Day 95 - Success with Chart Plotter

At nightfall there was a brilliant quarter+ moon and just to the right of it was our friend the comet, our companion for weeks now.

The evening's Sailmail brought good technical information from Arnold on the design of the Seatalk network. I ask his indulgence in my publishing his words which may be of help to others:

"The Seatalk bus consists of three wires that are connected to each device
on the net: 12V (red), GND (grey), and data (yellow).

The interface could not be more simple. A single wire (yellow) transmits
data between instruments. The wire is normally at 12V. When somebody talks,
the wire voltage bounces between 12V (binary 1) and GND (binary 0).
Technically, it's an open collector bus that is "pulled up" to 12V through
a resistor to indicate idle or 1, and is "pulled down" through a transistor
to indicate a 0.

I suggest you go to the Seatalk connectors in the comm closet and check the
following:

Red wires -- should be approximately 12V steady.

Grey wires -- should be 0V.

Yellow wires -- really need an oscilloscope for this. Check with the
multimeter anyway. If nobody is talking, it should be at 12V. If somebody
is taking it should be between 0V and 12V. After the DC check, change the
multimeter to AC function, low voltage. If somebody is talking I would
expect to see a small AC voltage.

The multimeter GND lead is connected to GND during measurements, of course.

If the red (12V) and grey (GND) wires look OK, I would try to disconnect
each device yellow (data) wire at the Seatalk junction box, one at a time,
to identify a bad device that may be corrupting the data bus."

Stephen sent me several messages containing the results of an internet search that he did for "Seatalk Failure" which were extremely useful and verified that I had been on the right track. (And a struggling amateur needs all of the validation that he can get!).

Several points stood out in my mind from this information: (1) ANY problem with a wire or connector can bring down the entire Seatalk network (2) I must heck that the autopilot black box is putting out 12V (3) Divide and conquer by isolating components from the Seatalk network.

With this information I went to bed determined to either fix or isolate the problem the next day, armed with a multimeter, wire cutting splicing equipment, fine sandpaper, the can of terminal cleaner, and lots of resolve. ("This ends NOW, Seatalk!") The ocean is a harsh environment for wiring and there was a good chance that corrosion had crept in somewhere.

The wind didn't even last the night. I was waken up by the heavy rolling of the boat, which meant insufficient wind and the headsail flogging back and forth. I got up and rolled it in and there we were, wallowing in the water. It was a a brilliantly clear night yet ahead of us on the horizon I could see regular flashes of lightning.

After 3 cups of coffee I began work on the Seatalk Problem.

I'll give the outcome up front so that anyone not interested in the technical details may skip "The Fix" section.

I would have to call it a total success with the chart plotter but a partial success with the system.

The chart plotter now displays position, heading, the COG/SOG data, the wind direction and speed, and the depth. So for the first time in weeks I am again able to monitor the boat at the nav station and no longer do I have to stick my head out through the companionway to look at the wind data.

The autopilot was the source of the problem and has been isolated from Seatalk. I will probably remove its 5A fuse to ensure that it is not energized.

To my surprise and disappointment the radar is still not functioning. It goes through the 60 second scanner warmup OK but when I give the TX command there is no rotation or imaging.

--------------- The Fix -----------------

I began the work with the principle that I need not investigate the wind & depth instruments or cabling because I had already isolated them with no result.

I probed the Seatalk network at the Raymarine junction box and got 9.8V on the red (R) and 0.73V on the yellow (Y). I switched on the multimeter to AC on low voltage as Arnold had suggested and got nothing on the red or yellow.

Going into the top of the junction box were two Seatalk cables: one from the autopilot (AP) and one from the front of the boat. The individual matching wires had been joined with solder and crimps. I withdrew the top wires from the junction box and found that the wires going into the lower end of the box were now dead. Going into the lower side were 3 Seatalk cables: one from the GPS at, one from the front of the boat, and one from the wind & depth instruments.

