Critical speeds

I was recently involved in a study where we were trying to identify some problematic vibration issues on a rather large boat (built in 1982).

At one point, I noticed a plaque in the operating room giving some technical specifications on the boat, and one line read : "Critical Speeds : 90-110 rpm".

Coincidentally, this happens to be the problematic zone of vibration.
Between those speeds, a frequency of about 7300 cpm (121 Hz) is excited in the longitudinal direction at the rear of the boat. An impact test on the rear bulkhead (right behind the 2 propellers) confirmed a natural frequency of 7280 cpm.

So I'm trying to understand the source of excitation of this natural frequency, which leads me to these questions:

What would be the most common reasons for critical speeds on boats (shaft rotation, structural resonance, turbulent flow, etc.)?

Do all boats have "Critical Speed" zones, or is it only older models before FEM was made available?

Thanks for reading!

Alex

 

It can on or several of those causes. Another common cause is the vibration caused by the propeller blades as they approach the hull. Sometimes it is possible to correct the problem. Otherwise, the operators need to avoid those critical speeds. In the case you mention, the bulkhead seems like the cause. It is interesting that the frequency is about 10-15% higher than the shaft RPM.

 

The excited frequency is very far from the shaft/vane pass frequency:
Natural frequency of bulkhead : 7300 cpm
Rotational speed of shafts : 100 rpm
Vane pass frequency (4x) : 400 rpm

It would take quite a bit a energy to excite at that frequency.
Cavitation is also a possible cause, but unlikely...

 

Can you explain what do you mean by excited in the longitudinal direction of the rear of the boat?

 

Critical speed in this case is the resonant frequency of the hull relative to the engine. At that RPM range the engine vibrations equal the natural frequency of the hull, or some part of it, and so the amplitude of the vibrations is amplified. The trick is to nope operate at those RPM ranges, or, if that is not possible, it might be possible to change the natural frequency of the part in question, but that's going to be a lot harder.

 

It may not be the machinery but something else. For example, the rudders vibrating at that speed. Does it make any difference with a head or a following strong wind?

 

Critical speed of the engine (shaft) is mostly determined by the shaft diameter. It is an harmonic frequency that is distructive if allowed to continue. If the prop shaft is fairly thin for the setup, it may be possible to add a plumbers block type bearing to help support the free shaft lengths.

 

I do not think the rotation of the shaft has much to do with the excited resonant frequency for 3 reasons:

1 - Shaft vibration (measured on the bearing supports) do not change through the rpm range and are quite low.... only the hull vibrates in those "critical speed" zones.
2 - The resonant frequency (122 Hz) is very far from roational frequency of the shafts (1.7 Hz).
3 - When both engines are running at 100 rpms and the vibration is at its worst, vibrations stop immediately when one of the engines (starboard or port) is shut off.

What I am trying to figure out is why the manufacturer identified these "critical speed" zones, and the possible causes.

Could the wake (or turbulent flow of the water) excite the natural frequency of the rear bulkhead?

 

Vibrations are predominant in the longitudinal (or axial or X) direction. Only the rear of the boat are subject to these vibrations.

 

There are a lot of other operating parts in the engine/drive trains than the shafts and props. The excitation can be a harmonic of the valve operation or injection pumps between the two engines. Even some other part of the machinery, but at 121 Hz, it is very likely a higher harmonic of whatever is causing it.

 

What type of ship are we talking about here? What are it's dimensions, draft and displacement? What type of engines does it have, how many cylinders for each engine, how many valves for each cylinder? What is the drivetrain transmission ratio? How long are the shafts, what is their diameter and how many bearings do they have? What is the prop diameter and how many blades do the props have?

Each of these factors play a role in the vibration analysis. Vibrations are still an ugly beast to fight, despite of all the FEM tools we have.

The plate with the warning about ship's critical speed is there for a good purpose. It means that the ship has been trialed, it's critical speed has been measured, and as a result of the trial the plate has been exposed as a warning to the crew that rpms between 90 and 110 have to be avoided in the continuous operation.

 

That only the rear of the boat vibrates does not mean the vibration is in a longitudinal direction, only that it is created in that area. They didn't correct the problem, but gave you a way to avoid it.

 

There's quite a lot of energy in the exhaust blasts before the mufflers. Is the frequency of the exhaust blasts close to 121 Hz? At 1200 engine rpm the exhaust blast frequency should be 60 Hz for a straight 6. With two engines running at the same rpm, the combined exhaust would generate 120 Hz. This would also explain why the vibrations go away when one engine is shut off.

Erik

 

We are talking about a 98m (322') Class III Arctic Ice breaker.

The boat is propelled directly by 2 independend electric motors (fed by 6 diesel engines located in the middle of the boat). Shaft size is about 20" with 4 blade props.

I beleive there are many possible causes for these specific vibrations, but right now I'm trying to understand the causes of boat critical speeds in general, designed and fabricated in the early 80s (the reason being excessive vibrations).

Another thing I'm trying to figure out is if boats designed nowadays have these same critical speeds that are to be avoided... or has modern technology or design trial and errors corrected these issues of the past.

 

That's a good candidate for a high energy source exciting high frequencies... but test were conducted with 2-3-4-6 diesel engines in different combinations and did not affect the natural frequency in the slightest. So we've ruled out the diesel engines as potential culprits.