The Offshore Voyaging Reference Site

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

14 Jan 2019: We have now updated the information in this article.

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

In fact, Phyllis’ and my enthusiasm for this technology has led us to change our COB 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.

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Dick Stevenson

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

richard stanard

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



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!

Philippe Meloni

So true!


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.


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…

Marc Dacey

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…

Marc Dacey

Ah, thanks, John…this occured to me after I posted. Your point is correct. I mean, SH encourages me to register my handheld VHFs…

Richard Ritchie

In UK you should register your MOB-1 with OFCOM onto your radio licence, as an additional piece of radio equipment. I guess this relates to the gmdss radio alert part.

Dick Stevenson

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

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.


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 ( 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.

Rick Gleason

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.


I second all those who have thanked John for this and all the work he does.
This post explores some of the potential problems of a safety item’s deployment (an AIS beacon) that itself relies, in some cases, on the deployment of another safety item (an auto-inflate flotation device) that John has also shown in previous posts to have its own potential problems.
To me that begs the question of just how far down the road of system integration we should go, perhaps especially for safety equipment.


This is an interesting and very important post. Let me add some experiences:
(1) I bought two PLB1’s late 2015 in the Netherlands. No plastic caps, nor any mentioning of them in the manuals. We had them ‘armed’ inside our Secumar vests.
(2) Our fault: we stored a humid lifevest after a foggy crossing to Nova Scotia and it went off inside a locker during the night at anchor. Next calm and sunny day sailing along the coast I discovered the vest and flashing PLB1. Took it out of the locker and took more than 15 seconds to disarm. Had no VHF on (fault number 2) but within minutes the Canadian Coast Guard called my Dutch GSM (!!!). Things were solved immediately. Lessons learned: (a) The PLB1 worked flawless outside the locker. (b) Do register your PLB1 with your MMSI in your country. It saved the Coastguard some very expensive helicopter movements. (c) Always have your VHF on when moving. The Coastguard had called us relentlessly. Hats off!
(3) The battery of the PLB seemed not replaceable, as Ocean Signal told me in Canada. But is is.


It is MOB1 and not PLB1 in my post above. Apologies.


John —

I looked through the POB Prevention and Recovery book, and I could not find where you discussed your choice of POB device and why you chose it (admittedly, this may be user error). Anything you would care to share?

I am currently leaning towards the WeatherDock easyONE-DSC because it uses a water activation system similar to that used in our lifevests. That said, it is a bit bulky, and I am concerned about how it will wear once it is mounted.

Thanx!! Les


Hi John,

Great article. We just got delivery of two MOB1 units yesterday. The instructions were dated 02/21/2016 v1.03 (current PDF v1.06), but the boxes included the clear plate. The tethers were unchanged and so I shall be writing to OS for an update. No manuals talk of the 15s delay in transmission as part of training/ testing. Just that it takes 15s of pushing the test button to put the unit into programming mode. As for programming the unit, no success with the downloaded program, but both units programmed first time with the browser. Completely agree with your comments on registration of the beacons! Staggering they don’t have a place to do so on OS, especially given their 5 year warranty.




Hi John,
We have 2 MOB1, bought in 2015. I had exactly the same problem and discovered during routine maintenance that the sliders had dropped on both units without activation. After doing some internet research I found that later models were shipped with the protection plastic cover. I got them F.O.C from oceansignal. Two lessons from this: 1/ Product development standards in the boating industry is very poor even for safety product. 2/ Don’t bet your life on any of those ie: don’t fall overboard.
Thank you for this good article.

Peter Mannerstråle

One thing that’s not mentiond or not showen in any installations videos are the attachment of the fixation lanyard so you don’t loose the unit after activation


Hi John,

thank you very much for that article!

I bought two of this devices in Germany in July this year. They shipped with the plastic cover and a ribbon with a loop on one side.

After reading your article I contaced OC and this is what Debbie Heath answered (with her kind permission to publish it here):

The alternative activation tape and fitting method described within the article is a non-standard installation method designed for specific Spinlock Deckvest LITE lifejackets only.

The activation tape that you have and the installation method described in the MOB1 manual is correct. The activation tapes we supply are manufactured to a high specification standard which includes durability and endurance thresholds and are not prone to failure.

It is good that you should check your equipment prior to sailing and as company specialising in life-saving equipment, safety is our number-one priority. There should be no problems with your MOB1 or any of the accessories it is supplied with, therefore when inspecting your units, if you have any concerns at all or believe that any part has been damaged in any way, please let me know and we will be happy to help resolve all issues and give you peace of mind.



