Possible Auto-Activation Failure of The MOB1 AIS Person Overboard Beacon

As most of you already know, I believe that AIS person overboard beacons are the biggest advance in person overboard (POB) recovery in my lifetime.

In fact, Phyllis’ and my enthusiasm for this technology has led us to change our POB recovery strategy to be 100% reliant on the Ocean Signal MOB1 beacons that we bought in the spring of 2017.

That said, we recently discovered that for much (maybe most) of the first season after we fitted them to the Spinlock lifejacket/harnesses that we wear at pretty much all times when underway, they would not have self-activated.

And, while much of the fault lies with us, our experience does bring to light two potential problems that others relying on the auto-activation features of this beacon, particularly those who bought before mid-2018, need to be aware of.

How Auto-Activation Works

To understand the problems, we need to first look at the mechanics of how the MOB1 activates:

A small plastic piece called the “activation slide” contains a magnet and has a ribbon attached to it that is wrapped around the bladder of the lifejacket, so that when the jacket auto-inflates, the slide is pulled off the MOB1, thereby inducing an activation signal.


To clarify all this we have made a short video. Please watch it before reading on. We have also included rearming, as the process can be a bit intimidating with only the written instructions for reference.

Potential Auto-Activation Failure

So that’s how activation should work, but here’s where it gets scary. In the fall of 2017, when I checked on the beacons installed on Phyllis’ and my Spinlock lifejackets, I found that the slider was hanging loose on both beacons, so that neither would have auto-activated if one of us went overboard.

On finding this, my first reaction was to question why the beacon had not activated after the slider fell off (probably just from normal movement around the boat), and thereby warned us of the problem, both from the strobe light flashing on the beacon, which I would expect to be visible through the fabric of the jacket, and the alarm on the boat going off, not to speak of all the VHF calls we should have received asking if we had a POB.

The answer is that the slider must be removed rapidly to activate (confirmed by a phone call to Ocean Signal) and ours had fallen off gently—I have since confirmed by testing that it’s quite easy to remove the slider without activating the beacon.

Our Mistake

Now, most of the reason for this problem was our fault. That said, it’s a mistake others could have made, so I’m going to go into some detail.

When we installed our MOB1 beacons back in the spring of 2017, we neglected to install the small plastic clip-on cover with “ARMED” stamped into it (shown in the image at the start of the article and in the video), which prevents the activation slide from being removed gently.

Relevant page from the manual that came with our MOB1 beacons shipped from Spinlock in early 2017.

Stupid mistake? Well, yes, but there’s more to know. We bought our beacons from Spinlock and at that time they came with different instructions for installation on their lifejacket than those provided directly by Ocean Signal.

And, in addition, Spinlock had a video on their website showing how to install the MOB1 on their lifejackets that I’m near certain made no mention of the plastic cover.

I say “near certain” because since that time Spinlock have removed said video, both from their site and from YouTube, and simply link to the instructions from Ocean Signal.

Screen shot from Spinlock site taken 13-10-2018

But lest you think I made this up to make Phyllis and I look less stupid for making this error, you will note that the product images for the installed MOB1 on the Spinlock site still do not show the plastic cap in place.

That said, there is no question in my mind that Phyllis and I should have followed up with Spinlock and/or Ocean Signal to find out what the plastic cap was for and whether or not we should install it—big mistake and ours alone—particularly since I have now verified that the Ocean Signal instructions packed with our beacons do mention the plastic cap.

Still, when doing this sort of tricky installation, it is always tempting to just watch and follow the video, as we did, and so I fear others could have made the same mistake.

Shipped Without Plastic Cap?

Also, from reading between the lines of what I was told in conversations with Ocean Signal, I think that some early beacons may have been shipped without a plastic cap at all.


I strongly recommend that all owners of MOB1 beacons, but particularly those who bought early on in the product’s life, or who used the Spinlock installation video, immediately check for the presence of the plastic cap.

And Ocean Signal did tell me that they would mail free plastic caps to anyone who asks for them.

Failed Ribbon

Wait, there’s more. I also discovered when I opened one of our jackets, that the Spinlock-branded activation ribbon had broken at the small added loop made of thick thread.

So even if we had installed the plastic cover, one of our MOB1 beacons would still have failed to auto-activate.

