How accurate are mechanical RPM meters on MAN engines ?

[BartW]

Or can they go/read wrong?

The SB MAN engine on Blue Angel (70ft 20yo boat) gave some white smoke
Re this thread, post #47
http://www.ybw.com/forums/showthread.php?t=281034&page=5

Now we don’t know which meters we can trust,
Initially we had adjusted the remote analog RPM meters (inside helm and FB helm) using the mechanical meters in the E/R as the reference.

Also the temp indicators were all different, but after using some contact spray on the inside and FB dash panel multi connectors, the temp meters on the remote helm positions became stable,
But the SB engine was always reading more then the P engine,
While in the engine room the temp on the port engine was higher

Then we discovered that the SB engine feels hotter then the P engine, overall (approx 10°), feeling by hand, on cooling water tubes, exhaust tubes, above turbo’s, etc…
So we decided to run the P engine app 300RPM more then the SB engine, (looking at the mechanical RPM meters in the engine room)

After doing this, the result was:
- Both engines feel the same temp
- The white smoke is gone
- The remote temp gauches show 80°, all 4 (2 inside helm and 2 on FB helm)
- The rudder indicator is now pointing exactly in center to go straight, (while before he was always’s a few° turned to SB)

So as a summery, if the mechanical RPM meters could be wrong,
and read 300RPM difference, while real RPM are equal (?)
Then all other systems are OK,
(Except the old temp meters in the E/R, remember all systems are 20yo)

To be sure about RPM’s, I could do a RPM measurement with a strobe meter on the shafts,
But the next time when I’m at the boat is only end September, then I’m there a full week, perhaps I better hire a MAN engine specialist, to have a full checkup/ adjustment,
Any advice or know how is welcome

 

[hlb]

My experience is that, all rev counters are suspect. On two boats I've had different readings upstairs and down. Some times one RC will go bezerk.

 

[MapisM]

can they go/read wrong?
...
To be sure about RPM’s, I could do a RPM measurement with a strobe meter on the shafts

Yes Bart, I'm afraid that they can.
And yes, a strobe meter is a very accurate way to measure the engines RPM - provided that you know the gearbox ratio, which I assume you do.

But in the meantime, there's another couple of tricks you might be interested in.
Neither can give you an accurate measurement of the engines RPM, but they allow you to check if they're running in sync or not, which at the end of the day is what you're really interested in.

1) While cruising - at whatever speed suits you - just try to bring the engines at the same speed. Most likely, you will NOT hear a constant engine sound, as if it would be coming from a single block. You will rather hear a cyclic sound, as if one of the engines would try to catch up with the other. This indicates that they're not in sync. By adjusting one of the throttles, you can reduce the frequency of that cyclic sound, up to the point when you will not be able to hear it anymore.
Believe it or not, after some practice with this system, you can synchronise your engines within a max difference of 10 (not a typo, I really mean TEN) RPM or so! Eat your heart out, electronic controls.

2) If you have an autopilot which automatically senses the optimal center position of the rudders - as most a/p do - just check after a couple of minutes of cruising at constant speed the "calculated" rudder center. If it reports zero degrees, bingo, your engines are synchronised. If it reports say 2 degrees of steering to port, it means that the port engines is pushing more than the stbd one, hence the a/p must correct that by slightly steering to port. Either reduce the port engine or increase the stbd engine RPM to correct that, and check again after a while. The other way round if the a/p reports X degrees of correction to stbd, of course.
Mind, this second method, in spite of the fact that it can appear more scientific, relies on the fact that the A/P is perfectly calibrated, i.e. that the a/p "reads" the rudders as perfectly centered when they actually are.
Which is not necessarily true, particularly with a 20 yo vessel. And I've seen poorly calibrated A/Ps also on brand new boats, anyway.

 

[Latestarter1]

Bart,

Twenty year old MAN's......Assume that you are talking about 28 Series motors, dependant on power rating between 2,100 and 2,300 rated speed.

Approaching this problem without gauges, strobe, etc is as much use as one legged man at ass kicking party. Feel touch hearing, pretty much worthless, you cannot perform trouble shooting without accurate tachos.

