MOB1 type device Vs Portable VHF with panic button
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jssailem
SBO Weather and Forecasting Forum Jim & John
- Oct 22, 2014
- 22,825
Never falling off the boat solves the problem. On that one event when you step the wrong direction without the tether is when the Mob1 and the PFD become important.
I saw an AIS MOB go off in Maine a few years ago. It was in Castine harbor (a false alarm). I was 8 miles away on the other side of Vinalyhaven island and my chart plotter went nuts with a flashing stick figure icon of a drowning man. I assume it had such a long range because my AIS antenna is at the masthead 60ft off the water. But in any case, I've never seen a handheld VHF carry more than 3 miles. Anyways, in less than two minutes the CG, a harbormaster, BoatUS, and six other boats came up on VHF 16 and were responding. And these days I suspect there are more boats with AIS than working DSC. The AIS MOB is also much smaller - hardly noticeable when packed in the inflatable lifejacket. I don't know of any VHF radio that's small enough that you're going to want to wear it on your person at all times.
I was so impressed by the Maine experience above that now when my wife and I are alone on the boat we wear our AIS MOB's. When we're in coastal waters with other boats I tell her if I fall over she should just get on the VHF and make sure someone's received the AIS MOB signal and are on their way to pick me up - preferably a professional in a low sided fast powerboat. No watching me constantly. No throwing stuff overboard. No fancy maneuvers under sail designed for a full racing crew. I've never been confident that in a MOB that my wife alone could maneuver our high sided, high windage sailboat under power near a swimming person in open water without someone getting hurt. Frankly, I'm not sure I could either.
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Anyway, I believe the panic button on a newer DSC enabled VHFs essentially do the same thing as the MOB1 device, but at a considerably higher transmitting power. By the way, how did you know the signal you saw was not someone pressing the panic button on a DSC enabled VHF and not a MOB1 device? Or maybe the MOB signal was sent by the mothership or someone else repeating the distress signal.
While it is true that the MOB1 device is smaller, the Standard Horizon HX890 and the Icom m93d are not too big
at all. They both have built in GPS's, transmit at 5 Watts (m93d) and 6 Watts (HX890) and cost approximately $200 each. The MOB1 will automatically start sending a distress call when an inflatable PFD is activated. I wonder if it also activates with a non-inflatable PFD. (I personally dislike inflatable PFDs; but that's a subject for another thread.) One nice feature of the HX890 is that it has a strobe light that is enabled on contact with water to help locate you.
Anyway, here is a URL that has links to US Coastguard DSC coverage maps. There is also a similar page that shows the Canadian coverage. From the The US Coast maps web page:
"Charts showing predicted areas of VHF National Distress System coverage can be downloaded in .jpg format. Coverage plots assume a mobile transmitter power of 1 watt or greater at sea level over water."
By the way, the MOB1 transmits the DSC signal at 0.5 Watts and the AIS signal at 1 Watt, according to its documentation. That just seems low.
jssailem
SBO Weather and Forecasting Forum Jim & John
- Oct 22, 2014
- 22,825
I think both have value the other does not.
With the VHF radio you make the call and get to tell the CG who you are and ask for help. The challenge is you need to be awake and able to push the buttons.
The Mob1 operates without action by the person struggling in the water.
With the VHF radio you make the call and get to tell the CG who you are and ask for help. The challenge is you need to be awake and able to push the buttons.
The Mob1 operates without action by the person struggling in the water.
No they don't. The panic button on a VHF sends a digital distress call to all stations within range and repeats that signal until other stations come on and respond. It sends the location of the boat, not the person overboard. The coordinates are sent along with a code that describes the emergency, if the sender takes the time to enter the code. There is substantial, minutes long, wait time between rebroadcasts.
The MOB1 sends a AIS distress signal which is different. It shows up on all AIS receivers within range. The broadcasts occur seconds apart, not minutes, although it will take a minute or two for the gps to establish location. The MOB1 sends a DSC call to a preprogrammed VHF with an MMSI. The MMSI should be the mothership's MMSI.
This link explains how the MOB1 works once it is activated.
It depends, if the radio happens to be a portable unit attached to the person overboard then it gives their location.
If the radio has a gps.
Let's play this out. I fall overboard and manage to remain conscious and to hold onto a handheld VHF. At water level I hit the DCS panic button and the radio sends out a distress message and the GPS coordinates. All DSC enabled radios that are turned on receive the message and the coordinates. The USCG will wait almost 3 minutes before responding, and they will call the station sending the distress signal to inquire about the emergency. In the meantime, if any boat has heard the distress signal, they are trying to copy down the coordinates and then enter them into the chartplotter or gps to see the location of the distress call. And now, assuming no transcription errors, the boat can begin navigating to the distress location. In the meantime, the crew (likely your wife) is on the boat trying to get it under control and copying the coordinates and entering them into the chart plotter, while remaining calm and collected so she doesn't make an error.
Let's take the other scenario. I fall overboard, my inflatable PFD inflates, 15 seconds later an AIS distress signal is sent. It will continue sending distress signals 4 times a minute. At 2 minutes after activation the MOB1 is now sending accurate GPS coordinates along with the distress signal. It has also sent a call back to the mothership on a DSC channel to alert the crew that I'm in the water. By 2 minutes all AIS recievers within range will show my location. The MOB1 continues sending my location until the battery dies or I turn it off.
