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Old 06-08-2018, 00:41   #1
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Single point of failure

I have been upgrading my electronics and removing miles of cable. There seemed to be a separate cable for everything with the old system.

OFC the new system is N2K with a single backbone cable and most things running off a small spur.

One of the old timers around the local boat yard lamented how a failure in the backbone would take out the whole system - and there have been similar posts on here recently, but how likely is this?

A few things come to mind when I thought about this:

1. If I get hit by lightening - backbone cable or not - everything will be pretty much fried.
2. If for example one of the multi function display units (Raymarine i70s) died - they wouldn't take other things on the network with them.
3. Again if the plotter died it would take the radar with it (I accept that), but the autopilot and AIS can all still be controlled from the i70. Also I can get the N2K output via my Vesper onto OpenCPN - so the plotter has a backup.
4. How likely is it that once installed a actual backbone cable will fail - and even if it did, and I needed to carry a spare, it would be one of the cheapest spares on the boat.

Thats the way I am looking at it at the moment - but interested to hear if this reasoning is flawed.

Al
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Old 06-08-2018, 00:49   #2
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Re: Single point of failure

I think your reasoning is sensible.

In my mind a N2K set up is ultimately significantly more reliable than NMEA0183, with its thin, often improperly specified cabling, crimped ends etc.

Once the backbone is in, and presuming it is properly anchored and protected from chafing, I would consider a failure of the backbone itself to be highly improbable.
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Old 06-08-2018, 01:59   #3
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Re: Single point of failure

It is a good point, but it is not hard to design a system with sufficient redundancy with a little care.

You can install non NMEA 2000 equipment as a back up. For example, on our new boat we have NMEA 2000 depth as well as non NMEA 2000 depth.

It is also helpful to realise that from the power injection point there are essentially two NMEA 2000 backbones. With a suitable junction box in the event of a failure it is easy to screw in a termination resistor isolating one side.
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Old 06-08-2018, 02:22   #4
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Re: Single point of failure

My systems we pretty much stand alone.. when I got hit my lightning they all were effected as well as the bow thruster...
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Old 06-08-2018, 04:41   #5
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Re: Single point of failure

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Originally Posted by noelex 77 View Post
It is also helpful to realise that from the power injection point there are essentially two NMEA 2000 backbones. With a suitable junction box in the event of a failure it is easy to screw in a termination resistor isolating one side.

This is a good point and following the guiding logic you can fairly easily build in redundancy and have tactics and spares set to use if you have a problem.

I had an issue recently on my new N2K system that ended up being my fault during the install. However, what was concerning was how one iffy connection (the electronic compass) ended up bringing down the entire network.

That said, its light years ahead of 0183 in simplicity and its easy enough to troubleshoot once you have some understanding of it.

Its hard to imagine a backbone cable going bad short of it getting physically damaged but I suppose it happens. Far more likely are issues with the connectors or faults in the electronics themselves.
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Old 06-08-2018, 08:19   #6
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Re: Single point of failure

Met a fella just a few days ago. He'd just purchased his 45' sailboat with "full", up-to-date electronics. He was taking it from the point of purchase to his home base and got hit by lightning. He was insured, but has a $12,000 deductible. Plus, until he gets somewhere with available stuff, several weeks away, he has to do without-no lead line, no paper charts, no hand-held GPS or sextant, no nothing. He thought all those electronics would suffice.
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Old 06-08-2018, 08:25   #7
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Re: Single point of failure

I don't think it's likely if your system is designed well. Sure the main feed cable could burn up... but it's unlikely and if it does a fire extinguisher is going to be your most important item or you'll be swimming! Even that shouldnt happen though if you fuse the main cable as ABYC recommends now. Anything fed off that will melt the small wire first or trip the fuse/breaker.
A simple easy access on off switch on the main cable would be nice to put in.
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Old 06-08-2018, 09:31   #8
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Re: Single point of failure

Rather than be concerned about a few instruments when you do have paper charts, how about that new computer controlled engine?

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Old 06-08-2018, 10:51   #9
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Re: Single point of failure

Your "Single Point of Failure" concept isn't really accurate when describing a backbone setup, in that all the connected components are actually acting individually and the backbone is just how they communicate.
That being said, losing said communication can still be major setback. I suggest it would be wise to understand your backbone cables routing and connection points and be prepared to implement a jury rig when needed.

e.g: A typical network often has most components grouped at a steering station or pedestal, with a possible secondary grouping at a nav station. More distantly placed network components would be masthead wind instruments or a heading compass, which is usually located forward and at waterline level. Radar is generally high up but the newer digital units don't use the backbone, using a faster ethernet (e.g. "RayNet") cable instead to handle the higher video data rate.

So a quick switch out of a damaged backbone cable and termination would allow restoring MOST functions at the pedestal or nav station, or a combination of both.