The Ribbon Has Changed, Twice

I called Ocean Signal to share this finding and they sent me two replacement ribbons without the Spinlock branding and with a loop in only one end.

However, I have now found that the latest MOB1 fitting instructions (see image above) show an activation ribbon with no loops at all and a new locking toggle, which would seem to indicate that Ocean Signal and/or Spinlock have decided that the thread loops were a fundamental weakness—I would agree.


I strongly recommend that all owners of MOB1 beacons check if the activating ribbon has loops on one or both ends and, if so, replace it with the new ribbon with no loops and a locking toggle.

I have not checked with Ocean Signal, but I assume (and hope) that they will supply these new ribbons for free to all who request them.

Not a Witch Hunt

Now, at this point, some of you are probably thinking that my purpose in revealing all this is to in some way punish or get back at Spinlock and/or Ocean Signal.

Nothing could be further from the truth. I have a huge admiration for both companies and a deep appreciation for how difficult it is to design and build advanced safety equipment, particularly for a small market like offshore sailing that will never generate big revenue.

Bottom line, there will always be weaknesses in any safety product (particularly first generation ones like the MOB1), and so it’s our job as responsible boat owners to not assume perfection, but rather take the trouble to understand the products we buy to the point that we can identify and manage those inevitable weaknesses.

But Disappointed

That said, I am disappointed that despite the fact that I brought both of these weaknesses up in a conversation with an Ocean Signal staff member in June of 2018, that, although they have fixed the problems, they have not, at least as far as I can find, issued any sort of public warning to existing owners. Or enlisted the media to help spread the message as far and as quickly as possible.

Crew Training

In addition, based on this experience, Phyllis and I feel that, even with the improvements to the MOB1 lifejacket installation suggested above, auto-activation is by no means 100% guaranteed.

MOB1 with activation slide removed to show manual activation button.

Therefore, we strongly recommend that all crew members on MOB1-equipped boats should regularly handle and activate the beacon with the goal of firmly fixing in their minds the two methods:

  1. By rapidly pulling the slider off.
  2. By pressing and holding the activation button if the slider is not present or if removing it fails to activate the beacon.
You have 15 seconds after activation to deactivate the beacon by pressing and holding the Test button until the light flashes red twice, before it transmits.

Suggestions To Ocean Signal and Spinlock

As I said above, this is a first-generation product for both companies, so expecting them to get it perfect is simply unreasonable. That said, I would make the following suggestions for the MOB1 going forward:

  1. Both companies should immediately issue press releases to the media detailing these issues and offering free caps and activation ribbons, without loops and with toggles, to anyone who asks for them.
  2. Both companies should immediately set up an online registration screen on their web sites so that owners can register their products. As it is, neither product even ships with a registration card.
  3. If improvements are made to the products in the future, an email should be immediately sent to all registered owners.
  4. All products should be shipped with a card urging owners to register.

Recommendations two and three are trivial to set up using an email service like the one we use here at AAC (Mailchimp). I’m talking seriously-trivial, no more than half-a-day’s work.

Bottom line, it is simply unacceptable for companies making safety equipment to not have a way to contact their customers if they have a product problem.

Possible Product Improvements

As I said, the MOB1 is Ocean Signal’s first AIS POB beacon and, I think I’m right in saying, the first in the industry to include automatic activation when installed on an auto-inflating life jacket. Given that, I think they did a great job.

That said, I have two suggestions for Ocean Signal and one for Spinlock, to think about when designing future products:

Ocean Signal

  1. Manual activation if slider activation fails should be way easier than on the MOB1, where even finding the red activation button, particularly taking into account the realities of having recently hit the water, probably with cold and maybe gloved hands, could be very difficult.
  2. Reliable auto-activation should not depend on an add-on plastic cap.


Lifejackets that are intended to have AIS POB beacons installed on them should have a transparent window to allow crew members to easily inspect the status of the beacon without the need to unpack and repack the bladder, a process that is both fiddly and time consuming, and therefore unlikely to get done often enough, particularly at sea.

Worthy Competitors?

We are now seeing more beacons coming to market, and some may incorporate my two recommendations above.

Here are some I found as the result of a quick Google search:

That said, I need to make clear that I have not evaluated any of these beacons, and am only including the above links so that others can investigate before making a purchase decision. Also, I’m sure there are a bunch more options and more coming on the market all the time—all to the good.


