VHF Interference from Autopilot

I noticed in the web log stats that someone had searched for “VHF interference tiller pilot”. This reminded me that I had a problem which occurred soon after the new electrical panel was wired: whenever I turned on the autopilot, there was a constant buzzing interference audible on all VHF channels.

The source was as I had expected: an earthing problem. I used the Seatalk cable to connect to a NMEA multiplexer and the NMEA input to connect to my chartplotter, both of which were earthed, but the primary power input negative (earth) was not connected (d’oh!). The resulting high currents on the signal earths for Seatalk and NMEA caused RFI which was picked up by the VHF antenna.

Many years ago I built an audio (hi-fi) amplifier which had earth loops because of multiple return paths to earth – the AC mains hum was unbearable! Fortunately I remembered that lesson.

The best strategy is always to run all earths (and power supply grounds) to a single earthing point such as the main negative terminal on your electrical distribution panel.

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Of cooking and fires at sea…

While at anchor at Whitehaven Beach in the Whitsundays, we had the misfortune of our alcohol stove catching fire, nearly burning us down to the waterline. If we weren’t actually watching the stove at the time, it could have easily turned out a nightmare. Here’s what happened…

Sunny Spells is fitted with a MaxiMarine alcohol stove (two burner with grill) of fairly recent vintage, installed on gimbals. I’ve always been uncomfortable with the procedure for lighting the stove, which feels a bit like starting a barely controlled fire. A little methylated spirit (metho) is dripped into a tray around the burner and lit. This liquid metho burns and heats up the burner itself. Once the burner is hot enough to vaporize the metho, the alcohol flow is turned on again and the metho vaporizes and burns cleanly, a bit like a propane stove.

Our MaxiMarine alcohol stove

Getting just enough metho into the tray initially is a bit tricky, and you either don’t have enough or the alcohol burns quite vigorously while you watch it (very nervously).

On this occasion, we had all three burners going, including the grill. For some reason which I still can’t determine, the metho started pouring out of one of the burners (I suspect the grill), flooding the bottom of the stove and then running onto the cabin sole. When this caught alight, it was seconds before we had a roaring fire!

Fortunately, the fire blanket was close to hand and this was quickly thrown over the fire and I then closed all the control knobs on the stove. Unfortunately, the fire was still burning and getting out of control. I then grabbed the fire extinguisher, took a couple of seconds to remove the safety pin and work out which way to point it, and then discharged it into the fire under the fire blanket… The fire went out instantaneously – to our combined relief! The fire blanket had helped to slow down the fire, but in future I’ll be grabbing the fire extinguisher at the same time!

To say we were all a bit shaken, if not stirred, is an understatement. In the seconds before using the extinguisher I was about to urge everyone to abandon ship, it was that close…

The inside of the boat was covered in powder, a substance we found pretty difficult to clean up. We took the stove outside, cleaned it and then started the two burners (not the grill) on deck, where it operated flawlessly. The stove was reinstalled and used for the rest of the trip without any incident.

The problem now, of course, is that I don’t trust the devil…

I’ve learned a lot from the experience though:

  • In my initial safety briefing I concentrated on sea safety (MOB procedures, harnesses etc..) but we didn’t cover the fire-extinguishers, which I left for a later briefing which never happened. When I briefed the crew on the use of the stove I eplained the use of the fire-blanket, but the most experienced crew member, who had used an alcohol stove before, was on the helm at the time. Of course, she was the one doing the cooking when the stove decided to go up in flames!

    Lesson 1: make sure the crew is briefed on fire fighting and that they all get the opportunity to handle the fire blanket and fire extinguishers.

  • I was intrigued by my subconscious resistance to firing a $20 fire extinguisher while the boat was going up in flames…

    Lesson 2: Be mentally prepared to use the fire extinguisher.

  • I fumbled with the fire extinguisher: first I couldn’t get it out of its bracket, then I had trouble getting the safety pin out and, finally, I had to force myself to loook at it carefully to find the trigger and make sure the nozzle was pointing the right way (while having a very urgent, flaming distraction in front of me).

    Lesson3: Familiarize yourself and the crew with the fire extinguishers so you can use them with your eyes shut.

It does raise the old question again about cooking on board: propane vs alcohol? I don’t know the answer to this one. I do feel a bit biased against the alcohol stove now though, but is an explosion better than a fire?

Freedom from the “Tyrrany of the Tiller”

I have an ST4000+ wheelpilot on my 33ft Dick Carter sail boat. IMHO it is an essential piece of safety equipment when you are sailing shorthanded, or even just with a smaller crew (i.e. less than 4 people).

I recently had a challenging night offshore after splitting the mainsail in a squall (just after putting the 3rd reef in). Even though I had two crew, they were both too sea-sick to be of much use, and the autopilot steered perfectly under motor while I lashed the main to the boom and set the storm tri-sail.

The benefits in freeing you from the “Tyrrany of the Tiller” is obvious.

It is vitally important though that you:

  • fit a rudder angle sensor, otherwise the Autohelm performance will be mediocre at best, but probably next to useless;

  • select the location for the fluxgate compass very carefully to avoid magnetic and RF interference.

  • Getting it as close as posible to the keel will also minimise unecessary movement of the fluxgate, making the Autohelm more stable;
  • make sure all wiring is done professionally, cable joins are avoided as far as possible and cables are routed in dry areas (keep them out of the bilge if possible).

The Autohelm on Sunny Spells was pretty useless when I bought her, but after fixing the problems listed above, she now helms perfectly downwind for hours on end. I’ve had an 11 hour run downwind with poled out genoa and main by the lee without touching the autopilot controls (on wind-vane mode) other than accepting wind shift alarms.

The best $1000 you can spend!

Navbus/NMEA and Displaying Wind Data

Mystery solved…

After a lot of correspondence, telephone calls and hours in the boat, in front of the laptop, decoding NMEA0183 sentences (did I really need to know all this stuff??), the mystery is finally solved…

It appears that the chartplotter firmware is the culprit. If Navbus is enabled on the chartplotter, it will only accept wind data from Navbus, not the NMEA0183 input. Disable Navbus, and the wind data is displayed!

It seems like a quick fix, but it comes at a price. The biggest issue is that DSC capability (i.e. automatic communication with the VHF to receive and transmit DSC messages) is lost. Also, the barometric pressure and air temperature data sensed by the VHF is now not available at the helm… Navman’s John Dusting said that he would request a firmware update to solve this issue, but it could be a while…