The Offshore Voyaging Reference Site

AIS Crew Overboard Beacons—Setting Up The Boat Alarms Right

When working toward making our boats safer, it’s easy to just buy a piece of gear and then pat ourselves on the back and move on. And I have to confess that’s pretty much what we did in the spring of 2017 when we bought two MOB1 AIS crew overboard (COB) beacons.

But Phyllis and I have learned over years of going offshore, that voyaging safely is a lot more about good installation and developing procedures taking into account the gear’s strengths and weaknesses, then it is about just buying lots of cool safety gear.

So last winter we really thought about how the one of us left on the boat would use an AIS COB beacon to help recover the other in the water, and then documented that in great detail (see Further Reading).

And this spring we dug into how these beacons actually work. In the process we learned some important things, and that’s what this and the next two chapters are about.

Yes, a lot about just one piece of gear, but I make no apology for that since I firmly believe that AIS COB beacons are the most important advance in COB recovery gear of my lifetime.

Let’s start off this chapter by looking at the boat side of the equation, and more specifically what alarms will, or maybe will not, sound in the event of a crew overboard.

And we even have a video at the end to show what a full-on (not test) AIS COB beacon alarm looks like on Morgan’s Cloud.

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Denis Foster

On Panbo website : Lessons learned by causing a false AIS MOB alert

This a detailed story of false activation of a Mc Murdo S10 Ais beacon by electromagnetic field and by the way some flaws with NMEA2000 and a Vesper xb8000.

Lessons learned :

Not fool proof device, the battery would of gone dead after a few days of activation.
The Nmea2000 didn allow the Ais alarm and icon to show up.

On our HR46 we both have Vesperxb8000 with Nmea 183 and 2000 so I will check. Our life jackets have Mc Murdo s20 and Oceansignal mob1 so does our MOM ocean safety Jon buoy.

Thanks for this article that reminds us to check installation and its potential week points.

Dick Stevenson

Hi John,
Once again, I am very admiring of the quality and quantity of work that goes into these reports. You do the offshore cruising (and racing) community a great service.
I am curious about whether you feel that the setting up of the POB system to work effectively had to be such an effort (I hesitate to call it an ordeal, but I suspect that would have been my experience). From my outside observation, you were executing R&D (research and development) which should have been done by the company (ies) involved. I do recognize that there are a number of companies involved (ICOM, Vesper, Ocean Signal, etc.), but it does appear that OS had some obligation to be clear which ancillary units their product plays nicely with, the ones that need tweaking etc. possibly even working with the other companies to develop a (more) seamless package.
My best, Dick Stevenson, s/v Alchemy

Bill Wakefield


Thanks for your continuing efforts for the betterment of all.

We went through a couple of iterations with hardware and device upgrades, and feel comfortable with, and confident of our current MOB alert system set-up. [We still have earlier generation AIS only MOB beacons.]

We test the beacons routinely, but as you point out, that isn’t the same as testing an actual MOB event.

With current generation Vesper AIS units [we replaced our Watchmate 850 with a Vision last year] we are able to test our instruments reaction to an actual MOB alert transmission without having to activate a beacon. [True MOB transmission (over our NMEA networks) from the Vesper, but in a test only mode.]

This is truly useful for confirming things still work as expected– especially after upgrades and tweaks along the way.

Ben Ellison at Panbo just happened to publish an article regarding this feature today:

I thank you and Phyllis, once again, for all your efforts.

Cheers! Bill

Marc Dacey

Very good, John. You do the sailing community a real service with these sort of tests. Certainly AIS is in the early days, and I suspect “virtual buoyage” based on AIS will change things even further. But the POB issue is a compelling one. I think AIS tags combined with these sort of alarms (and assuming one has crew that find one charming!) have the potential to superannuate the older PLB tech. I have a ACR ResQFix with 406 GPS broadcast ability that I bought for an offshore delivery in 2009, but it’s the size of a cellphone from 1991 and I wouldn’t mind retiring it. This tech gets us closer.


Thanks John,
Excellent Article! I have been considering how exactly to set-up our system. We have the Vesper 8000 AIS with both NMEA 0183 & 2000 wired to our system. I didn’t know Ocean Signal had the Alarm Box… thanks!!!
I agree: being able to use the AIS as a MOB recovery tool is the biggest safety innovation I have seen as well.

Marcus Ward


Are you worried about creating a single point of failure that would bring your whole network to a halt if the OS alarm box failed?

Tom Hanaway

We just purchased our ocean signal alarm box and two MOB ais beacons.
Has anyone overcome the need to manually turn on the alarm box each time it is powered on?
Tom Hanaway

Tom Hanaway

Following your guidelines, I’m posting this here. Just purchased two ocean signal mob devices and the alarm box.
Our present configuration uses a vesper 8000 which shows AIS info on our garmin chart plotter.
I’m reasonably comfortable with wiring electronic instruments (having done complete wiring and installation of avionics on two separate airplanes), but for the life of me I can’t determine how these different systems are hardwired.
I.E., what connection goes to where between the Vesper 8000, chartplotter and alarm box.
Once I have this sorted out, I’ll do the various configurations.
Do you have a schematic that shows how the Vesper, chartplotter and alarm box are wired together?
Thanks in advance,
Tom Hanaway

Tom Hanaway

Gee, it’s so simple when you see it. Thanks. I guess I was thinking that I needed to run an independent 0183 cable somehow incorporating the vesper blue alarm wire in addition to the existing wiring. My current hookup is 0183 from Vesper to chartplotter. So, an easy reroute.

