In the channels of southern Chile, wellboat traffic — vessels up to 100 metres long carrying salmon between farm sites — runs around the clock, often in limited visibility and at 10-14 knots. For a 35-foot sailboat without AIS, the only way to be seen is if someone happens to be watching the radar. With AIS, you appear on their screen as an identified target with a name, course and speed. The difference can be literal life and death.
How AIS works
AIS operates on two dedicated VHF channels (87B and 88B) and automatically broadcasts a data packet that includes: MMSI identifier, vessel name, GPS position, speed over ground (SOG), true course (COG), vessel dimensions and vessel type. This information is received by any AIS-equipped unit in the area and can be displayed on a plotter or radar with AIS overlay.
Like VHF, AIS range is line-of-sight — typically 20-40 nautical miles in open water, less in areas with complex terrain. Services like MarineTraffic and VesselFinder pick up AIS signals through a network of shore-based receiving stations and publish them online, letting you see maritime traffic from any internet-connected device.
AIS Class A vs Class B: what you need
For recreational vessels in Chile, the relevant equipment is AIS Class B. It transmits your position every 30 seconds at 2 watts of power, which is more than enough to be detected by commercial ships and coastal receiving stations. Class A is designed for cargo ships, tankers and ferries — fitting it on a small vessel adds no real advantage and costs significantly more.
Receiver vs transponder: the decision that matters most
An AIS receiver (also called a passive AIS unit) lets you see other AIS-equipped vessels on your screen. It's useful for situational awareness in areas with heavy traffic. The entry price is low.
An AIS transponder (Class B CSTDMA or SOTDMA) does the same thing but also transmits your position to others. In low-visibility conditions, at night, or in high-traffic areas, being visible matters as much as seeing. For offshore navigation or the southern channels, I always recommend a transponder.
Plotter integration: AIS connected to your chartplotter via NMEA 0183 or NMEA 2000 shows AIS targets directly on the electronic chart, with speed vectors and CPA (Closest Point of Approach) alerts. When the system detects that an AIS target will pass within X miles in under Y minutes, an alarm sounds. Configure it correctly before you sail.
What AIS cannot do
AIS only detects vessels equipped with AIS. In the channels of southern Chile, a large share of traffic — artisanal fishing boats, local community launches, rafts, drifting logs — has no AIS and will not appear on your screen. Using AIS as your only collision-avoidance system is a mistake that can end in an accident. AIS complements radar and visual watch; it never replaces them.
The AIS blind spot: A large commercial vessel can have an AIS shadow sector under its bow where a small boat won't appear on its screen even while transmitting. Never assume that because you're transmitting AIS, the other vessel has "seen" you. The rules of the road and good seamanship remain your responsibility.
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