The Offshore Voyaging Reference Site

Review of Ocean Signal PLB3 Against MOB1

Ocean Signal1 have released the PLB3 Personal Locator Beacon that has both AIS and 406 MHz capability, as well a lot of other new features.

Let’s compare it to the Ocean Signal MOB1 that Phyllis and I have used since it came out and that I have written about extensively over the years (see Further Reading).

Feature / Specification MOB1 PLB3
AIS Alerting Yes Yes
406 MHz Alerting (satellite, same system as EPIRBs and COBs) No Yes
Return Link Service (confirms to the user that the distress signal has been received by the ground station; does not mean that a SAR mission has been launched) No Yes
121.5 MHz (older aviation distress frequency sometimes used by SAR assets to home in on an EPIRB) No Yes
VHF DSC alert (in countries where this is allowed) Yes No
Clunky, but usable, programming Yes No
Program and Monitoring with phone using Near-Field Communication No Yes
Visible Light Yes Yes
Strobe Yes Yes (IR)
Operational Life (hours) 24+ 24+
Height (mm) 134 200
Height (in) 5.27 7.9
Weight (grams) 92 160
Weight (lbs) .2 .42
Requires Registration No Yes
Street Price (US$) ~$335 ~$600
I have used bold type for the differences that I think are particularly important.

Ok, the new beacon has way more features and even works with our phones—queue gasps of delight—so it must be better.

Let’s all replace our MOB1s with PLB3s, and anyone who buys an MOB1 going forward is an idiot, right?

Nope, that’s the typical irrational decision making of us guys (women seem to have more sense) in which we automatically assume anything new and shiny with more features has got to be better, particularly if it has a companion phone app.

Let’s dig deeper:

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George L

You say it at the bottom, if you also want the EPIRB function, carry the MOB1 and the PLB1.

The PLB3 looks like a real clunker …

The DSC function is one of the most important ones, so I wouldn’t give up on in. I’d rather pay the fine if it deploys in a country that doesn’t allow it than be dead ….

And yes, an upgrade of the MOB 1 is loooooong overdue. The programming is absurd and those things ought to be rechargeable.

Matt Marsh

You may be overestimating the technical difficulty of making it rechargeable. An off-the-shelf BMS for up to four 18650 Li-ion cells is a $0.67 part that weighs one gram and takes up less than one cubic centimetre. They only get complicated and expensive when you start looking at larger multi-cell packs.
That said, relying on the user to keep something like this charged is non-ideal. There is something to be said for just sealing a lithium thionyl chloride battery in there and saying “yup, it’s good for 10 years at any temperature down to minus 55° or so.”
What I don’t see any justification for is permanently-sealed, non-replaceable batteries. If the device is reasonably well designed, anyone with a modicum of electronics repair skill should be able to open it up, replace the (non-rechargeable) primary cell, verify that it’s good, and seal the device back up for another 10 years of use.

Jean-Louis Alixant

Hi John,
Happy New Year to Phillis, you, AAC and all members!

Thank you for bringing up the PLB3 to complement the set of articles on MOB1 and PLB1. I went through the analysis when the PLB3 was revealed and reached conclusions similar to yours: I kept our six MOB1s (which I have installed in each of our PFDs).
But I stress that I also kept our two PLB1s, which are held in a Spinlock side pouch that is fitted to the relevant PFD when sailing solo or double-handed. When there’s more crew on board, we simply exchange the pouch with the PLB1 at crew shifts; this is done in less than a minute, unlike removing and refitting a MOB1, or PLB3 for that matter.
I mention the numbers because it is obviously a big investment. If I wanted to maintain the AIS capability on the vests we potentially use, which I absolutely do, I’d be looking at six PLB3 permanently-fitted devices, ouch!

But I never had to confront the economics of it: I simply do not want to replace our MOB1 + PLB1 setup with a PLB3 for reasons you mention in the article. Most importantly, I do not want the automatic activation of the 406 MHz, and I want the DSC feature (even though in our legislation, it is restricted to an Individual Call to the on-board VHF). And lastly, the PLB3 is too bulky.
I would suggest adding a column to the comparison table, to include MOB1 + PLB1. That may be a more meaningful comparison for prospective buyers.