Of the two cables passing to the front of the boat, one had to go to the C120 and the other had to go to the depth transducer, though at this point I wasn't sure which was which.

I removed the 5A fuse from the AP and probed the fuse gap and got 12V.

I then did my first wire cuts: the R, Y, and G (ground) wires from the AP. I stripped the R and Y and probed to find 0.36V from the R.

I then cut and stripped the top wires going to the front and got a reading of 11.38V on the red. This had to be the C120 cable, and the source of power to the Seatalk network.

Leaving the three bottom sets in place (i.e. GPS, depth transducer, and wind & depth) I connected only the Seatalk wires from the C120 cable at the top, leaving out the AP wires. The chart plotter came alive, displaying everything.

I then decided to push my luck to see if reconnecting the AP wires would bring back its functionality and allow the C120 to once again see the flux gate compass and for the AP to work. It was worth a try because I had cut out several cable joins. I connected the AP cables and once again the chart plotter went dead. I withdrew the AP Y (data) wire and still the chart plotter was dead. Next I withdrew the R (power) wire and still the chart plotter was dead. Finally I withdrew G (ground) wire and the chart plotter came alive. Why the Seatalk ground cable of the AP causes a problem is beyond me. Perhaps something has gone wrong with the AP's power supply.

Back to the radar, I opened the access panel behind the C120 to make sure that I had not disturbed the radar cable when I looked into the area the previous day, but the plug is a heavy duty screw on type and solid as a rock. The area is so protected and dry (The cables are dusty.) that I saw no point in disturbing the connection. I had not run the radar since the hard passage around the Horn and it is possible that the heavy weather caused a problem up at the radome, or maybe even the cable running down the mast, though it is in its own conduit.

------------------- End of Fix Account ---------------------

So that has left me with full functionality of the C120 except for the heading, but no autopilot or radar. Most importantly, I have no reason for doubting that the AIS is working since it is a 3rd party product interfacing into the C120 via an NMEA 0183 interface and totally independent of Seatalk, which is working anyway. (I'll know when we encounter the next ship.)

I give my thanks to both Arnold and Stephen for their indispensable help in getting this vital system up and running again. Thanks fellows for your support.

As if on cue the wind began to get lively just as I had finished. Soon we were moving NE with a gentle wind and I went back below to put all of the tools and materials away. Within the hour it was drizzling and the wind had picked up to the mid 20's. I reduced sail and started the engine because the House bank was down to 12.0V.

The starboard water tank started making gurgling sounds as though it was about to go dry. I had been expecting it and I considered every day of water from that tank since the Horn a gift. The 140 liter starboard tank combined the water from La Paz and the rainwater that I had captured in the doldrums had supported me for 95 days. Now I had 140 liters of water in the port tank and 15 liters of La Paz water in reserve to get me Cape Town. I was fairly sure that I would be able to collect more rainwater once my mainsail was back up, but for now I had to assume the worst. I would reinstate the system of using the jug to hold myself to 2.5 liters of water a day, giving me 56 days at sea. Certainly the morale sustaining indulgence of all of the hot coffee and chocolate that I could drink which sustained me past the Horn would have to cease, since all that seemed to do was to push the fluids straight through me.

At noon our position was 48S34, 50W53, giving us a n-n distance of 69 miles in the direction 061T. In the last 3 days I had averaged a paltry 64 miles per day. At this rate it would take 50 days to make Cape Town. I needed to do better and I expected to.

I had been drizzling for hours and at one point I saw some serious looking clouds approaching. I dug out the spare shower curtain and spread it over the cockpit well in front of the binnacle to see if I could capture even a few liters of water. Unfortunately as all the other so-called rain since the doldrums it turned out to be more weak drizzle.

As darkness approached the sky began to clear and I was eagerly awaiting a predicted backing of the wind so that our course would alter more to the north than our current 100T.

----------
radio email processed by SailMail
for information see: http://www.sailmail.com