Simple answer is yes the standard VDO tachos can be out, they should always be checked as part of sea trial survey. Suspect the units on your engines, and we can only guess as we lack the data, are mag pick style counting ring gear teeth, not the more hit and miss alternator sensed ones. Sometimes clueless tec's mess with the mag pick up, rule is screw in until sensor touches the ring gear then back off 3/4 of a turn. Experienced tec will CAREFULLY screw in sensor with engine at idle, feel when it is JUST touching and then back of a wee bit, signal will be super strong. Do not try this at home!

Stop farting about trying to do mental gynastics with the shaft speed, take belt guard off and strobe the crank damper. If tachos are out earlier VDO style simple as wheelbarrow to alter calibration using dill switch in the back of unit. Test at idle 1,000 1,500 and rated rpm. Get hold of MAN data sheet it will have correct high idle range for your specicic model.

Problems are fixed with reliable data.

 

[BartW]

But in the meantime, there's another couple of tricks you might be interested in.
Neither can give you an accurate measurement of the engines RPM, but they allow you to check if they're running in sync or not, which at the end of the day is what you're really interested in.

great advise Mapism,

1) I know perfectly what you mean, in fact I have been triing to synchronise the engines a few times, (at lower rpms where the RPM meters are completely wrong),
but forgot about this trick when we were testing different RPM P/SB at cruising speed

2) there is an auto pilot that I have used quite often, and I can confirm that I alway's had to introduce sort of an offset (Alfonzo thought this is normal as it is not correctly calibrated anymore. )
I don't know if it can sense the optimum center pos of the rudders, I guess it does, but I have found / started to read that manual only at the end of the trip. moreover the LCD display of that Furuno unit is badly faded out by the sunlight, (it still works) but there is a more proper one on the lower helm position, might have a look there.

thanks for the advice, shoud have asked this question one week ago

 

[BartW]

Bart,

Twenty year old MAN's......Assume that you are talking about 28 Series motors, dependant on power rating between 2,100 and 2,300 rated speed.

yes, they are 2842LE402, 12cyl, 2300max RPM, 1100HP

Approaching this problem without gauges, strobe, etc is as much use as one legged man at ass kicking party. Feel touch hearing, pretty much worthless, you cannot perform trouble shooting without accurate tachos.

agree,
will organise a MAN engineer to have a look at it, and do the adjustments,
and at the same time do the first maintenace on these engines,
they have now ran 80hrs sinds I own them.

thanks for the advice.

 

[MapisM]

I alway's had to introduce sort of an offset (Alfonzo thought this is normal as it is not correctly calibrated anymore. )
I don't know if it can sense the optimum center pos of the rudders, I guess it does, but I have found / started to read that manual only at the end of the trip.

Bart, if it's a Furuno, it's possibly a FAP-330, which is the same I have on my boat. Great bit of kit, possibly the best in its days.
With the "technician mode", the configuration possibilities are endless, but be careful if you wish to play with that, because it's easy to spoil a perfectly working configuration.
Re. the offset, not only it allows to "force" it manually, but it calculates it automatically if you allow him to do so (hence giving you the opportunity to check also the engines sync, as per my point 2 above). And you can also control the a/p speed of response in doing such self-adjustment.
Though as I breafly mentioned in the previous post, before messing with the a/p "software" you must be 100% sure that, with the rudders physically centered, the a/p reports an angle of zero degrees. If it doesn't (as I suspect, based on what Alfonso told you), there should be a rudder reference unit, attached to the actuator (inside the rudders compartment) which must be mechanically adjusted.
Been there, done that.
If your a/p is indeed the 330, I can send you the PDF version of the manual.
Much better that the paper version imho, both for quickly searching what you're looking for and keeping it handy on your portable PC.

 

[BartW]

the a/p is a Furuno FAP-50 see picture below
its the lower unit from the 3 display's : Sonar / GPS position display / Auto pilot

this is an old picture from the inside helm position,
the old radar screen has been replaced by a Furuno Nav net VX2 with 1833C Radar, together with a new Icom 630(?) VHF, a few years ago

 

[MapisM]

the a/p is a Furuno FAP-50

Aha, I see.
It must be the predecessor of the 330, which maybe was not yet around by the time BA was built.
Anyway, I just found on the web an electronic version of the manual, and had a quick look at it.
It's actually very similar to the 330, and all I previously said applies 100% also to your a/p.
The self-centering function I was talking about is what they call "AUTO-TRIM", and in the version of the operator's manual which I found online it's explained at page 13.
Glad to help you in more details if you wish, just email me.
Good luck with your troubleshooting anyway!