One word to balance the comment on the “on-screen” programming of the DSC feature of the MOB1: I have found it simple, quick and effective; I have done it using OceanSignal’s website page and only had to restart once out of six devices. It’s generally a one-off, or at least rare, process. I therefore qualify the NFC capability as “nice to have”.

All that said, I do agree with the liferaft and solo applications you highlight for the PLB3. And wholly support the features you promote for a Mk 2 MOB1.

PS: A comment I recently posted on MOB1 at the end of an older article may be useful to readers interested in these devices.

George L

the programming works ok if you have access to a normal computer screen.

but if you only have an Ipad on board, its a pain.

Nick Dawbarn

Having just outfitted new everything I ended up reaching the same conclusions. The cost of buying the two vs one device was a wash, and I could have even saved money buying a MOB1 and a PLB if I got a reasonable PLB (I went top of the line with RLS with screen and combined with a MOB1 it wasn’t any more expensive than the PLB3). In exchange I get DSC, better lifejacket fitment (can’t say there is a heap of spare space in my spinlock with a MOB1) and a detachable PLB that I can use for other outdoor persuits or lend to someone if needed.

As I tend to move around boats a bit, better programing would move me. But I’d really want to be reprogramming DSC for the boat I am on, so even there the PLB3 doesn’t really offer me much in practice given it’s missing the all important DSC.

The idea of packing one in the liferaft does seem like a great idea though and I’ll file that one away for next round. Have already decided the next round of EPIRBs will have AIS, but that could be a great stopgap as an add-on in the mean time.

Michael Lambert

I can attest to the false alarm issue. I woke up one morning and saw that my pfd(with MOB) had inflated during the night while hung upright on a winch. Apparently too much rain on it despite being under the Bimini….. We were near plenty of anchored boats in the cove at Jewell island in Casco bay, a very popular spot, and when I turned on the plotter there were plenty of boats on ais going about their business. I suppose given my location everyone knew what was not up. But it was nice to see the alarm and mob marker on the plotter, so more of an interesting than bad experience. If satellites were involved, no bueno.

Michael Jack

Excellent stuff, John as usual. I was mostly considering the PLB for, as you so gently put it, body recovery while single handing. In my current sailing area, I won’t last long in the water even in a dry suit. The North Sea has a lot of traffic (shipping, oil rig tenders, and the rapidly increasing wind farm maintenance) so DSC is still a fairly good option for a remote chance of an actual rescue. Loss of DSC in the PLB is the biggest issue for me in fact. The others are a minor annoyance (I relish fighting again with the Spinlock to get the dam thing in…not). But like you and Jean-Louis, I think I will get one and stick it in a pouch (can either of you post the link to the pouch you are using as there seems to be quite a few). Ok, now back to making sure I don’t fall off in the first place.

Jean-Louis Alixant

Hi Michael,
I use the Spinlock Chest Pack, which has three attachment toggles that fit right into loops built into most Spinlock lifevests. In addition to the PLB1, and depending on the activity, I add energy bars, fluorescein…
Like you, we really emphasize staying aboard in the first place and we actually call the thing a harness, rather than a lifevest.

Michael Jack

Excellent than you, Jean-Louis. Makes sense to keep other things in it as well.

Jesse Falsone

I have a MOB1 but I’m ashamed to say that I have yet to install it in my Mustang EP38 despite numerous passages where it would be prudent equipment. Could you please share a good Youtube video of the correct installation procedure? I have not found one that was particularly intuitive.

Iain Dell

I was thrown into the water off the Falkland Islands when the ‘rigid’ and the ‘inflatable’ bits suddenly decided to part company in bad conditions returning to a warship. I later became involved in research on sea survival at the Institute of Naval Medicine to examine theory against practical reality.

My own experience taught me how rapidly you lose the ability to think, never mind be able to act. Although the conditions were extreme, I was wearing a layered immersion suit, was very fit and professionally trained for such conditions, not just a one day jolly in a warm swimming pool. The difference between deliberately going over and being thrown over the side is profound; even falling into warm water people can die from a number of different causes and you can’t really judge how you or your crew will react or cope no matter what their personality is like. There have been tragic cases where the casualties have been unable to even pull the sprayhood over their heads, despite being conscious.

