I went and checked and the 70% is not in SOLAS, just class rule via IACS. But it isn’t a power figure, it’s revolutions corresponding to max ahead continuous power revs. Think it has more to do with auxiliaries but I don’t have a reference to show it. There’s a separate rule for steam turbine but comparable and typically the astern turbine is lower capacity/size than full ahead… I’m not digging out the tech background books… is what it is.
MB executed, per the chartlet, emergency astern for two minutes at the end, following full ahead at time of crane strake.
I sailed on tankers with steam turbine propulsion but as I remember it the built-in astern turbine gave of up to only 50% astern output power as that of the ahead turbine. It made some captains, who were used to 12 knots motor ships, very nervous sailing with 20 knots and having only half the astern power. This was the thrust but with the low efficiency of the propellor when going astern it was said that the total astern power was no more then 20%…
In this case backing power doesn’t seem like it was an important factor.
Having said that, with a diesel going astern requires that the engine is stopped and restarted with the opposite rotation so the amount of power available is the same. So in theory the only issue would be the reduced efficiency of the propeller.
In practice, as was mentioned a low-speed diesel engine cannot be started in reverse against the force of the propeller which would be freewheeling unless the speed is under around 6 kts or so. Attempts to do so would likely just result in running out of start air.
Not sure about the case of a crash stop in which case I think it just dumps all the air in a single all or nothing attempt?
Optimisation of the crash-stop manoeuvre of vessels employing slow-speed two-stroke engines and fixed pitch propellers
The combination of a slow-speed two-stroke diesel engine and a fixed pitch propeller is the most favourable propulsion system for large seagoing vessels, since both components feature outstanding simplicity and efficiency. However, they are disadvantageous where manoeuvrability is concerned. The crash-stop performance of such vessels is characterised by relatively long stopping distances.
Question remains if going around in a tight circle, with the help of the tugs, while losing most of the speed, would have been possible. There seems to be room for that maneuver apart from traffic. What are your thoughts on that?
Don’t know, I’d think the investigation is going to have to involve a simulator.
In most European ports in tight quarters most likely you’d see the two tugs made up on the centerline fore and aft on short leads. High-horsepower tugs set up like that can really muscle a ship around. Even set up like that however I’ve had up to four tugs on a windy day.
Both the captain and pilot are required to have a quantitative, rather than an intuitive exchange of information, based on experience, understanding of the stopping distance and the time required to stop.
Crash stop data gathered during sea trials should be available on each ship as outlined and required by IMO, also on board the Milano Bridge. Manoeuvers required during sea trials by IMO standards include turning circle, zig-zag and full astern stopping tests.
1.2.2.14 The “crash-stop” or “crash-astern” manoeuvre is mainly a test of engine functioning and propeller reversal. The stopping distance is essentially a function of the ratio of astern power to ship displacement. A test for the stopping distance from full speed has been included in the Standards in order to allow a comparison with hard-over turning results in terms of initial speed drop and lateral deviations.
In view of the Rules of the Road it is important that the ship turns to starboard.
The stopping distance should not exceed 15 ship lengths. (SOLAS Chapter II-1, Reg. 28). The stopping distances are measured during a crash stop test, which is carried out for every ship during sea trials.
With a steam turbine the stopping distance is reduced if the propeller is held stopped by using the astern turbine until the speed drops below about 8 knots.
In an actual trial with a conventional low speed diesel of 18,500kw and 30,000 dwt a 70 tonne bollard pull tractor tug on the centre lead aft was able to stop the ship from just over 4 knots with the engine at dead slow ahead.
With all due respect, a simulator will more than likely come up with similar results as what us old timers in our recliners observed from the very start. Turned late, was going too fast, and got set with the wind. No room to bail. Extra tug would have been helpful, even then, way too fast approach, and too close… JMHO
If a simulator run gives the same results that would be ideal.
What would be interesting is to adjust the parameters and watch the change in the outcome.