  1. Has anyone else found on inspection that their MOB1 beacons were not going to activate properly?
  2. Does anyone have first-hand experience with alternative auto-AIS POB beacons other than the MOB1? (If you are considering suggesting the Weems and Plath CrewWatcher, or other smart phone-based “beacons”, please read this first.)


We are publishing this article for free as a service to the offshore sailing community. That said, testing, researching and writing about these kinds of issues is very time consuming, and it’s our members (not advertisers) who pay for this work.

So if you are not a member, please join us. The price is as little as US$19.99 a year for full access. Not only will you be supporting work like this, you also get a bunch of other benefits.

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Meet the Author


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 12 years, as co-editor/publisher of AAC.

19 comments… add one
  • Dick Stevenson Nov 8, 2018, 3:15 pm

    Hi John,
    Once again, thanks for all the hard work.
    I posted when you first wrote about the MOB1 that, on ours, I found the initial set-up fussy in a way that produced errors and dead ends which we finally sorted.
    I also looked at the thick thread on the loop with mis-giving, but it passed my tug test, so I went with it. I will definitely pursue the upgrade and again, thanks for making this happen.
    I also appreciated the video as I looked at the wind-up tool that came with the MOB1 and its directions with mis-givings also.
    Some directions just give me the willies as they scream at me that I am going to do something wrong.
    I think there might be an argument for a “rule”: the fussier an operation is, the more likely something will go awry.
    Finally, I know these are companies/products you admire so I appreciate and support your taking them to task for actions not (yet) taken.
    My best, Dick Stevenson, s/v Alchemy

    • John Nov 9, 2018, 8:37 am

      Hi Dick,

      Glad it was useful. I too was intimidated by all the installation stuff and the fear of doing something wrong with these units. That said, once I found out that I had 15 seconds before transmission, I felt a lot more confident in messing with them.

      And I like your rule a lot! In fact I think it might be a natural law.

      Hopefully the next generation of these things will address some of these issues.

  • richard stanard Nov 9, 2018, 3:59 am

    impressive post…i now have a new respect for your expertise and your thoroughness 😊

    • John Nov 9, 2018, 8:39 am

      Hi Richard,

      Thanks for your kind words, that said, your respect would be more warranted if I had dug in and sorted things as soon as I got the units, and not waited six months.

  • Les Nov 9, 2018, 7:18 am


    I have just read your comments on the MOB1. Very informative and slightly concerning!
    I have two MOB1’s purchased in the UK in April 2015 and neither had a plastic cover or any reference to it. Following your article I have emailed Ocean Signal to ask for advice.
    The only consolation I do have is that one of mine activated perfectly when I snagged my lifejacket on reaching into a deep locker!

    • John Nov 9, 2018, 8:41 am

      Hi Les,

      Thanks for coming up. And good to hear it worked.

      That said, your experience confirms that the original units were shipped without the plastic cover, so Ocean Signal is way out of line for not actively publicizing this. When will these companies learn that bad news does not get better with age?

      • Philippe Meloni Nov 9, 2018, 9:40 am

        So true!

  • Guillaume Nov 9, 2018, 11:52 am

    Hi John, good call. I also bought mine in 2017 and there was no protective plastic cap. Just off the phone with Ocean Safety sales service in the UK that seems unaware of the issue. That being said, looking at the protective plastic cap, it is not attached to the Mob by any strap. So you would lose it in case of an emergency activation.

    • John Nov 10, 2018, 8:22 am

      Hi Guillaume,

      Yes, the plastic cap would definitely be lost in an emergency activation, but I can’t see how that matters much since the beacon would have done it’s job. Also, after use the beacon would need to be returned to Ocean Signal for a new battery so a new cap could be acquired then.

      That said, given that they were not shipping the plastic cap originally with the unit, it is clearly an after thought modification to fix a problem.

  • Guillaume Nov 9, 2018, 12:19 pm

    Hilarious. The gentlemen at Ocean Safety is telling me the cap is an extra optional part priced at £1.75 + VAT. One would hope the PLB activates quicker than the information circulates within Ocean Safety…

    • John Nov 10, 2018, 8:26 am

      Hi Guillaume,

      That’s very disturbing. The person I talked to who knew about it is Debbie Heath debbie.heath@oceansignal.com

      (Normally I would not publish someone’s email address like that, but to be blunt if Ocean Signal are not going to get their act together on this, they deserve an avalanche of emails.)

