Rudder indicators

[Guest 000]

This new vessel of ours has two helm stations, each with identical instrumentation which includes Vetus rudder indicators. There is a lot of close quarter manouvering here on the canals so the indicators are quite useful, or would be if they worked properly.

The indicator in the wheelhouse is very precise in showing rudder angle but the one in the top helm position shows a bias to port by about 15 degrees. Both take their input from the same rudder mounted sender via the ignition switch. When the ignition switch is off both rudder indicators are at rest in the hard to starboard position. Since the wheelhouse indicator is beside the ignition switch and the top indicator has a cable run of 5 metres I assume there is some sort of voltage differential, but then the wiring is very heavy gauge so...

Any help please, and keep it simple, I find electricity difficult.

[Island Time 1,231]

There must be a way to set zero on each instrument. Which model are they?

[Guest 000]

Sorry, done a mistake. They are VDO rudder indicators and I can't see any model numbers. There is a brand new spare instrument on board so I assume that the previous owner bought it to fix the problem. I tried it but it shows the same degree of port bias. There is no adjustment anywhere on the gauge, which is why I thought it may have something to do with input voltage. The new instrument has a wiring loom in the box so may pay to try wiring up that?

[wheels 543]

Hi Chris. Pretty sure those units are a very simple Potentiometer that works just like a Fuel tanks sender. Basically a linear resistor with a wiper arm running along it. If you connect two gauges to the same terminals, that will likely upset the reading. Although I would have expected the error to be seen on both gauges.
Anyway, they use a Two station sender if you need two stations. That will mean that on the sender unit, (mounted on the rudder quadrant), there will be three terminals if it is twin station. If there is only two terminals, it is only single station.
SO if there are three terminals, then one will be common and the other two for each separate gauge. Try swapping the two and see if the fault then switches to the other gauge. That will determine if the fault is at the sender itself, or between the sender and gauge.
Calibration was simply setting the rudder to midships, then adjusting the sender unit till the gauge/s read midships.

[mattm 98]

Not necessarily correct on the dual station part wheels. All the VDO rudder pos senders I’ve seen, dual station ones that is, are two terminals. One is earthed, and the other goes to the S (signal) position in both of the gauges.

 

To adjust the reading you do just rotate the base of the RPS. which does not help when you have two, reading differently. I’d try swapping the gauges over and see if the issue moves or not, to tell you if it’s a gauge or the wiring. Remember, if it’s a gauge, it could be the ‘correct’ one, and the RPS has been adjusted to allow for it. Good luck.

[wheels 543]

Thanks for correcting that Matt.

[Guest 000]

Thanks for the replies.

To recap, when the wheelhouse indicator shows straight ahead the top helm indicator shows about 15 degrees to port. The wheelhouse indicator is correct as the vessel runs straight when this indicator is in zero helm position. I have changed the top indicator with a new one found on board but it shows the same error.

Now this is where it starts to get out of my comfort zone.

Turn on the ignition and centre the rudder according to the wheelhouse indicator, the top indicator will show about a 3-5 degree bias to port. Start the engine and the top indicator will slowly increase to a 15 degree port bias whilst the wheelhouse indicator remains in midships helm position. I can imagine all sorts of scenarios going on here.

[ex TL systems 63]

Bend the needle until it shows you what you want to see !

[Island Time 1,231]

Nah Chris, Matt told you the right stuff. Swap the upper and lower units over and see if the issue remains on the upper one. If it does, the issue is resistance in the wiring to the top unit....

[wheels 543]

Two things to check. Is the cable tinned wire or plain copper. If plain copper, the copper could be corroded and need replacing. Second thing is check the terminal connectors are well crimped to the wire and make sure the terminals are clean and tight. Ypou need to do this at both ends. Also, of the gauges are engaged and disengaged by the Key, check the terminals t the Key switch also. As IT says, the fact the fault still remains when you change the gauge, shows the fault is in the wire, not the gauges.
The issue of the gauge moving with Key and then engine starting suggests a faulty connection somewhere. Possibly earth.
These things work very simply with a voltage passed down the cable and the wiping resistor in the sender unit alters that voltage. This in turn swings the dial from 0 to fullscale. The 0 will be say hard to Port and the Fullscale will be full to starboard. The centre position reads midships. So the issue is that the upstairs gauge is not getting the full voltage required to deflect it to the correct position.

