A Fundamental Danger Of NMEA 2000 Networks

NMEA 2000

I  was just reading an article on the NMEA 2000 marine network standard over at the excellent Ocean Navigator blog. For those of you who are not aware, NMEA is a standard backbone cabling system that allows you to connect every piece of electronic gear on your boat together, regardless of what company manufactured each piece. Most every techy type in the marine journalism community has been waxing eloquent about how cool this is. And they are right…but.

You knew there was a “but” coming, right?

You see, the thing is that many devices use the NMEA backbone not only to communicate with other devices, but to communicate with their own sensors. And therein lies the problem that I have not seen one commentator mention: If there is a significant problem with the NMEA 2000 backbone, every piece of gear on the boat dependant on it stops working. What does that mean for a fully NMEA 2000 boat?

  • The engine instruments don’t work.
  • All of the devices that require a GPS position, including the plotter, stop working.
  • There is no read out from the fluxgate compass.
  • The multi function display in the cockpit that displays radar and AIS information goes black.
  • The autopilot stops working because it has no input from the compass.
  • The speedo/log, depth sounder, and wind instruments stop working.

Now let’s put this into prospective. Mr. Murphy will make absolutely certain that all of this happens just as you are making an approach to a tricky harbour crawling with traffic…in the fog. That’s going to be fun! As a famous ocean racing skipper was heard to say after nearly running aground on Cape Sable in the fog, “only your laundry knows for sure”.

So how likely is it that this will happen? Let me quote from the ON article (emphasis mine).

Because cabling problems are especially troubling for bus or linear topology networks such as NMEA 2000, the cabling should be checked first if any malfunctions start to manifest. This is because if there is a short or open in the trunk or drop lines it can affect the entire network, downgrading overall performance or even taking down the network. The problem, however, could be as simple as a missing/damaged terminator or loose/dirty connectors. If cleaning and reseating connectors or terminators cannot solve a problem, then by all means seek the professional help of an NMEA-certified electronics technician. Follow this link to find the closest CET to your homeport.

Hum, this is comforting. There are yards of cable with dozens of connectors snaking all over your boat and routed through inaccessible and damp places. And all it takes is one of those connectors to fail and it’s lights out.

I’m an electronics and computer technician by trade and I can tell you that network problems can be some of the most frustrating and time consuming problems to troubleshoot because there is no easy way to logically zero in on the failure point. No, all you can do is check every single connector and terminator until you find the bad one. And if the problem is intermittent, as it very likely will be, then the trouble shooting is going to be ten times more difficult.

But not to worry, all you need is a “NMEA-certified electronics technician”. Good luck with that in most of the places we voyage to. Even if you can find one, think about what his or her charge rate is likely to be. And think about the number of hours solving a difficult problem could take—ouch!

Now don’t get me wrong. I’m not saying that you should not use NMEA 2000 or that it’s a bad protocol. In fact I really like the idea of replacing the hundreds of different wires running hither and yon through most modern boats with a single network backbone.

But what I am saying is that we need to think about what a network failure will do to us on a dark night in a tricky situation and have a backup plan. And because of the issues with trouble shooting these complex systems in remote places, it would be good to have a plan for continuing our cruise without all this interconnection too.

I, for one, will make sure that:

The point of all this being that if we are going to be safe and competent, we need to really think about new technology and what its intrinsic vulnerabilities are and not just get swept up in the hype.

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John was born and brought up in Bermuda and started sailing as a child, racing locally and offshore before turning to cruising. He has sailed over 100,000 miles, most of it on his McCurdy & Rhodes 56, Morgan's Cloud, including eight ocean races to Bermuda, culminating in winning his class twice in the Newport Bermuda Race. He has skippered a series of voyages in the North Atlantic, the majority of which have been to the high latitudes. John has been helping others go voyaging by sharing his experience for twenty years, first in yachting magazines and, for the last 18 years, as co-editor/publisher of AAC.

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