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.
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Does seem that is more likely the case. If 65-68 RPM was just the barred range seems unlikely that fact would be passed on to Synergy.
That rpm restriction is imposed to prevent torsional vibrations from damaging the crankshaft counterweights on some engines. The weights move slightly to counter torsional loads.
In my experience, I have not seen any counterweights on crankshafts of slow speed 2 stroke diesels. Essentially when the cylinder fires, the load on the crankpin is a shock load (to turn or rotate) the crankshaft. This shock load deflects the crankpin in the direction of rotation (well within elastic limits) and the reaction is it bounces back past its normal no load position in the opposite direction. This is the torsional vibration. Frequency or number or cycles every rotation is damped or reduced by the torsional vibration damper fitted at the free end of the engine to limit the cyclical load that may lead to fatigue failure of the crankshaft. Older engine with a large pressure differential between the compression and max (or firing) pressure was quite pronounced (about 30 bar). It is much less on the newer engines (less than 10 bar and a much smoother pressure rise) such as MC, ME, RTA, etc and thus the torsional vibrations are much less of an issue.
It is also fair to say that the torsional vibration of the engine or restricted/barred rpm range does not have a bearing on hull or structural vibrations. Excitation from the axial and transverse vibrations from the main engine and/or prop induced vibrations if they are close to the natural frequency of the structure resonance type enhanced vibrations could be experienced.
PS: Counter weights commonly found on medium speeds (that are not supposed to move) have fretting on the mating surfaces (due to micro movements and a hammer effect on the mating surfaces). These are generally not maintenance items. Not easy to address in place without removing the crank. I have seen engines with counterweights thrown out of the crankcase some with catastrophic damage to the engine block as well.
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The counterweights are on the airplane engines in the previous post. Some airplane tachs have a ādonāt stay hereā range because of that.
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I have no experience or any knowledge on aircraft radial engines - but admire the engineering. Variable speed and variable load introduces another dimension of complexity.
Anecdote: Alan Mulaly when he came in as CEO of Ford (after being CEO of Boeing), initially was not accepted as a ācar guyā. Supposedly in their 7 am world wide call, he asked the group how many moving parts in a car and the proud group replied - about 40K. Alan replied - interesting - on aircrafts it is about 120K ā¦ and we need to ensure they keep performing when they are up in the air. Needless to say he got the respect that was deserved almost immediately after this episode.
I should have mentioned that my response was to yacht-sailorās post about aircraft engines and not all engines in general.
It is interesting you mentioned radial engines. They have just 2 counterweights opposite the single crankpin for each row of up to 9 cylinders while the more common opposed piston crankshaft has 2 diametrically opposed counterweights for each pair of cylinders.
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Asking legal eagles of gCaptain forum for opinion and/or comment.
Have just come across an interesting Trade Winds podcast regarding loose electrical wire on Dali. Speakers there suggest that the very cable can save Owners quite a lot of money as far as claims are concerned.
Pls comment.