Would not have thought a 1 foot range of tide would cause much of a current even mid tide.
Just to put it in perspective we needed a tidal height of around 7 meters to generate a 5 KT current
Otherwise your theory seems plausible. Was there strong current caused by heavy rainfall? Don’t suppose anyone without detailed local knowledge would know
The salvage divers working on the bridge reported currents in the order of 1 knot.
Very interesting reading from the experienced pilots on steerage (or lack thereof) with engines off and the SG is more or less useless. What bothers me is that there is not even the slightest course change to the left. And then there is the last minute right heading straight to the bridge. NTSB report does say the SG3 came on and that the speed is slow and not very effective. Intriguing if this theory is correct - probably an eye opener to many Masters (and pilots).
I don’t know that I would consider 9 knots to be high speed, especially on a slow bell.
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Chief MAKOi mentions the possibly of low fuel oil pressure which was an issue during the blackout while alongside. Also mentions the possibility of procedural errors.
That’s along with the known issues with the switchboard / breaker. It’s often the case that when there more than one cause involved it can be a real puzzle.
Added the transcript here.
KC,
Yes, he also makes particular mention of the fact that the engineers were closing breakers manually. Perhaps the PMS had issues or they were attempting to minimise the outage timeframe by going manual. AV2013 indicated that the switchboard issues may be a combination of both design and procedure.
The fact that No.2 (standby) generator fired up automatically and remained running, being mindful that it was on the common fuel line to No.s 3 & 4 doesn’t lend weight to the supposition of low fuel tank levels.
The NTSB were wise to involve the Korean technicians at an early stage and the tamper proof seals placed on the HV switchboard makes somewhat of a statement.
Perhaps the preliminary report content has been structured around the forecast tsunami of future litigation.
It is indeed puzzling.
Bank effect would not even be a minor issue for transiting vessels with fully functioning engine and rudder at high or low speed. But our subject is a vessel without engine power or steering. In that case bank effect becomes a major issue. Neither wind nor current have been identified as factors.
The question was raised by several knowledgeable participants in the discussion as to why the Dali began turning to the right instead of continuing in a straight line and why left rudder, if applied, had no visible effect. My post suggested a possible explanation.
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1 knot current would be typical in the Chesapeake bay. 1.5 considered a lot. I have never seen bay current measured at 2 knots. Certainly don’t know them all but the wide shallow bays without obstructions soak up tides effects.
Boats3,
Many thanks. I suspected that may have been the case. A genuine stab in the dark.
One Eighteen,
I stand corrected.
A bit more than I would have expected I must say.
Thanks for the information.
I was a bit sceptical about the divers reports of 1 KT currents as bubblheads always over egg it.
Facts: LOA=299.0 m . > Lpp is abt 287 m, B= 48 m .
As per report : T= F=A= 39.9 ft even keel. = 12.16 m. Depth of the channel H= 50 ft= 15.24 m.
Some coeficients. H/T= 50/39.9= 1.2531
Lpp/B= 287/48= 5.98 very low Lpp/B ratio and bluff hull form
Have made another inventory of comments on M/V Dali and discovered with regret , that experienced pilots and some captains elect to elaborate on unknowns instead of on “knowns” . Above and below are “knowns” and it would be great if some would comment on Lpp/B ratio and it’s influence on ship’s maneuverability as well as other known factors afecting vessell behaviour in confined pilotage waters .
Below material may indicate there is a lot to chew on the road of discovery as to why Dali turned where it turned and hit what she hit.
Likewise I am still waiting for some competent engineer to explain exactly what it takes to perform " emcy crash stop" when vessel is on DSAH in " maneuvering" mode with " bridge control" . Acc To Chief Eng Makoi it is a problem to reverse M/E when the vessel is at 8 kts and M/E is stopped but the prop is still turning. It appears as per SOLAS it should rather not be a problem . So where is the problem then?
It has been observed that a vessel with a block coefficient above 0.7, that is, if the vessel is considerably bluff ( see rem.)in hull-from , there is a tendency to trim forward, assuming other conditions as fixed and the vessel previously on even keel. Conversely, for vessels that are finer in form (block coefficient < 0.7), there is a trimming tendency aftward.