The relevance to this particular and very interesting article is that any plan that relies on the casualty being able to operate fiddly bits of kit when in the water is, as Captain Blackadder would say, ‘Bollocks’.

As John has pointed out throughout this site, no one subject should be viewed in isolation. The choice between PLB or MOB1 is simply one sub-element in a system that includes staying on the boat, staying alive in the water, gaining help, safe recovery and post-recovery survival. Anyone considering which item to buy must regard this choice as part of an overall strategy. Perhaps surprisingly (or not…!) research has shown that the worst people to make meaningful risk assessments are those sailors with the most qualifications and the most experience. Over-familiarity, over-confidence and ‘its never happened in a gazillion sea miles’ are cited as reasons.

Whatever individual bit of kit is chosen, I’d stress through bitter experience as well as research that:

all safety kit must be able to be used by the people who might have to use it in the conditions in which it might have to be used.

So many read the instructions once then never look at them again. Training, training, and then more training is paramount, preferably in as realistic and hostile an environment as you can safely stand until your actions become automatic. Even that ‘jolly in a warm pool’ seriously increases chances of survival particularly if you can take whatever kit you’ve bought in with you and play with it. We all to have to make our own assessments on what might work best for our own personalities on our particular boat in the conditions in which we might be sailing.

Sorry for a bit of a ramble but hope this might be helpful. Having read my own words, I’m off to read those excellent articles on jackstay alternatives again…..

Iain Dell

You’ve shaken a great deal of complacency out of folk in these columns – none more so than me!

Whitall Stokes

During a shorthanded 165 mile race off California in March, the very loud DSC alert on my VHF radio went off. Lat/Long provided, skipper taking on water and not able to stop it.

The USCG and competitor response was swift and impressive. CG was in immediate communication over VHF and had a helicopter over the vessel in 15 minutes. Even if there was no USCG, the competitors would have taken care of things.

The entire area of the race, even though it runs 70 miles offshore was covered by VHF. I’d guess easily 90% of the time our needs are covered by DSC for most of us offshore types. Not sure about DSC alert response times in other countries.

So yeah, I won’t be investing in a PLB3.

But obviously off the shelf or other remote areas, the PLB/EPIRB system comes into it’s own.

Jim Schulz

In regard to the size of the PLB3, I’ve had one in my Spinlock vest for the past year and I don’t find it cumbersome at all, in fact it’s hardly noticeable. It was a little tricky to install the first time but I’ve re-installed twice and it gets much easier the second time.

Terence Thatcher

Thanks for the review. We use the MOB1 and will continue to do so. When I repack the life raft this year, I may buy the PLB to place there. I too worry about getting the big beacon and the ditch bag into the raft. Last year my son was wrestling with a head sail and it hit his life jacket, setting off the MOB. It was momentarily annoying, but at least my wife did not get a call from the Coast Guard about a beacon emergency. We try not to worry her when we are offshore.

Jim Schulz

That’s actually been one of my main concerns – inadvertent inflation and not getting it shut off in time. I hadn’t even considered the lack of DSC which was a pretty big oversight on my part. I’ve thought about carrying it in a pouch but agree with Ian that activating it in the water would be a stretch. I’m leaning toward the idea of putting it in the life raft and getting a DSC enabled MOB1 for the vest.

Eric Klem

Hi John,

If only I had read your article before purchasing ACR’s version of a PLB3 this summer, I am not sure that it would have changed what I purchased but I would have understood it better.. One thing that I completely missed was the lack of a DSC Alert. My own impression of DSC alerts is that they don’t seem to work as well as I would hope around here, maybe other areas treat them differently. I am amazed how often we pick them up and the coast guard doesn’t. The radio traffic that follows is often a little cringeworthy and it seems most people don’t know anything besides having heard an alert. I am not sure that I have yet to hear one that isn’t a false alarm but I do hear them maybe 10 times a year. All that aside, I did feel that the PLB type functionality was important for a few reasons. For one, I am often places with the life jacket where the boat’s EPIRB is not with me and I may not have access to it. Also, in an abandon ship situation, my lifejacket is highly probable to be with me although your solution is even better for those going the life raft route. As you have pointed out, a dedicated PLB could also serve these functions but I am trying to keep things as simple as possible and have a single unit.

