ST60 log only reads when engine is running

Could there be electrical interference or a signal generated from the engine? Electrical output could increase with rpm giving a false increase in speed. Engine off - no electrical charge from engine or alternator

Therefore problem could be that it has never "worked" it's just reacting to engine rpm?

Just a thought - perhaps someone more knowledgeable could expand on the science (or more likely say I'm talking cobblers)
 
searj said:
The boat has only just gone back in the water smooth, barnacle free etc. Also, surely a barnacle would interfere with readings all the time, rather than being a variable issue? The log does register our speed when motoring, seemingly accurately, but that said we motor at about 3 knots to get off the mooring then the engines off so it might need some more exploration under motor which Ill do as soon as I get the chance!

Just to clarify, the other day when the log went down to 0, I started the engine in neutral and the speed immediately climbed up to 4 or 5 knots, I killed the engine then the speed started dropping to 0 straight away, and I think its not just us being rubbish sailors? Just to show how pronounced the correlation with the engine running is!
Click to expand...

That's compelling. So it’s not a barnacle blocking the paddle wheel, and it’s not the supply voltage to the ST60 unit.

The way to test the paddle wheel is below. But we know your paddle wheel is working when the engine is running. So I wonder about the wiring to the paddle wheel - if a really crap connection is resulting in a voltage drop along the cable between ST60 and paddle wheel. How many ohms' resistance does your meter show across the paddle wheel cable when the paddle contact is closed? Then finally, on reconnecting the cable to the back of the ST60 display, check the connections for corrosion.

- Verify that the transducer’s paddle wheel is free of marine growth and rotates freely on its shaft

- Power down the instrument

- Disconnect the GREEN and SHIELD transducer leads from the instrument display or instrument pod

- Connect a multimeter to the GREEN and SHIELD transducer leads and configure the multimeter to measure continuity or resistance

- Slowly rotate the transducer paddle wheel by hand. The multimeter should alternate between an open circuit and a closed circuit with each quarter turn. One revolution of the paddle wheel should open and close the circuit four times.

​
 
Homer J said:
Could there be electrical interference or a signal generated from the engine? Electrical output could increase with rpm giving a false increase in speed. Engine off - no electrical charge from engine or alternator

Therefore problem could be that it has never "worked" it's just reacting to engine rpm?

Just a thought - perhaps someone more knowledgeable could expand on the science (or more likely say I'm talking cobblers)
Click to expand...

Good thinking Homer! If there is a "screen" on the signal wire and it is not connected at either end it could be picking up a frequency signal from the alternator or rev. counter. The "screen" must be connected to "ground", normally only at one end. You may pick it up with the test referred to by Belle Serene.
 
Homer J said:
Could there be electrical interference or a signal generated from the engine? Electrical output could increase with rpm giving a false increase in speed. Engine off - no electrical charge from engine or alternator

Therefore problem could be that it has never "worked" it's just reacting to engine rpm?

Just a thought - perhaps someone more knowledgeable could expand on the science (or more likely say I'm talking cobblers)
Click to expand...

I happen to have been using an oscilloscope to diagnose something completely different on my alternator, and noticed that a substantial signal can be picked up with the oscilloscope probes not connected to anything. So I'd echo the possibility that the log is picking up a signal from the alternator rather than from the log impellor,
 
Alex_Blackwood said:
Good thinking Homer! If there is a "screen" on the signal wire and it is not connected at either end it could be picking up a frequency signal from the alternator or rev. counter. The "screen" must be connected to "ground", normally only at one end. You may pick it up with the test referred to by Belle Serene.
Click to expand...

It's a great idea. Although the OP did say at post 5 that his instrument recorded boatspeed when he removed the paddle wheel and span it by hand. Either way, the continuity test would pick it up.
 
Good thinking Homer! If there is a "screen" on the signal wire and it is not connected at either end it could be picking up a frequency signal from the alternator or rev. counter. The "screen" must be connected to "ground", normally only at one end. You may pick it up with the test referred to by Belle Serene.

It's a great idea. Although the OP did say at post 5 that his instrument recorded boatspeed when he removed the paddle wheel and span it by hand. Either way, the continuity test would pick it up.

Yes, I wonder if moving the transducer around while spinning the paddle would see any alteration in read-out. could be there is an intermittent fault with a connection, or in the cable. He has power to unit so Battery OK. Unit does not read with transducer in place with vessel underway. Unit reads with engine running. Points to transducer or cable fault. Would be interesting to note if reading changes with engine speed as alternator frequency will increase with RPM. Even if it is picking up peaks in the DC it should alter.
Trouble with online fault finding is you cannot see the fault.
 
Last edited:
Ok so some updates today -

Paddle wheel recorded a speed when tested in the cockpit, but not when in the forepeak where it is fitted. Worked when engine was running predictably, as before but after a few tests it stopped working regardless of engine running

Engine tacho started playing up when the engine and switch panel were both off, hovering between 1 and 3krpm for no apparent reason. Something strange going on inside the boat!

I had coiled up the slack of the transducer cable (quite a lot of it) and cable tied it away in the battery compartment and am thinking that whatever magnetism is coming off the engine or alternator could be generating or amplifying small signals through this coil perhaps? It was around a meter from the alternator so very close!

I checked continuity across the transducer again, and by the end of the experiments there was no continuity across green and unsheilded. I chopped the wire off near the transducer and measured again and found the same fault so I reckon there is a breakage inside the transducer that was causing a temperamental failure so it looks like new transducer time! Still very confusing, lots of weird happenings.
May have something to do with holding the transducer upside down when spinning the paddle, but not working when the correct way up perhaps? anyway very confusing, not sure what was going on with the rev counter that was borderline creepy to see that having a mind of its own. Almost seemed to mimic wind strength and gusts.....

Thanks for the input all, i think this is put to bed even if not fully resolved. Ill look out for a transducer as the next step!
 
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