What needs to change to make that turn? For example:
keep everything the same including the speed but change pilot orders to wheel, helm and tugs, use full rudder at the beginning for example, set the ship up for the turn better etc.
increase the draft to normal “in ballast” condition
shift tugs to centerline leads fore and aft
decrease LOA to around 200 meters.
add a tug or two / increase tug power
I agree there has been a lot good observations here but it would be interesting to see them put to the test.
It would be interesting to see the simulation. I would like to see a simulation where the tugs are placed one centreline aft and one starboard shoulder. Engine dead slow well before the turn. The turn started with full starboard rudder at least 1 cable before the actual turn which would bleed off headway. As she was steadied on the approach the after tug was instructed to lay back to keep the speed down to 4 knots. she should pull up with little drama with a dead slow astern off the berth. The bow thruster together with a bit of transverse thrust and the onshore breeze would move her onto the berth. the two tugs in a position to lift off if the lateral movement was too fast and keeping the ship parallel with the quayside.
The Milano Bridge LOA is 364.95 meters - the three ship in the document above are 400 meters.
Dead slow for the three ULCS is listed as 10.4/6.3/5.0 kts.
The Astern power is given as abt 35% of ahead
Advance is given as 3/3.1/2.4 units being LPP which is given as about 400 meters. Advance would be 3*LPP or about 1200 meters.
At 7 meters draft the area above the water line in m2 is 10,670 / below the water line 2770
At the position off the yellow “C” buoy there is roughly 610 meters or 365/610 = 1.7 ship lengths to make the turn which is about half of what’s required.
So the tugs would need to roughly double the turn rate.
According to a report published on May 7 by the South Korea’s Ministry of Oceans and Fisheries (MOF) the container ship Milano Bridge did not reduce its speed as it approached a pier in Busan New Port on 6 April 2020, causing it to knock into and demolish a gantry crane, investigations showed. That is one of the conclusions.
KMST noted that Milano Bridge sailed towards pier #2 at a speed of 8 knots, which was higher than the usual speed of 6 knots when berthing. Wind speed at the time was 5 to 8 metres per second, which is considered normal.
I am somewhat surprised about the berthing speed of 6 knots that was mentioned as allowable, while I think that this is still much too high.
I always thought the ‘Milano Bridge’ arrived from a yard just around the corner, from Busan or Geoje.
In a hurry during this short movement, they could have overlooked some essential tests or preparations before docking.
This report says, they arrived from Zhousan in China, around 500 NM away.
Hence, they had all the necessary time to test and prepare their ship before arriving at Busan port.
From Dutchie’s link >>>
…entered the port with about one-third of its propeller exposed above the water surface because it was not carrying sufficient ballast water. ‘Milano Bridge’ was ballasting at the time…
Does this say, they ballasted only, when they had seen the ship’s poor reactions to rudder and propeller, already inside the Busan port?
Engine and rudder. What else is needed to move a ship?
I missed this detail.
That’s a day’s run. Assume ballasting from shipyard to “in ballast” would take several hours, I’d guess about 10hrs. Any number of reasons why it wasn’t done. Crew could have had other things to deal with… Last time we left the yard we finished the sea trial late at night, everyone was exhausted. Way over work/rest requirements.
Presumably when they say “berthing speed” they are talking about earlier in the approach, when the ship was doing 8 kts. For actually berthing fwd speed is going to be close to zero.
The operations of Milano Bridge at the time of the accident were also simulated, using the voyage data and statements from related persons, as well as similar weather, tidal, and wind conditions.
The simulations showed that when the propeller is fully under water, the ship had better manoeuvrability and a lower risk of accidents.
KMST also calculated that the accident could have been avoided if Milano Bridge had slowed to less than 7 knots when approaching the pier.
So they did run a simulation. The way I read it the error of coming in too hot could have been corrected had they had full immersion or the error of the failure to ballast could have been corrected had they been going slower.
Just wanted to toss out something that came up when I was involved in new buildings. One of the companies I dealt with was a german company. I was talking to one of their superintendents and he mentioned one of the issues they had with their big containerships was operating at dead slow was often still too fast even with multiple cylinders cut out.