Your concerns of false activations are unfortunately well founded. I was confident that I did the right thing when installing in my lifejacket the first time but a few minutes later as we were leaving in the dinghy, a coast guard boat approached and asked if we had an EPIRB or PLB that could be going off. Indeed, it was going off. Thankfully we were anchored only a few hundred feet from a coast guard base so they just jumped in a boat and came and asked. They were very pleasant about it but I felt quite bad and was thankful that there were no real distress calls when this happened.

The near field communication has also been very problematic for me, I have it on my list for this winter to try to figure it out better.

The unit I got did not ship with the adhesive for lifejackets with welded in bladders and ACR was pretty slow to respond but after several follow ups I did finally get them to ship it. They also sent directions to me marked draft which were not publicly available. It seems that the call was made to commercially release this before they had really gotten everything ready.


Dick Stevenson

Hi John and all,
Excellent article and excellent series of comments: not only the content, but exemplary of how to think through decisions.
One thought below on the prediction of false alarms which do seem more likely. The number of false alarms you report are disturbingly high as it is.
My concern has been, over the years, the number of boats going to sea where the boat is not fit for the duty or the boat is prepared poorly or the skipper is not up to the outing. This may be a variation of the thinking you worried about: rescue too easy to come by can lead to poor preparation and a certain abdication of responsibility. Call outs to Search and Rescue services should be for bad luck and not for poor preparation or poor judgment.
I say the above as every call-out, even the most benign, presents a degree of danger to SAR crew: even with their superb training. The false alarms will just escalate the likelihood that statistics will prevail and an SAR crew will get badly hurt or worse in a call-out.
It would be awful to have a tragedy occur going the rescue of a boat that was poorly prepared (think about reporting to the loved ones), but equally awful to have a tragedy occur for a false alarm. Both should not happen.
Our “deal” with SAR services (to my mind) is that we, as recreational sailors, go to sea prepared to meet the likely (and sometimes bad) conditions we might encounter and call on SAR for bad luck. Manufacturers should be very diligent that their designs are not going to be inviting SAR crew into danger needlessly.
My best, Dick Stevenson, s/v Alchemy

Stein Varjord

Hi John,
I agree with all of our thinking on this. I’d perhaps go even further on the same line.
lSay I could choose between two devices:
1. One will 100% certainly reach every satellite, ship, plane and whatnot on the planet, immediately and certainly, but not the mothership.
2. The other will never reach any of those nr 1 reaches, but only the mothership, and only with realistic probability.

For a PLB I find nr 1 totally useless and would go for nr 2 any day of the week. That’s true no matter what price or other issues.

If the PLB doesn’t help the mothership in the retrieval, it’s a total fail. That’s its ONLY job. Doing more is nice, but never at the cost of the main point! It’s kinda like giving a new phone model the ability to project videos on a wall, but then it can’t be used for any type of communication anymore. What was the point of a phone again? 🙂

I think it’s far better to have nothing than something dysfunctional. The latter gives a feeling of false safety, which is inevitably increased danger. The close to zero times the no 1 device could save a life is certainly far less than the number of people dying from feeling safe by that useless device.

Another issue. Sivil disobedience. When authorities make stupid rules, I inform them about my observation in no unclear words. In the case not allowed to program a general AIS alarm to all ships, it’s qualified, of course. I will do the illegal, inform the authorities that I did it, and ask them to try make me reverse it. If they are stupid enough to bite that hook, they will be displayed in the news shortly after. It’s surprisingly easy. News channels love this stuff. However, the acting authority is rarely the stupid part. The rules makers are.

Jonathan Durning

I recently called and spoke with a very helpful Ocean Signal employee because I am getting outfitted to cross the Pacific. I will have 4 crew for this crossing. I was informed that you are limited to a maximum of 2 PLB3’s per vessel ( per MMSI number ) and that PLB3’s can only be programmed at their factory. Thus, reprogramming your PLB3 to sail on a friend’s boat would in theory require sending your unit to the factory both before and after your day sail! In contrast the MOB1 can be reprogrammed by the user as often as needed. Thank you for your great article.