Andre Langevin

I was excited to purchase an Alarm box and was surprised to find it come with NMEA 0183-HS … not NMEA0183. These are not the same protocols at all one being at 4800 bauds and the other 38400 baud and no electrical compatibility between the two. I ended up purchasing an Actisense ND-4 to convert 0183 to 0183HS.

Modern boat have more networks than the median enterprise 20 years ago 🙂

Andre Langevin

Well on their web site it is clearly said:
NMEA0183-HS (38400baud) input
and the HS is important when anybody talks about NMEA. I am a member of NMEA and a “certifier installer” so i have access to the full standards. Originally when i read out on the AAC site that its was an 0183 thing i just went out to buy one… and on my boat since its all Furuno based it is a native NMEA 2000 and Ethernet boat . The only 0183 on the boat is
for providing basic position and time sentences to the VHF to support DSC operations. NMEA 0183-HS was never on the boat.

Upon receiving the box i clearly saw that i had to work some more to have it installed because i clearly recognized the value of it. The result is a little bit of work ton interconnect a new ICOM radio (because the old ICOM 602 doesn’t support DSC). It ended up like this :

Since i have not “yet” been able to configure DSC on the MOB1 (will work on it) because of its Canadian birth certificate or whatever, i have documented how i had it working on AIS:

Hope it can be useful to anybody contemplating the installation of the Ocean Signal Alarm Box on a non NMEA 0183-HS boat.

Andre Langevin

Sorry for the tone of the post, a paragraph is missing and also some edit, i have been caught short while editing. Please consider it as an unfinished translation.

Tom Hanaway

Boy, I thought I was all set to install the alarm box between my vesper ais and the chartplotter. Both are presently wired 0183. Now I read Andre’s comments re: 0183HS. I took a look at the schematic Andre attached and I don’t see a connection from the AIS to the alarm box.

So am I to understand that a vesper AIS to alarm box to chartplotter all on 0183 will not work? If not, where is the AIS connected to the 0183HS adapter.
Tom Hanaway

Tom Hanaway

Just checked the Vesper AIS and it does put out 38,400 baud so no adapter needed.

Andre Langevin

Indeed Tom i checked most of current Vesper AIS and they support NMEA 0183-HS so you are good to go.

Andre Langevin

Hi Tom the VDM sentence is coming on NMEA2000 from the Furuno FA50 in my case, so it transit through the NMEA0183 port of the Navnet 3D and then to the Actisense ND4 where it is converted to NMEA0183-HS.

If you are 0183 it will not work. 0183 and 0183-HS are not compatible “per se”. This is why is had to purchase an Actisense ND4 which has great value is resuming all the NMEA0183 into a single box and permitting to have multiple signals with the same message not overloading the output.

I’ve written to Ocean Signal to see if there is any way to make sure the Alarm Box is receiving any NMEA data on NMEA0183-HS …i only see a green light and no diagnostic on the PC application used to program it. No response yet but i will post if i can get one.

Andre Langevin

Just wondering if someone has successfuly been able to get an alarm from the Ocean Signal AIS Alarm fed from a Furuno FA-50 (through NMEA0183HS). I have a case open at Ocean Signal because i have tried two units and neither has been able to do a sound altough the !AIVDM message is going through and plot correctly on both the Navnet chartplotter and the OpenCPN software. I have supplied an NMEA trace to Ocean Signal and waiting for reproduction on their side but if someone did install with Furuno AIS i’d be glad to know. Or maybe i was just unlucky…

Robert Krinner

Hi! I recently tested my AIS-MOB euipement and found some serious issues altough the B&G support is not of any help.

1. I activate my McMurdo S10 smartfind AIS-MOB.

2. My B&G Zeus 3, software april 21, pops up a window acknowledging the AIS-SART with the position giving the options of ignore, store a waypoint or activate MOB on the Zeus.

3. What does not happen is a audible alarm on the Zeus. The alarms on the Zeus are weak anyway, but at least there should be one.

B&G support claims, that there is an alarm as long as the „siren enabled“ is ticked, which I have, but it isn‘t.

To my understanding this option only activated the external alarm, currently none wired, but any way will only repeat an internal one.

B&G support, after ignoring my last question, insists, that it is a faul of my AIS receiver, the MOB beacon for sending an incomplete sentence and a crooked software on the MFD.

As an experienced sailor and user I know that this is the usual blamegame.

Q: Can anyone run a test with teir equipement and let us know the outcome ?

My setup: Zeus3, software april21, mc murdoch S10 smartfind, vespermarine XB-8000 smartAIS.

Christophe Herbinger

Hello John, Great info as usual.
My Garmin AIS 600 is connected to my Garmin chartplotter through Nmea 2000. The AIS 600 also has a NMEA 0183-HS port which currently is only used to provide power to the unit.
If I were to supply the NMEA0183 Tx+ and Tx- signal from my AIS 600 to the Rx+ and Rx- of the Ocean signal AIS Alarm, but without connecting the AIS Alarm to my chartplotter, would it work?
Your schematics shows the AIS Alarm as a pass-through between the AIS and the Chartplotter, which would not be the case in my situation.


Christophe Herbinger

Thanks John. You are right I would not be alerted to the fact that I forgot to turn on the COB. Unfortunately It would not be easy for me to connect Ocean Signal Alarm to my chartplotter through Nmea 0183. The AIS and the future Alarm are by the Navigation table inside the boat, while my chart plotter is some distance away by the wheel, connected through a Nmea 2000 backbone. Anyway thanks for your help and all the very good insights that you provide us all.