I would disagree. If the ship is on a load program from 60 RPM I would expect the critical to be well below that.
I agree, that it seems high for a critical speed, ours was around 38-43 IIRC, between slow and half speed. Load up started at about 65 or thereabouts. But full sea speed was 95+ RPM.
Soā¦nothing definitive, maybe someone else has better information. Is 65-68 RPM a reasonable, normal sea speed?
Also - the note says the engine cannot be run at that speed so that would imply that the engine was not run at those speeds.
Forgive my ignorance of giant diesels. Is this āvibration rangeā like some airplanes that have a no-go zone on the tach you can pass through either direction, but you canāt stay there due to destructive vibration? Not a common thing, but some engine-prop combinations in some airframes have this.
One would hope a ship would not be cruising around in this RPM range shaking everything apart.
Critical speeds to are very common on motorships. I have seen them on steamships as well but thatās not the norm.
I would interpret that ācan not be runā as āit shakes too fucking much so we canāt run it there.ā
I worked on a ship that, at the RPM for Full Ahead, the ship shook like crazy. We had a lot of growth on the hull. The solution was either 2 RPM below or above the Full number, and all was well.
high speed diesels can have a critical speed as well
Plenty of bolted together but not engineered drive trains have this but you dont know till it destroys the couplings or rips the 'torsional dampers apart"
Critical speeds for the main engine is usually associated with torsional vibrations. If the engine has this so called ābarred rangeā then the load program would be designed with the ācritical speed jumpā. I have only seen this on Sulzer RND engines and above the full maneuvering ahead rpm.
On the other hand, on this vessel it looks like there is some sort of resonance at the 60 rpm and does not really diminish at full sea speed rpm. Natural freq is sq root of āKā (stiffness) divided by mass. Looks like the surrounding structure around the engine room is quite flimsy and excitation from the engine/prop at that rpm is close to the natural freq of the structure. Owner/management company should have investigated this. DnV has pretty good vibration analysis staff.
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Iāve wondered about the same thing. Having some experience with vibration analysis I wonder about the quality of the steel used in construction.It would take a lot of study to see if that was the case, metallurgical tests etc but I think the case will be closed well before any thing of that nature happens
Possible. High tensile steel is widely used on modern container ships. So less steel means less mass - (but also as an aside, no good for fatigue life).
Sometimes involving the engine maker to see if the firing order is revised or ādesignā some imbalance in the engine such that the first order of vibrations excitations are not āconsistentā. Kind of confuse the surrounding structure.
With the cutthroat rates, most of these 2nd or 3rd tier owners do not have the vision or the means to conduct any studies. The so called āflushing pumpā is a prime example.
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Pretty well all the slow speed diesels I have sailed with have had a critical rev range and it has been typically lower than manoeuvring full ahead. The engine management system steps through the rev range as quick as it can.
A critical rev range also existed on a class of steam turbine powered frigates at about 17 to 18 knots and they bounced up and down briefly as they passed through the rev range. There was one frigate of the same class that swayed from side to side. One admiral on an inspection, was heard to remark āthis.ship has spent too long in the Far East.ā
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Funny! 
Dali revs at 90% MCR above .Source NK Class .
From 60 to 82.5 load program up/down , takes about 35 to 40 min abt 1.5 to 2 minutes per 1 revolution.Down is abt half of this .
So from 65 to 68 revs it is six minutes of vibrations that may cause the loss of radar antena or other parts. It had to have the ācritical speed jumpā if not one may cancell load program and and jump over the range manualy with the telegraph handle.
But have never sailed on such slow engines. Critical revs were always btw dsah and sah in manoeuvring mode well marked on the telegraph and highlighted in red on a plaque with revs nearby .
Vibrations also depend on loading and ballst condition . You can not do anything with cargo but can experiment with ballast. It often worked well reducing the vibrations.
Cheers
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Main engine is 9S90ME. The S is for super long stroke and as such low revs due to the limitation on the MPS (mean piston speed). Prop efficiency is probably better. I think it was first intended for tankers and guess found favor with container ships as well.
PS: I do not think Dali has a critical speed. The heavy vibrations around 60 is crew experience and info being passed on the hand over.