Accuracy of Analogue Speed (Through water) Log v. GPS SOG

[dawntreader]

I keep my speed wheel clean and remove the transducer between passages if I won't be moving for a while. It has become increasingly obvious that the analogue instrument, originally quite accurate, has become significantly different (under reporting) - both speed and distance - to the digital SOG and 'distance to go' reported from the GPS/chartplotter.

I have read earlier posts on this topic and, in particular, Salty mentions "....you need to adjust your log". I have the Raymarine ST60 and C80 chartplotter. How can the analogue wheel be 'adjusted' or does it have to be replaced?

[Lyra]

Here is the manual for the ST60.
I believe the answer is in pages 31-33

[dawntreader]

Thanks very much Lyra - much appreciated. Not sure why it would have lost its calibration but will re-calibrate next good opportunity   

[Salty]

................................Not sure why it would have lost its calibration but will re-calibrate next good opportunity.......

One reason how this may have happened is that the speed transducer wheel may have had a direct hit from a power washer last time your boat was hauled out. Mine was, and now the speed reads something more like actual x 100. Since then I've misplaced any information that I used to have regarding calibration, so thanks Lyra, your info was very timely.

Salty.

[dawntreader]

................................Not sure why it would have lost its calibration but will re-calibrate next good opportunity.......

.... speed transducer wheel may have had a direct hit from a power washer last time your boat was hauled out...

Salty.

Always removed before lift out to prevent damage by straps 

[Salty]

................................Not sure why it would have lost its calibration but will re-calibrate next good opportunity.......

.... speed transducer wheel may have had a direct hit from a power washer last time your boat was hauled out...

Salty.

Always removed before lift out to prevent damage by straps 

Yes of course, but it wasn't the lifting straps that did the damage. The wheel is protected by ridges on each side of the housing, and on my boat the log is not located in way of where the lifting straps are placed. In addition no weight was put on the straps before they were exactly in the correct position, and if that had happened, then surely the most likely damage would reduce the speed of wheel rotation.

[dawntreader]

Salty, thanks for your input. It is my understanding that, like a bicycle computer, the paddle wheel is just a simple 'sender'. It has a magnet in one of the four paddles and this passes in front of a 'sender' (coil) located in the hull transducer. The movement of the paddle induces a current (pulses) into the coil as it turns and it is this pulsed current that is measured at the plotter (computer). A simple algorithm will be used to translate the pulses received from the sender to display the speed through the water. There is obviously some 'adjustment' available to this algorithm to allow for calibration. I can understand some small variation from this calibration but 100x ...? You may have a different issue.

[Salty]

Salty, thanks for your input. It is my understanding that, like a bicycle computer, the paddle wheel is just a simple 'sender'. It has a magnet in one of the four paddles and this passes in front of a 'sender' (coil) located in the hull transducer. The movement of the paddle induces a current (pulses) into the coil as it turns and it is this pulsed current that is measured at the plotter (computer). A simple algorithm will be used to translate the pulses received from the sender to display the speed through the water. There is obviously some 'adjustment' available to this algorithm to allow for calibration. I can understand some small variation from this calibration but 100x ...? You may have a different issue.

Yes, that's pretty much my understanding of how it works.
Last weekend and during a trip across to Howth and Malahide, we had speed over the ground of 5.5 to 7.5 knots, but the log was showing some 500 to 650 knots through the water. On the return leg, speeds over the ground were higher due mainly to more favourable wind, but the log speed through the water had reduced to around 350 to 400 knots (though we hadn't turned our hats back to front) !!
So yes there is another issue here, but it only started following that direct hit ashore from a pressure washer and where the sound of the paddle wheel turning was akin to that of a jet aircraft about to head off down the runway.

[MarkTheBike]

could it just be that the instrument is not showing the decimal point on the display? Can you change the accuracy from 2 d.p. down to 1 d.p. and back? Although rare, LCDs can lose a pixel or two and this might move the position of the d.p. to a different area on screen.

[Salty]

I don't know Mark, but I'm going over to the yacht club this afternoon for a committee meeting and will stay a couple of nights onboard. In the meantime I've downloaded the manual that Lyra posted earlier in this thread and will have a look at recalibrating the log tomorrow. Either way I'll let you know the results.

[MarkTheBike]

I'd certainly be interested. My paddle spins freely and is kept clean but most of the time, I get no boat speed displayed (although sometimes I do). I've tried the multimeter tests (no response but then it worked when I next went out) and I've checked and rechecked all the connections. It's baffled me for the last 3 years. The SOG works fine, of course, but that's not relying on the paddle. Don't particularly want to buy another paddle until I can confirm that's where the problem lies.

[Salty]

Hi Mark,
I used the information that Lyra provided, and while I think the job remains as a "work in progress," I've now got a display of speed through the water that is more in keeping with that of a B36 rather than something about to head through the sound barrier.
I tried to recalibrate, but in the end went to the factory settings where the display showed the number 15 at one point. This was changed down to 14, and the decimal place appeared 0.1
Returning to the normal screen showed my speed on the mooring had changed from around 650 knots to 0.0 knots. Starting the engine, and dropping the mooring I took the boat for a run around the harbour and speed increased up to 5.0 knots which I thought was a bit slow for the engine rpm, so I may need to take another look at it, but at least it's a move in a more appropriate direction.

[Mirror45184]

Large or zero speed readings could also be due to dirty plug contacts. I regularly unplug the instrument cables at the back of instruments, spray with RP7 or WD40, local equivalent water repellent magic spray and plug them back in. Removing and reinstalling the plug wipes off corrosion that may have built up and the WD40 helps to stop it returning.
Cheers

[MarkTheBike]

Hi Salty - great progress indeed. Since you first mentioned it, I've kept an ear out for sonic booms...! As far as mine is concerned it's possible that a squirt of WD40 might help but I suspect that there's a break somewhere in the cable.

[dawntreader]

Here is the manual for the ST60.
I believe the answer is in pages 31-33

Well, I managed to get out in calm conditions and with slack water at long last. I followed the instructions on how to calibrate from my own handbook supplied with the ST60. However, I'm not entirely satisfied with the calibration process where you are able to set analogue speed to be the same as SOG. Despite what the manual says, the analogue speed did not change to equal the displayed SOG. This was despite me following the instructions to the letter several times. However, it did change and, once taken out of Calibration mode, it did settle down and analogue speed is now similar to digital speed shown from the GPS so that is a great improvement. I was also unable to select the manual speed adjustment either so not sure what is going on there?