We have four MOB1s on board that we bought in 2015. All four have the grey activation cap and the thin activation strip still intact.
I just checked again on all four and one was indeed not properly armed (the red slider was not pulled down), which is of course our fault and I think I know when that happened:
Last year we had activated one of the MOB1 by accident while still inside the vest. I noticed that the signal on the chart plotter did not indicate a MOB, as the instructions claim, but simply a boat with an unknown to me MMSI and no name right on top of our boat. It took me a while to realize that this must be one of our MOB1s and when we checked the Spinlock vests we could see the blinking light through the vest. When we re-armed it, we probably forgot to pull down the arming slider. Earlier on that day I had clicked away the beeping distress relay on the VHF, not realizing it was our MOB1 that caused that.


Hi John
You are right, we don’t have those caps. I will try to get them asap.
Thanks again for the excellent article on this issue!

Julian Morgan

Dear John

We have the Weatherdock Easy One water activated AIS MOB devices in our lifejackets. Water activation means no activation strings to attach to the lifejacket etc. We sail as a couple and regard these as a vital component in our overall safety scheme. They can also be activated manually and have a test mode which we use to check them periodically. The only issue we have had is a crack developing in the clip that holds the activation cover and antenna in place. 2 out of 4 have failed but in my view this clip failure will not stop the devices from working. Our UK supplier is sending us replacement clips with a new design that should prevent the cracking issue.



David B


Thanks for an another thorough piece of work. I have just (March 2019) bought two MOB1s and they came with arming covers and open ended ribbon attachments although it is impossible to thread it the way it is shown in the instructions; I went mad trying to follow the looping order( version 01.06 20/06/2018). The UK radio licensing agency (OffCom) allows you to register the MOB1 MMSIs’ against ones’ account. Still no product registration process.

When I tested the units every alarm in the boat went crazy and my chart plotter and instruments jumped into life. If one of us sleeps through that cacophony we will have some serious explaining to do to the investigating authorities!


Bruce Kavanagh


Really useful article which I’ve just found via the link from your recent comparion with the Sea Angel device. I was pleased you still recommended the MOB1 having bought a couple of them a few months back!

Some observations from my own purchase and having used them on a charter boat in the Fastnet last year (they are now mandatory equipment for many RORC races like the Fastnet).

The way the activation ribbon has to be threaded in the activation slide is quite fiddly and also quite different from the very similar looking ladderlock buckles that we are all familiar with on rucsacs etc. but is critical to functioning of the auto activation. On our charter boat when moving one of the devices from a boat lifejacket to a personal one I found that the activation ribbon had not been threaded properly, such that when you pulled the part of the ribbon going around the bladder it just pulled through the “buckle” on the activation slide and so wouldn’t have activated when the lifejacket inflated. I checked the other lifjackets on the boat and found that only 2 out of 8 had been correctly threaded. The devices had apparently been “professionally” installed which was required since it was a coded charter boat so considered commercial. 2 key lessons from this:
1. Make sure you thread the ribbon correctly and that it locks when you pull on the part that goes around the blader.
2. Don’t assume that anyone else will have installed it correctly – check for yourself (nothing new there but just another example of why!)

My personal units purchased July this year had the activation ribbon with a loop in one end, and from Marc’s message above it sounds like Ocean Signal haven’t adopted the tape without a loop that Spinlock are using.



Juan Luis Picazo


Great article augmented by the excellent contribution of the members!

I own an MOB1 beacon that I acquired at the London Boat Show in 2017. I contacted Ocean Signal asking if they could provide me with the DSC Timing Chart of the MOB1, both “Individual Distress Call” and “All Ships Distress Alert”.
I was interested in gaining better understanding of the operational functionality of the DSC calls and potential implications of provisioning the MOB1 for a regional mode then navigating in a different region that regulatory does not allow it. 

Another area where I cannot get the rational is the “DSC Individual Distress Relay Call” as it implies that the MOB1 beacon relay a distress call originated by a third party (who is the third party? The POB who carries the MOB1??). I am aware that is regulated by the international organizations ITU/ETSI and has nothing to do with Ocean Signal.

As of today, I have not received response from Ocean Signal, but visiting today their web site I found the following link that I want to share, hoping that is useful in this forum:

Suddenly something went wrong, and the link seems broken. I was able to find this alternative link that you can try.

I will appreciate any comment, opinion, view… in the topic of MOB1 DSC Functionality and the rational of categorizing the DSC Individual Call as a Relay Call.

Congratulations for this great site!

David Shepherdson

I just had a disconcerting experience with my MOB-1 (purchased 2019) and remembered this post. My lifejacket went off accidentally recently (I think I snagged the manual release) but my MOB-1 did not go off. I think the ribbon was not tight enough around the bladder. My fault of course but I thought I had followed the instructions. I was unprotected for most of last year. I have very little faith that I could manually operate my MOB-1 at night and without glasses on, probably not even during the day under stress. Disturbing, I will be looking at some other alternatives now.

David Shepherdson

It was a spinlock, I purchased the lifejacket and MOB1 separately.

