View Full Version : In praise of AIS on transats - and Nimble Navigator

24-06-10, 10:58
Scuttlebutt is getting all het up bout this, altho it seems mostly the antis are just doodling around the Solent, so i thought i'd do a sepret post.

I crossed the atlantic once with no AIS, a few times just with a receiver, and last two times with receiver and separate (independent unit) transmitter+receiver.

The independent unit is branded TrueCentre, yellow box with own antenna and gps mushroom. It broadcasts MMSI number, bearing etc and sometimes seems to broadcast that I'm sailing, and sometimes that i'm motoring, it seems a bit random. Fortunately when stingo and i checked eac thr when a mile apart, he was motoring (whwn sailing) and i was listed as "sailing" haha

The AIs C receiever unit is integrated as part of (ugh) a raymarine setup. It's with this that i just have recieve-only (=AIS C i think) but the raymarine setup is a bit hopeless, although it looks fine to start. The main head unit is an E80, maybe later/more expensive units are better, but the targets are displayed as though you're only ever going to have a handful of them , and the only listing you have s in order of how near they are to you - not how near they are *going* to be to you (ie in CPA sequence) and not how long until a "nearness incident" happens (ie in TCPA sequence). One other rotten thing about the raymarine software is that it sounds a warning when there are no targets - it trggers an alarm. Jeez, so if well offshore and a distant target skims acorss beyond the horizon the alarm is often on a lot as it gets the target, loses it, get it again and so on.

Hence much better with additional independent unit. I supose i'd have this rather than the raymarine thing if i could choose just one. The output from the AIS transceiever arrives on rs232 then UBS and into a computer, and i display using Nimble Navigator, 95 bucks. Much better. It shows all targets, and sometimes around gibraltar, coast of Spain there might a couple of hundred or more. So the drill is to daydream at or around the cockpit helm, and if things start to look interesting, have a play indoors with Nimble Navigator on the computer. You can list in sequence of range, CPA and TCPA.

AIS C (receive-ony) is beter than nothing, but just not a patch on having the AIS B (transmit and receive) as well. When transmitting on long passage you can see ships ten miles away or more jigging to starboard just a couple of degrees to miss us by a mile instead of a couple of hundred yards, and since having this there haven't been any ooer surprise encounters ("he was a bit close eh?!") since we've become a grey triangle. Not as vital as some safety gear, but certainly i wouldn't want to be without it.

Actualy, a realy good thing about the two separate units is that you know for absolute certain that you're transmitting - cos it shows your own boat on the raymarine screen. Ok, first few days there's a cold sweat as a AND having a constant display there means the spurious "lost AIS target!" alarm doesn't go off. Yep, the bad thing is that the dangerous target alarm on the AIS has to be turned off, but i don't much take notce f tat anyway. Raymarine radar alarms allow lots to slip thru, the software can't calculate time = distance/speed reliably once the distance s over a few hundrd miles, and it calculate the CPA a bit wrong as well. It looks nice though.

Of course, once well inshore daysailing and pootling about or at anchor I turn the main AIS C off - i'll be changing course all over the place and so will everyone else. And one looks a bit of a nob with the AIS blasting away whilst in a little anchorage.

By the way, AIS C sort -of makes it that if you go westbound with the ARC, you're actualy potentially in more danger, not less. See, by default, ARC boats get daily positions to the second. In other words, it's a race. Yep, you're the skipper and can say well, we're not racing. Yeah, okay, and be a wet blanket why doncha. So, they don't do that, and they're racing a bit, sort-of. Specifically, they're sort-of half-racing against another identical boat, perhaps, him over there. BUT...if you're broadcasting your speed and position all the time via AIS C whilst the others aren't well, it's a bit of a giveaway innit? So (especially late on) people turnem off. Oh, and loads boats (about half i spose, since it equinoctial ish) arrive at (often) unfamilar st lucia coast at night, another not-very-safe idea really.