded the general alarm, and ordered everybody into the life-boats.”
Speed: 8 knots