      • Marc Dacey Nov 10, 2018, 5:17 pm

        Once again, I thank you and Phyllis for falling on the swords of early adoption of new and potentially game-changing technologies and for doing the meticulous work of actually testing this gear aboard your boat (well, in the saloon, anyway). I find the lack of a way to register this sort of device anomalous, given that every PLB I’ve ever heard of, the device that has the greatest functional resemblance to this one, has a registration process that involves boat and personal info, MMSI, and all sorts of data sent to the usual SAR facility. I was told this was to avoid situations where a PLB was chucked out into the waste stream and might involve a “false positive” on a garbage tip.

        Having read your piece, would you suggest buying these units from Spinlock or directly from Ocean Signal? I mean, I might give it a few months until they fix their string vs. ribbon issue…

        • John Nov 11, 2018, 8:26 am

          Hi Marc,

          Thanks for the kind words.

          As to registration, two different things. PLBs and EPIRB’s are registered with the applicable government authority because it is they who will receive (via satellite) and verify any activation. On the other hand, AIS beacons are short range devices (max 5 miles) so there would be little point in governmental authorities keeping track of them. What I was referring to in the post was my belief that all manufactures of any safety equipment should provide a way for buyers to register with them so that in the event of a product problem or improvement the manufacturer could inform the buyer via email.

          As to who you buy a MOB1 from, I don’t think it matters.

  • Dick Stevenson Nov 9, 2018, 12:23 pm

    Hi Guillaume,
    That is a disturbing report. Lets hope you just got a telephone answer-er who is just out of the loop. Dick

  • Alastair Currie Nov 11, 2018, 8:47 am

    This article is why I subscribe to the service you provide: independent, honest information, thought through and presented in a no nonsense manner. One of the key points you make from time to time, is that we the user need to take responsibility for understanding how our emergency equipment works – in detail. If we are not happy with the online or written instructions, then we should seek clarification; being cognisant that the retailer or professional might not always have the correct answer either, but may feel they have to give an answer. It does happen. Thanks for reporting on this as AIS POB locators will be purchased when I start extended cruising.

    • John Nov 12, 2018, 7:32 am

      Hi Alastair,

      Thanks for the kind words.

      And yes, that’s it, isn’t it? It’s just not smart to just buy some piece of gear and blindly trust it without taking the trouble to really understand how it works. That said, that’s exactly what I did for the first season. Not proud of that!

  • Cyrille Nov 11, 2018, 3:57 pm

    Thank you for this article. We debated a lot about which mob tracker to get and went with the mob1 instead of seaangel due to better chance of auto activation…
    We bought 2 mob1 a month ago (Amazon.com) and got the plastic ‘armed’ cover but not the updated ripple attachement, which should be there since May 2018 as the updated online doc is showing.
    This is strange as the paper document is from 20/06/2018 (version 01.06) but not aligned with the linked posted above on Spinlock.

    • John Nov 12, 2018, 7:35 am

      Hi Cyrille,

      Hum, the timing is interesting. I fear that Ocean Signal may have been using up old ribbon stock when they packed yours. Also, they sent me the older single loop ribbons in June of 18, so that kind of confirms that. Not good.

      The more I’m hearing from buyers like you, the less happy I’m getting.

  • Rick Gleason Nov 12, 2018, 8:13 am

    When I purchased an earlier version by Ocean Signal two years ago, it was clear that lawyers had gotten involved. The warnings and disclaimers were inside the box in a small pamphlet with small print, and it had warnings that the device would not work properly beyond restrictive conditions and legal statements, such as there were no warrantees once activated. There was a statement that the device complied with certain regulations.

    When I called to discuss this notice with Ocean Signal, because I simply wanted some assurance that it would work, provided I held the device above my head to get the signal above the waves, I did not get much assurance. I asked if there was any testing done and if there were any reports available and the answer was simply that there were no reports. I asked about the compliance testing and they advised to call the government. I found the compliance testing was not really about safety.

    I still wore the device but was not terribly convinced it would work properly, which is probably a good thing!

    I do hope that Ocean Signal has cleaned up their ridiculous legal antics, and others have reported on real world testing by now.

    Thank you for the report.

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