What does your Class B AIS transponder do for you?

[W32]

I was thinking of an AIS transponder purchase but on further inspection I am not certain of how good of a use of money it is, because it isn't cheap. The reason for my hesitance is that these things put out less than 4 watts of transmit power which doesn't seem very much to me. A ship traveling at 20 knots isn't going to be able to do much by the time it picks up your 4 watt transmission. I'm thinking of how limited a typical 5 watt hand held VHF is and even though there is a height advantage it still won't be impressive. Come to think of it, I haven't heard of even one instance where a Class B transmission has effectively alerted the crew of a fast moving MV that made any difference. I'd like to hear your experience with your transponder. Does it give you any peace of mind?

[Off Trail]

We have seen freighters divert for us at 10+ miles. Not to mention many, many cases where improved SA at shorter ranges was a benefit.
It's a no brainer IMO.

[HeywoodJ]

Wow, way to jump in and stir the pot.

Classic Class B AIS is limited to 2W transmit power, Class B SOTDMA has 5W/2W options depending on traffic.

We have a Vesper Watchmate 850 old-school Class B AIS transponder and I have watched on numerous occasions as a ship over the horizon from us alters course 2 or 3 degrees to keep clear of the WAFI up ahead. I can even tell you their company policy on CPA, in most cases the CPA changes to 2.0 miles, but some companies have other policies that make that a bit more or a bit less. Wouldn't be without it.

Also came up one night on a sailboat, we were overtaking from astern, and they saw us on their receiver and started hollering on the radio about our CPA. I already knew they were there from their stern light, but if they had had a transponder it would have removed one more sense of doubt as to actual CPA. They could see us (on the AIS) but were not comfortable that we could see their dinky stern light.

It is amazing how far a small packet of digital data can carry on a low power radio signal. Think about all the places where you can and receive SMS messages from a cell phone but can't make a voice call no matter how you hold the phone.

[Hartleyg]

We've had a Class B aboard (RayMarine AIS650) since 2014, started in San Francisco Bay, sailed to all four corners of the Continental USA, up & down the Eastern Seavoard for the last couple years and we wouldn't consider sailing without it.
These units put out 1/2 a watt (the new "B plus" units are 5w) - Cruise ships and cargo ships appear to see us within 10-12 miles or so (our antenna is at the radar mount, so 30' off the water).

We've been called by name several times by these vessels when they had concerns (or to reassure us they were not going to run us down). We've also noted that when we call them with concerns, they clearly know who & where we are.


So we would recommend an AIS transponder highly - offshore or coastal!


Hartley

S/V Atsa

[wholybee]

AIS transmitters a digital signal, which requires less power for the same range than an analog voice transmission. My class B transponder transmits at 2W, and has a range of 6-10 miles. The newer sotdma are 5W and will have much greater range, probably 20 miles+.

My AIS has always given plenty of time for a fast MV to respond. I can see their class A AIS at 30-50 miles, and can clearly see when they see me at 6 miles out and turn to avoid me. On rare occasion they have called me at 6 miles out to arrange a crossing.

I can't imagine any vessel that 6 miles isn't enough time to react.

[W32]

The fact that it a digital transmission escaped me. Yes, I can see how this could significantly affect range. And, as has been implied, if they're calling you by name your transponder is doing a pretty good job. I guess I'm buying a transponder.

[Sailer_Med]

I am a coastal cruiser and have found our AIS particularly useful in alerting me to fast approaching power boats astern. Hopefully my signal had also let them know what I was doing. I know the rules, but I still like the additional heads up in this regard.

I’m not aware of any commercial ships diverting away from us as a result of AIS. Maybe they have…

I also like the AIS as the on board link for our MOB devices.

[chrisr]

range for your AIS has more to do with the height of your antenna than the transmit power. if possible, locate your antenna at the masthead or use a splitter and the vhf antenna

a modern class B AIS with antenna say 20m above w/l is easily detected at 20nm

but that is only the transmit part of the game. you can receive ships at 50nm or more. if the cpa is a problem you can then take the appropriate action.

get an AIS. it's that simple

cheers,

[Off Trail]

Just to follow-up. Your initial intuition based on handheld performance is correct. Yes the low-rate digital link has better low-SNR performance. However the range achieved mostly comes from having good antenna gain and height. We use an active splitter to share the mast top antenna with the VHF.