Influence of the ship’s trim:
From all of the above parameters,the captain and pilot must focus attention on the ship’s trim, which varies with loading and ballast, since it influences both turning and directional stability.
A ship with a positive trim (by the ste rn) easily begins to turn under the rudder effect. The lack of squat in straight bow shapes reduces the acceleration of the turn caused by the establishment of transverse components of hull resistances. The ship turns more easily during the drift turning phase. Reversing the helm easily stops the turn.
A ship with a negative trim (by the bow) loses some of its rudder efficiency.
Conversely the squat of stralght bow shapes improves the hull’s capacity to counter drift. The hull resistance applied at Cd’ and the centrifugal force sharply accelerate the turn. Although the radius of turn is reduced, this acceleration may in some cases exceed the rudder’s turning capacity, which is no longer effective enough to stop the turning inertia.
The ship can lose its directional stability and become uncontrollable. It is preferable therefore to avoid loading the ship with a negative trim, especially if it is known that the hull’s confinement will increase the latter’s resistance effects (in a narrow channel, for instance).
REM: Underwater hullform and coefficients
For any ship operating in open water, carrying capacity must be traded off, to some extent, againts seakepping performance. Such hull can be considered as three seprate sections merging into each oter along the ship’s length. The forward and aft sections are finely tapered towards the bow and stern where as the midship parallel body section is box shaped. The proportion of this centre region to the total underwater lenghth varies from one design to another. Ships with fine lines have relatively short length of parallel body where as bluff, of full formed, hulls may be box shaped for over half of their length.
Cheers.
First, let me say I am, by no means, trying to blame bank effect for the accident. The cause is clearly whatever led to the blackout. I am only addressing other contributors questions about what might have caused the ship to sheer to starboard and why they couldn’t straighten her out with left rudder (if there was rudder available).
If the screenshots from AusMariner are correct they reinforce my point about bank effect. In the first image the ship is almost entirely in the left half of the channel. The centerline runs along the right side of the ship. I would guess that even with the engine running ok the Dali would have turned right if left to her own devices…. which, pretty quickly, she was.
Her position was not an issue until rudder was lost. For the non-pilots, imagine you’re driving on a two lane road that is slightly crowned in the middle for rain runoff. If you suddenly lose steering the car will likely start slowly moving away from center. If unchecked she’ll run into the ditch.
In the second screenshot the Dali has moved to a position where her bow is in the flare but her stern is still close to the bank on the port quarter. Bank effect is still a factor.
In the last screenshot the bank effect was probably negligible but she’s only a ship length from the bridge.
I hope no one finishes reading this and thinks bank effect caused the accident.
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Described bank effect could trigger swing to sbd , which in the absence of rudder and M/E action due to nonavailability , accelerated to the point ,when it exceeded the rudder capacity to stop the turning inertia , especialy when the prop wash was not available.
Given the fact , Dali could be trimming by the head , the contribution of bank effect can not be neglected.
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I will give it a try and explain this: reversing an engine which is still turning to Ahead direction due to ship inertia can only be done when the RPM reduces below a certain value, otherwise the reversing will not be successful. When the telegraph is put in Emergency Astern, the fuel is cut and nothing is happening until the RPM drops below the breaking air value. Only then the breaking air is released into the cylinders and the direction of rotation is reversed to Astern.
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Follow-up question: So, even if 8 kn is her minimal achievable speed going foreward [lower limit of RPM envelope], the ME RPM associated with this speed is not identical to the minimal RPM for air reversal? (Sorry, no experience with direct coupling)
Thank You for Your kindness and understanding. Very interesting.
As long as the engine RPM running Ahead is below the breaking air RPM, the reversal should be very fast, no waiting time. Now the air pressure is also a factor, if there is not enough the reversal may not be successful. Breaking air RPM should be 15-30% of MCR RPM, this is from an engine manual, I guess it should have been set during trials.
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I was told that the Maersk Triple E were originally made twin screw as there was not a single engine large enough combined with poor flow to a single propeller to give service speed required. The solution was 2 big engines with offset propellers. It then became apparent that economical steaming with one engine working efficiently was very beneficial.
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