 

[BartW]

Aha, I see.
I just found on the web an electronic version of the manual, and had a quick look at it.
It's actually very similar to the 330, and all I previously said applies 100% also to your a/p.
The self-centering function I was talking about is what they call "AUTO-TRIM", and in the version of the operator's manual which I found online it's explained at page 13.

yes I found it, thanks for that
exactly the same manual as I have on the boat

Glad to help you in more details if you wish

thanks !
your visit to BA becomes more and more unavoidable

 

[BartW]

Bart,

Twenty year old MAN's......Assume that you are talking about 28 Series motors, dependant on power rating between 2,100 and 2,300 rated speed.

Approaching this problem without gauges, strobe, etc is as much use as one legged man at ass kicking party. Feel touch hearing, pretty much worthless, you cannot perform trouble shooting without accurate tachos.

Simple answer is yes the standard VDO tachos can be out, they should always be checked as part of sea trial survey. Suspect the units on your engines, and we can only guess as we lack the data, are mag pick style counting ring gear teeth, not the more hit and miss alternator sensed ones. Sometimes clueless tec's mess with the mag pick up, rule is screw in until sensor touches the ring gear then back off 3/4 of a turn. Experienced tec will CAREFULLY screw in sensor with engine at idle, feel when it is JUST touching and then back of a wee bit, signal will be super strong. Do not try this at home!

Stop farting about trying to do mental gynastics with the shaft speed, take belt guard off and strobe the crank damper. If tachos are out earlier VDO style simple as wheelbarrow to alter calibration using dill switch in the back of unit. Test at idle 1,000 1,500 and rated rpm. Get hold of MAN data sheet it will have correct high idle range for your specicic model.

Problems are fixed with reliable data.

problem is sorted (not yet cured)
with a simple lazer meter I could measure the RPM's of the cranc shaft,
(placing a small reflective sticker on the large hex nut on the front)

turns out that the mechanical RPM gauche in the engine room, on the Port engine shows approx 2240 RPM when measuring 2000 with lazer meter,
or
this engine runs only at approx 1760 RPM when the mechanical gauche schows 2000 RPM,

so I'm afraid that we ran the engines at different RPM quite a lot this summer
now running both engines at real 2000 RPM, the white smoke /higher engine temp on SB engine is gone , both engines nicely 80°C

question:

I forgot to make a picture of the RPM pickup unit,
the unit has a mechanical output to the e/r Gauches, and a electric output to the helm positions (I believe)
is such a unit adjustable,(as discribed above) or do I need to order a new unit immediately,
I could swap port and SB to be sure.

To MapisM thanks again for the advice on the synchronised sound of both engines,
just one complain, now I'm obsessed to have them synchronised permanently


another observation,

I had a lot more oil consumption on the Port engine (lower load) then the SB engine after my summer trip.
rough estimation, oil consumption during 70 engine hours;
10l on Port engine
5l on SB engine
(it is a estimation, but there was a destinkt difference between P and SB)

now having had running the engines about 10 hours at equal RPM, oil consumption seems equal P and SB
(need to do some hours cruising to know for sure)

is it normal to have less oil consumption on the heavyer loaded / higher temp engine ?

 

[MapisM]

To MapisM thanks again for the advice on the synchronised sound of both engines,
just one complain, now I'm obsessed to have them synchronised permanently

No worries, you'll get used to that.
I also had similar feelings the first time my mechanic explained me the trick, but now it's as automatic as setting the a/p!
There's just one thing I still can't get my head round: that mechanic is also able to tell, looking neither at the dashboard nor at the boat wake, WHICH engine is running slower/faster.
Tried and tested, he recognises that every time.
And needless to say, that drives me nuts...!

Re. oil consumption, I've never heard of the difference you're talking about, and can't think of any reason why an engine running at lower load should use more oil.
But I'm glad to stand corrected if the experts will say otherwise.

 

[jimmcgee]

here is a program that could be used

http://www.tunelab-world.com/rpmsound.html

 

[david_bagshaw]

Re. oil consumption, I've never heard of the difference you're talking about, and can't think of any reason why an engine running at lower load should use more oil.
.

no expert but perhaps the rings dont seal as well as piston is cooler and smaller, with greater clearances.?