We've run across many boats with arch-mounted, dedicated AIS antennas that are only detected at a mile or two. So pay attention to install and VSWR.

[bergius]

I wouldn’t want to cruise without a transmitting AIS (we have class B+). I’ve seen many freighters do minor course adjustments to give us way.

When we were coming back from Sweden in the fall we sailed across the full traffic lane of the Baltic Sea (avoiding the traffic separation scheme, of course) without having to do anything about other vessels. Lovely broad reach most of the way, Any time I saw a ship with a somewhat near CPA, a few minutes later that CPA distance started getting longer. I think in the end we never were closer than 1NM from any shipping until we reached our destination port in Poland.

We got the benefit of AIS on daily basis. In comparison, we only used the VHF twice during the five month cruise, both to clear in/out of a commercial port where that was required. They would have also accepted a cellular call, but since we have a VHF we decided to use it. Of course having the DSC distress features available is kind of nice,

[thomm225]

I just have AIS Receive at the moment from my VHF. There are AIS Receivers available for around $150.

I can pickup ships, tugs, etc sometimes 30 miles away but easily 15-20 miles out.

My AIS receiver uses the VHF antenna which is at the mast top.

I at first displayed the AIS Data on various monitors hooked up to my raspberry pi computer using OpenCPN but now I just use my 17" Laptop running windows 10

The data comes in via NMEA and an USB RS232 (USB 422/485 Adapter) adapter to the laptop (or Raspberry pi)

https://www.digikey.com/en/products/...yABEgL65PD_BwE

We have lots of ships to deal with here both civilian and military plus the tugs

Still using Raspberry Pi 4 in video with 19" HDTV for the display

https://www.youtube.com/watch?v=-TOdSCDHb2s

[deblen]

AIS uses VHF frequencies in the marine band same as your VHF (communications) radio telephone. Signal propagation/distance will be similar to a VHF r-tel.
As an experiment,I suggest all coastal(less than 10nm) cruisers set their fixed VHF to 1watt for a day. Your transmission distance will be reduced to 10 miles or so-which is more than adequate for calling your buddy "over there" & your receive distance will not be affected at all.
Note:The coast guard have "super" rx antennae & repeaters-they can rx even a 1watt handheld a lot farther than your full blown TXCVR & mast top antenna can.


1watt hand held-buy a coax adapter so that you can remove the rubber duckie & connect your handheld to your mast top antenna.You will be impressed.
With radio of any kind, most of the performance is the result of yours' & the receiving stations' antenna system.
Cheers & Happy Holidays/Len

[Pete7]

Originally Posted by deblen

As an experiment,I suggest all coastal(less than 10nm) cruisers set their fixed VHF to 1watt for a day. Your transmission distance will be reduced to 10 miles or so-which is more than adequate for calling your buddy "over there" & your receive distance will not be affected at all.

Indeed, sailing with Dockhead on here a few years ago on his Moody 54, we crossed from England to France. We left France 5am on Sunday morning for the return trip. Whilst I motored out of the harbour, he called up Solent Coastguard in the UK to left them know of our trip. It was late in the season and the weather forecast later that afternoon dire. During the call Solent Coastguard stated that he was weak but readable. The distance some 60 miles. Slightly perplexed he wondered what the problem was and went back to the nav station to investigate. Turned out the radio was set to 1w, not 25w.

W42, I think you should fit an AIS transponder, but don't skimp on the antenna. There have been some good discussions about VHF antennas on here in the past and they are worth searching for. If you plan to split the existing VHF antenna, now is the time to think about renewing it if its a few years old. Oh and avoid those cheap kits in the chandlers with RG58 coax. You can do better for a little bit more expenditure.

Our antenna is on the pushpit with a long coax capable of reaching the fixed VFH in emergency. We received targets at 7 - 10 miles normally sailing in the busy waters of the English Channel.

[Talbot]

in a crowded waterway, Class B can be ignored by Class A receivers while dealing with all the other Class A responses.

I sail in such an area, and prefer to use an AIS receiver only. I can then set up so that the give-way vessel does not need to manoeuvre before vessels are in sight. I then don't have the stress of worrying if they have seen me and are altering appropriately.

[KWF]

Just never forget that not all boats transmit AIS.
Radar continues to have a place.