Jeff Eschman

Just wanted to add that I recently had the exact same experience as David. My Spinlock Deckvest 5D inflated while not being worn (possibly humidity + old sensor?), and my MOB1 did not activate. The ribbon was tight enough that it caused some constriction / necking down of the inflated bladder, but the activation clip had not pulled out. 

Trying to understand where I’d screwed up, I reviewed several Spinlock instruction videos (one of which I had followed when I originally installed our MOB1s) as well as those from other PFD manufacturers. NONE of the videos emphasize the need to really remove slack from the ribbon before final re-packing of the PFD. To my eye, they generally leave the ribbon fairly loose around the not-completely-folded bladder.

The Ocean Signal MOB1 User Manual has these less-than-helpful instructions: “Pull the tape tight with the free end of the tape, so that the bladder is free to inflate and remains folded in accordance with the life jacket manufacturer’s instructions. Do not over tighten the tape.” followed by and exclamation (!) and “Test for tightness by ensuring you can freely insert a finger in between tape and the bladder.” 

Clear as mud, right? Tight, but not too tight, but maybe 1-finger tight? My approach going forward will be to remove enough slack so that inserting one finger is easy (loose?), but there’s not so much that I can slip two or three fingers underneath the ribbon without risking pulling out the activation clip. Getting this right requires folding of the bladder first, and then adjusting the ribbon… which is quite fiddly to do with only two hands without accidentally activating the MOB1.

Anyway, TLDR: To ensure auto-activation, you probably need to snug the ribbon more than the Spinlock videos suggest!

Michael Jack

I was looking at the McMurdo SmartFind S20 device as an alternative (because I have a McMurdo EPIRB). According to the website, if it is fitted to a lifejacket professionally, it can also be activated automatically.

I know I know, don’t assume that “professionally installed” actually means that it really is installed properly, but it seems to me, given all the discussion about MOB1 installation problems, it makes sense for there to be a professional installation of such a critical device.

Anyone had any experience with the McMurdo SmartFind?

Michael Jack

Thanks, John. I have written to Spinlock to see what they say about which device they recommend as one being suited to auto activation on their lifejackets. The page I found on the subject (through a search and not in the menus) seems quite old so I am not convinced they are really committing to any particular device. Let’s see what their answer is if any (sales support seems to be hit and miss in the sailing business):

Michael Jack

A quick answer from Spinlock:

To date, we have not approved the SmartFind S20 with the Deckvest 6D and are not planning to. We prefer the Ocean Signal MOB1 as it was designed from the start to be automatically activated and sit inside the cover. It’s slightly smaller and lighter than most other models.”

So, my feeling is, since it is the Spinlock 6D that I want, I will stick with the MOB1 but take the advice from the comments here on installing it properly.

Many thanks for this great article and the book. It was a tough read but a necessary one as I setup my newly purchased yacht.

Back in the 80s when I last lived on cruising yachts, we had no safety gear at all. I mean no lifejackets, no harnesses, no life raft, no radio, no electronic navigation, nothing. Hell, our engine didn’t even work that often and our lighting and cooking was all kerosene.

It was skin of the teeth stuff and great fun (no tragedies thank god). But I am somewhat more risk averse these days so this book has been a real eye-opener for me.

Thanks again.

Michael Jack

Ok just to finish this off, McMurdo is discontinuing the SmartFind S20 and replacing it with the Crew1 model (nothing on their website about it yet but I have a datasheet if anyone is interested).

Michael Jack

Hi, John. Just installing my MOB1 devices which I bought this year. I can tell you that mine have the thread style ribbon. The instructions do mention putting the plastic cap on. The Spinlock video is back and has both the non-thread and thread types in the video (which was confusing at first because it failed to mention that it was instructions for two different types):

Mark Hamlin

John is very kind to OceanSignal. I just bought one Sept 2023. The instructions had a number of flaws, but with about two hours of my time and help from some YouTube videos, I did get the MMSI programmed in. Next problem was physically installing the MOB1 into the Spinlock Vito vest. Again the changing parts of the MOB1, and the changing parts of the Spinlock vest, nothing really fits quite right regarding the instructions on YouTube or the paper that they provided. Today in testing, the product DSC signal made it through, and although the AIS signal was greenlighted on the MOB1 it didn’t seem to show up in our electronics. This is complicated stuff. It doesn’t help that the instructions are inconsistent with the products you pick up in the store. I certainly want this to work as it’s a great advance and safety, if it actually works when you need it. I agree with John. It’s a disappointment that you can’t contact anybody at all OceanSignal except by email.

Terence Thatcher

We have been using the MOB1 for a couple of years. No overboard situations. One alarm on the AIS when a crew member’s MOB 1 was triggered wrestling with a genoa. I found the instructions hard to follow, but I believe I figured it out. Now I have to go back to check the tightness of the ribbon. What happened to the video you previously had on this thread? I would like to view it. Your work is invaluable.