In truth, only a backbone cable susceptible to mechanical damage, connections vulnerable to saltwater intrusion or, yes, lightning are likely to upset your network and the first two can be mitigated with some forethought and preparation.
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Old 06-08-2018, 11:04   #10
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Re: Single point of failure

The problem is that as you go down in wire gauge diameter from a "branch" you must then fuse the wire at the source of the change. Lets say for example you go from 50 amp "branch" to a circuit wired for 15 amps, you are then required to have a 15 amp breaker or fuse where the positive side of the wire connects to the branch. This puts these fuses in all sorts of strange places on your boat. This is versus having one or two DC panels in one or two places....depending on the size of your boat. The same of course applies to AC circuits.

Personally, if the boat is small enough that it needs only one breaker panel then I would rather have all the fuses for the circuits in one place. At a glance, you can see which breakers have popped. The old-timers are right.
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Old 06-08-2018, 14:36   #11
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Re: Single point of failure

Get a tablet that has GPS and a navigation app. You may need also secondary charging machinery to charge it somehow after the normal sources of energy have been fried.
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Old 06-08-2018, 14:54   #12
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Re: Single point of failure

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Originally Posted by Briney Bug View Post
Met a fella just a few days ago. He'd just purchased his 45' sailboat with "full", up-to-date electronics. He was taking it from the point of purchase to his home base and got hit by lightning. He was insured, but has a $12,000 deductible. Plus, until he gets somewhere with available stuff, several weeks away, he has to do without-no lead line, no paper charts, no hand-held GPS or sextant, no nothing. He thought all those electronics would suffice.
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Didn't he have even a smart-phone for GPS?

Note to myself: in any case of even remote chance of lightning, disconnect nav station PC from everything, and put PC and smart phone, in a metal enclosure, even just wrap in Al foil. PC with charting program and chart data loaded, plus phone for GPS position.
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Old 06-08-2018, 15:53   #13
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Re: Single point of failure

Quote:
Originally Posted by Briney Bug View Post
Met a fella just a few days ago. He'd just purchased his 45' sailboat with "full", up-to-date electronics. He was taking it from the point of purchase to his home base and got hit by lightning. He was insured, but has a $12,000 deductible. Plus, until he gets somewhere with available stuff, several weeks away, he has to do without-no lead line, no paper charts, no hand-held GPS or sextant, no nothing. He thought all those electronics would suffice.
Rudy Sechez
I have oodles of gadgets on my boat: Class A AIS, HF radio, sat phone, Delorme tracker, hand-held radios, satellite radio, pactor modem, flux gate compass, navigation reference unit, chart plotters (in cabin and in cockpit) ... lots of stuff.

I was also a broadcast engineer who's seen what lightning can do. I assume all that stuff is toast in a lightning hit (besides whatever I can cram in the microwave oven before the lightning gets to me). So... I also have a full set of paper charts, sextant, the current year's astronomical almanac, and a decent mechanical clock that's set every day. Don't forget to toss the hand-sighting compass in the oven - lightning can depolarize your ship's main compass - and practice at least taking noon sightings with your sextant. It's easy with a little practice. I strongly recommend watching this series of videos:

N2K has a significant reliability advantage over NMEA 0183: it's balanced as opposed to 0183, which is unbalanced. The N2K buss is consequently less vulnerable to voltage transients, RFI, and noise, because all of those noise sources are "common mode noise" that's rejected well on a balanced buss. Up to a point anyway: all bets are off in a lightning hit.


Be careful when you rip out all those wires: each device needs its own ground return, ideally to a single common-point ground. Even with a steel hull - because rust never sleeps. Otherwise, you'll get ground loops that will drive you batty when troubleshooting the consequences.
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Old 06-08-2018, 16:57   #14
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Re: Single point of failure

Lightning jumps open switch contacts as if they weren't even there !
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Old 06-08-2018, 17:20   #15
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Re: Single point of failure

It’s about deciding upon what’s critical. Most of my equipment and systems aren’t actually mission critical, and I’m not going to spend thousands duplicating those. What do you actually need in case of a failure? Me, I’d say accurate gps position would be most important, and a functioning chartplotter after that. So make sure you have at least two and preferably three completely independent ones of those, and at least one but preferably two that work with a complete electrical failure. Then I’d want a vhf, so I make sure the handheld is always charged ready as a backup. Third most important is the autopilot, which is a lot harder to get duplication of. If you have modern equipment you should be able to substitute the controller, but you can’t do much about the course computer nor drive unit unless you carry spares on board. At the very least you need to know the wiring schematic so you can isolate the entire autopilot from anything else that might affect it. An independent windvane system with the ability to be an emergency rudder is probably the ideal for ocean passages.

Depth and wind speed I can worry about when I’m next in port.
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