The boat that is most likely to save you if you fall off is the boat you fell off of. The faster that boat can find you, the more likely you are to survive. If you are short handed and if the crew is below decks, no one is going to see you fall off or maintain a spotter on you. If there are any waves or swells, you will quickly become invisible to the crew. At 5 knots, the boat will travel 500 feet in a minute, almost the length of 2 football fields. That's 8 feet a second or a boat length every 4 or 5 seconds. In the 2 minutes to get a firm gps fix from the MOB1, the boat will have traveled 1000 feet.
It is a simple choice.
I do appreciate your thoughts, I do have a question though. Suppose I am wearing a standard sort of PFD that is not inflatable. Will the MOB1 still send a distress signal? I ask because there was a situation in the 2018 Mackinaw race where an inflatable PFD, equipped with a AIS beacon, failed to inflate and the sailor drowned. There have been other incidents like this as well. I have personally had a inflatable PFD inflate while I was crewing and dry on the boat. How embarrassing! Anyway, that is one issue for me. I do not like inflatable PFDs.
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jssailem
SBO Weather and Forecasting Forum Jim & John
- Oct 22, 2014
- 22,825
All that Dave identifies here is helpful and accurate. One last twist is, how did the owner of the VHF radio set up the DSC feature. Like many, the VHF radio is using the sailboats MMSI number. The data in the file is for a boat not a person.
The confusion is worse when the searchers looking for a boat get intermittent radio signals from a person in the water riding up and down the waves.
I am of the opinion that both the automatic MOB1 and a VHF radio are reasonable to have on your person should you be sailing solo or with a crew member down below in the cabin.
The confusion is worse when the searchers looking for a boat get intermittent radio signals from a person in the water riding up and down the waves.
I am of the opinion that both the automatic MOB1 and a VHF radio are reasonable to have on your person should you be sailing solo or with a crew member down below in the cabin.
jssailem
SBO Weather and Forecasting Forum Jim & John
- Oct 22, 2014
- 22,825
The MOB1 relies on the inflation of the PFD tubes to work automatically. If no inflation tubes, then the MOB1 works upon activation of the user, just like the VHF radio. You just grip and rip the activation cord. It is a Gross Motor skill. The VHF radio requires your fine motor skills (likely seriously hampered in cold water immersion) to flip a cover and press the tiny button to initiate the emergency DSC call.
Was that the WingNuts accident? My understanding was some of the crew was below decks wearing the inflatable. When the boat capsized the vests inflated and they were trapped below.
There is no 100% effective solution for all cases. There are always tradeoff, the best we can do is prepare for the most likely situations.
There have been improvements in inflateable technology. I once had a vest inflate on the front seat of my truck at 70 mph.
There 2 types of inflaters, a pill type and hydrostatic. The hydrostatic ones will only inflate if they have been submerged, the water pressure triggers them. The other pill type have pills that are less sensitive to moisture and dampness than the old ones. Mine went off because I had been wearing it in the rain and threw it and wet foul weather gear into the front seat.
He fell into the water, the PFD did not inflate, the crew tried to rescue him, the MOB device failed and he drowned. I do think a discussion of PFD types is a valid one. But, perhaps not in this thread.
Lifeline netting?
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jssailem
SBO Weather and Forecasting Forum Jim & John
- Oct 22, 2014
- 22,825
Yes Lifeline netting is a practical add on, especially to aid in keeping toddlers on board. It provides a guard for the space between the lifelines.
It is only as good as the way you secure it and maintain it. The line used to tie it to the lifelines is under constant exposure to UV. So it will require replacement in time.
It has a look to it that appeals to some boat owners. I believe it is not better than a proper installed and used Jackline, tether and harness.
It is only as good as the way you secure it and maintain it. The line used to tie it to the lifelines is under constant exposure to UV. So it will require replacement in time.
It has a look to it that appeals to some boat owners. I believe it is not better than a proper installed and used Jackline, tether and harness.
No small boat sailor should use the pill type of automatic inflating lifejacket. We spend too much time wet. I just wear a manual inflating one and take the chance that I'll still be conscious after falling overboard (most automatic inflating PDF'S can be converted)
And yes, an AIS MOB that doesn't go off when the life jacket inflates can be easily triggered by tugging on a cap or string.
If you are willing to always wear a conventional lifejacket and carry a charged VHF that's an option. But most people find that uncomfortable or cumbersome - especially when sitting - so they are likely to have neither when they unexpectedly fall overboard.
And yes, an AIS MOB that doesn't go off when the life jacket inflates can be easily triggered by tugging on a cap or string.
If you are willing to always wear a conventional lifejacket and carry a charged VHF that's an option. But most people find that uncomfortable or cumbersome - especially when sitting - so they are likely to have neither when they unexpectedly fall overboard.
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The OP has a 31 foot boat. Big enough for an auto inflater. However, your point is well taken. Small boat sailing, paddling, etc is not the place to wear an auto inflater. If for no other reason than it will cost $30 -$50 every time you fall in.
Well, my boat's 55ft but I still just use a manual inflating PDF. The odds that I would fall over AND be immediately unconscious are so low that I don't worry about it. And if I fell off the dinghy, I probably don't want it to inflate. This is especially true with the AIS-MOB attached as I don't want to send a false alarm.