There's quite a flurry of debate going on right now about automatic identification system (AIS) equipment. The question of the month is about who sees what: if you fit your boat with a class B AIS transponder, will big ships carrying class A equipment set their display filters to hide your much smaller vessel from their radar displays?

A bit of background on the debate, firstly: An anonymous post at Sailing Anarchy, quickly brought up for more serious discussion at Panbo, suggested that AIS systems on big ships had a "big red ignore class B button". The idea was quickly put to rest; a ship's transceiver won't just drop AIS targets on account of their class. It'll receive and track all properly formatted AIS signals, of any type. But then the Dashews started experimenting with upgrades to their commercial-grade Furuno 2117 radar, and- lo and behold- found a nice blue and yellow button to filter class B signals from displaying on the radar overlay. This was of course debunked, then sort-of confirmed, on Panbo, and the debate rages on at Cruiser's Forum and other sites.

For those unfamiliar with AIS, some technical background is in order. Class A AIS transponders are, or soon should be, fitted to most ships over 300 tons and most commercial passenger ships. They transmit, every few seconds, a short burst of digital data on VHF channels 87B or 88B. The data includes the ship's position, heading, speed, identifying parameters, and other data that authorities and other ships might find useful. A class B unit, meant for smaller vessels, transmits a similar, but much simpler and lower power (shorter range) message at intervals of 30 seconds or 2 minutes.

What's come out of the wash, so far, is this:

• Ships do need the ability to filter class B (or even some class A) transmissions from their displays, otherwise they would be unable to enter port without an alarm sounding every few seconds as they pass other boats.
• Ships' bridge electronics are not supposed to stop receiving or tracking class B AIS targets when the display filters are on, even though they may not be shown on the radar overlay.
• Most ship crews are professional and competent, and use these suppression filters only when absolutely necessary.
• It is possible that a less than careful crew may leave AIS display filters active once the ship is out in the open.
• Most ship crews will not alter course unless they have good reason to believe that a collision is likely.
• Nobody actually wants a collision, and all prudent mariners will do everything possible to prevent one.

A considerable amount of the commentary on the AIS issue seems to be along the lines of "if I show up on the ship's AIS, he will change course to avoid me. Therefore, class B sucks because he might not see me and will run me down." The basic premise is valid, in that the purpose of having an AIS transponder (instead of just a receiver) is to be seen by ships, and that a class A unit is superior in this respect to a class B. Where the argument breaks down is in the assumption that, because you appear on a ship's display, he will avoid you. The ship is several thousand times larger than your boat and likely travels at two or three times your speed, possibly much more. If your intent is not clear, the ship captain would much rather keep going in a straight line at a constant speed, instead of making course and speed changes to try to dodge a boat that might be on an erratic course and is probably not in VHF contact with him.

COLREGS, as far as pleasure craft near ships are concerned, are almost as much about "who was at fault in this collision" as they are about "how do we prevent this collision". It's almost always better to stay far enough away from the ship that COLREGS are irrelevant. This is where AIS shines: it lets you, the small boat skipper, to see the ship, to identify it by name and MMSI, to hail its captain and confirm he knows about you, and to help plan a safe course that keeps you well clear of the ship. He doesn't want to change course or hit you, and you don't want to come anywhere near him. Ergo, you identify him on AIS, hail him on VHF, and check that you understand each other's intent.

Assuming that your AIS signal is automatically received, acknowledged and acted upon by every ship you see is much like using a Civic to bully trucks on the highway under the assumption that because you flashed your turn signal, they'll slow down and move over for you. It's a dangerous, stupid thing to assume. The ship captain doesn't want a collision and will do everything possible to prevent one, but evasive manoeuvres are much easier for you than for him. By the time he can figure out your (smaller) boat's location and intents, it may well be too late for him to get clear of you- a large ship doing twenty knots might need several miles to initiate a turn. You are in a small, agile vessel; the ship captain is not. Stay out of the ship's way. Period.

There are, of course, a few problems with this plan. What about the situation where the weather's come up, you're stuck running with the seas, and can't make appreciable speed or course changes? Well, this is where AIS really comes into its own, from the small-boat perspective. You can hail that ship (by name or MMSI, thanks to the AIS data, so you're less likely to be ignored) and ensure that its helmsman sees you. You can tell them you can't change course and will be at risk of collision unless they do. Since they now know you're there, they'll look for your AIS target and can calculate an appropriate course to avoid hitting you.

What about single-handing, though? Well, here you're relying purely on luck. If you don't have eyes on watch, you're in violation of COLREGS, and you accept the added risks involved. Transmitting AIS might boost your luck, a little, but don't trust your life and your vessel to it.

In summary: If you're going to be sharing waters with big ships, fit a class A transponder if you can afford one and feel it's worth the cost, a class B if you can't. Have it programmed with your boat's name, MMSI and key parameters. Never assume that anyone else sees its signal until you've confirmed it with them on VHF. Use your AIS unit to identify ships along your course and to hail them by name or MMSI. Tell them you are transmitting an AIS signal, and ask if you will pose a hazard to them on your present course. If you do, adjust your course to stay clear of the ship. If you can't adjust course, tell that to the ship captain and, if nothing else, at least he now knows where you're going and to look out for you.