[Black Panther 1,581]

My rudder angle indicator lies. So do my wind instruments. I find it helps not to look at them.

[Guest 000]

Thanks BP, I agree that it's a wiring issue. The problem is that the power off position of the needle is hard to starboard. Powered up, the mis-reading gauge shows 15 degrees to port with rudder midships which would then indicate excessive voltage?

I wonder if the top deck rudder system is over-earthed, possible? A wiring diagram I found online shows the system earthed at the rudder sender unit with an optional earth at the gauge. My set up has both positions earthed.

[wheels 543]

No it is likely that it is "under earthed" err for use of a bad description. This maybe difficult to explain, but in effect, the Earth pulls back the voltage.
You couldn't have a gauge system where the Main Ships voltage is across the Gauge and Resistor as such. Because then when the wiper reached Earth, there would be a dead short to ground and the wiring would melt. So there is a small signal voltage applied to the system parts, which is called "signal". Sometimes gauges tend to work in reverse to what you would think, in that full scale deflection is actually the lowest voltage and that the high voltage actually pulls the gauge back. So a poor earth ends up reading high.
So, if you are experiencing fluctuations with the Key and engine running etc, then this actually makes a little sense now that you say the system is earth at two points. It may not be the Gauge Earth that is the issue, but the main Earth feed itself. Check that the main Earth to the instruments is properly connected at both ends. There is a "voltage drop" on the feed for some reason. Turning the Key is placing a load on the Earth, but the earth is not able to pull right down to zero.
I hope that made sense.

[Guest 000]

No, I'm afraid it doesn't but that's my fault, not yours. Electricity is confusing! However, I will follow your advice and check out the earth connections.

Joke says why don't I save myself all the heartache and just scratch a line on the upper indicator bezel where the instrument shows the midships helm position.

She's right and it makes sense, it's just that misbehaving equipment drives me crazy.

[Jon 360]

I was about to suggest you put a little arrow sticker on center for the one that’s out but I was beaten by Joke

[wheels 543]

And as the connection worsens, the needle is going to move across further and eventually you are not going to be able to read it with all the updated scratch marks.
Sorry to have made that explanation confusing. To summerise, sounds like a bad earth. check all Earth connections.

You may even find that other gauges could shift their position if they were all attached to the same Earth.

[mattm 98]

I’d eye ball the wiring sure, but my first actual task after that would be swapping the gauges.

 

I think the bit you missed Chris, was that either gauge could be misreading. If you rotate the base of the RPS 15 deg to starboard, the ‘wrong’ gauge will be right, and the ‘right’ gauge will read 15deg to starboard and now be wrong. Assuming it’s the gauge for a minute, what you want is two gauges that read the same at any given resistance, then you set the RPS to read center when is centered.

The simplest check you can do, with limited understanding of power, is swap the gauges over and report the findings. Or, perhaps fitting the spare gauge in the position of the ‘correct’ gauge would tell a story also, wouldn’t be my preference though unless the gauges are a PITA to remove, which often they can be.

[Guest 000]

I am pleased to advise that the rudder indicator issue is now resolved. As with a lot of these type problems the issue was one of human error, essentially mine.

I don't know how I missed it but the two installed gauges were slightly different which made me think different gauges, perhaps different signal voltages. Changing the odd-man-out gauge and installing the spare to match the other one installed solved the problem. Both reading the same now, Thank goodness.

[Winter 42]

Glad to hear it is solved.

some of the vdo indicators do actually have an adjustment pot, however it is set at the factory and not accessible.

if you carefully drill a hole in the side of the housing you can shine a light in and look, you can then workout where on the housing the pot is, and drill another hole right above that.

 

I have a VDO voltmeter I adjusted in this way

[Guest 000]

Thanks Winter. That's a handy bit of info.