Something to puzzle over

[Revolver]

Engine, Wartsila 6L38, is physically running at proper RPM and set at fixed speed; shaft generator has proper Hz. And the engine monitoring system, the computer in the ctrl room, is reading proper 599-601 rpm.

But the local control panel reading, and analogue reading in control room/bridge, reads roughly 580rpm not 600.
Seems like something may be robbing some mA from the signal, and looks like the robbing of a few mA is causing the external stby pumps to kick on even though pressures are normal.

What's clever too is; it was a problem, and then during operations for the past 2 weeks the problem wasn't there, then it came back...

Only Differences I can currently think of:
Salt Water vs Fresh water.
No problem in salt water.
We're sitting deeper in fresh water...or difference in conductivity...
And we were in various states of loaded and unloaded during salt water operations and no issue. Currently loaded and into fresh water.

An interesting puzzle...
Any thoughts?

[Merlyn]

This a genset?
RPM signal from the CAS?
When you say shaft generator do you mean Crankshaft generator or output shaft generator?

[JK]

minor ground somewhere?

[Big Pete]

I am assuming from what you say that the local and Bridge RPM signals are analogue, while the other readings are all Digital.

The thought about the Conductivity of SW versus that of FW is interesting, but you are losing indicated RPM /mAs /Volts in Fresh Water which is an insulator so it appears a bit back to front, so I am not sure how that would work. That is not to say that that isn't the cause, but if that is the cause it will be through an INDIRECT mechanism.

You used to be able to adjust the readings on analogue Tachos with a small potentiometer on the back of the gauge, which would solve the problem with the readouts, but not the standby pumps kicking in. Is there any way of adjusting the RPM at which the stand by pumps kick in and out? Or is it all pre-programmed in the magic computer?

Good luck and let us know what you find.

BP.

[Revolver]

Main Engine, Red. Gear box - shaft generator.
Soon as the M.E Starts, the SG starts turning and so does the CPP. Once the engine is at 100% RPM setting it's banging at 600 RPM, we go to fixed pitch and synch the SG online.

ECC and Bridge gauge IS analogue - on the local control panel it's a digital reading. (And I do believe that's where the Analogues get their signal).
My same thoughts about SW vs FW conductivity, BP; it was just the only difference in conditions that was sticking out.

All pre-progammed in the magic computer.
The stby pumps kick in based on pressure signals, but it seems if a couple mA are being skimmed from them even though the actual pressures are normal they get a lower signal telling them to start up thinking the pressure is lower than it is.

While off watch of course I was dreaming about the problem; a ground was running through my mind. I see JK mentioned ground.
But remember, this happened in fresh water on the lakes. Then we were in salt water, no problem. Now back in fresh water and it's back again. I would have figured if it was a ground in the electronics it would have been consistent.


Things also mulling round in my mind relating to the difference SW v FW:
Shaft earthing device...not really looking too much into that. Band looks nice and clean, should be doing it's job.
ICCP system - when in salt water, system is comfortable doing it's thing. Now back in fresh water it's in alarm state and not as happy.


*That ICCP system operating in FW brings up another question from me though.
When in FW instead of salt water and putting charge into the hull - could that result in the brittlement of hull steel and actually be more of a detriment than protection.
Or maybe my galvanic and electrolytic action thoughts were just my dreams running away from me hah

[Revolver]

Also on an unfortunate note, because curiosity kills cats, we're running now and I'm going home next port so I won't be able to explore the issue like I want to.
I'll be sure to give any updates when I can.

[Big Pete]

Maybe the impressed current takes the line of least resistance,ie through the Sea when in Salt Water but when in Fresh it finds that the path of least resistance is through your electrical control systems?
As you say the ICPP won,t be doing much in Fresh Water, I would check the makers manual, the ICPP systems should certainly be shut off when you are alongside another steel structure and possibly in Fresh Water too.
It is well worth shutting of the ICPP when in Fresh water to see if that solves the problem.
Without any corrosion protection you have a big "Battery" formed by the steel Hull and Bronze Propeller, the ICPP should bring the steel Hull potential Volts to the same as the Bronze Prop and stop the flow of current between them which deposits steel ions on the surface of the prop, where they are promptly washed away. It may also have links to the shaft Earthing, you may have nearly cracked this one on your own,
In Salt Water you use Zinc sacrificial anodes on steel Hulls but in Fresh water they don't work and you use Magnesium sacrificial anodes, because the metals react differently with each other in Fresh and Salt Water.

Good luck with the problem solving and have a good leave.

BP.

[D Winsor]

I was on a couple of vessels with an ICPP system that was installed after the vessel was built and we would shut the system off when the ship went into the Lakes. We were also told by the manufacturer that it would serve no real purpose while in fresh water because the system required salt in the water to induce current to the hull from the anode. If there is a faulty insulator on the anode could possibly cause issues with grounded DC electronics such as some analog gauges, meters or low voltage pump control systems that work on DC. The last time I was on one of the vessels fitted with the ICPP systems it was disabled due to from what I was told issues with the system possibly the anode
I'd be tempted to shut the system off and see what happenes

[Merlyn]

The mystery of the missing Milli Amps may now never be solved methinks.
I'm holding my breath here.