AIS for Cruisers: What It Does, What to Buy, and How to Use It

AIS — the Automatic Identification System — is one of the most undersold pieces of marine safety gear in the recreational market. For under $1,500 installed, you get the ability to see (and sometimes be seen by) every commercial ship and AIS-equipped vessel within VHF range, with their exact heading, speed, name, and closest-point-of-approach calculations laid right on top of your chartplotter. If you do any boating in commercial harbors, near shipping lanes, or at night, it changes the calculus on collision avoidance.

This is a practical guide to what AIS does, what type you need, and how to actually use what it shows you.

What AIS is, briefly

AIS is a VHF-frequency digital broadcast where vessels share their identity and movement data. Every commercial ship over 300 gross tons is required to broadcast AIS continuously. A growing number of recreational vessels (those with transceivers, not just receivers) broadcast too.

Your AIS receiver picks up these broadcasts within roughly VHF range (typically 20-40 nm depending on antenna height) and shows you each vessel as a target on your chartplotter — with name, type, speed, course, and a calculated CPA (closest point of approach) and TCPA (time to CPA).

For a sailboat or cruising powerboat at night in shipping lanes, this is the difference between a foggy "is that ship going to hit us?" and "the Maersk container ship Caoba is 7 nm out, heading 175°, will pass 1.4 nm to our south in 22 minutes."

Receive-only vs. transceiver

There are two flavors:

Receive-only AIS ($150-$400): you see them, they don't see you. Cheap, simple, no installation complications.

Class B transceiver ($600-$1,500): you see them AND they see you. Class B is the recreational standard (Class A is required commercial gear). Transceivers broadcast your position, name, dimensions, heading, and speed roughly every 30 seconds. Modern Class B SOTDMA units broadcast more frequently and at higher power than older CSTDMA Class B units — worth the extra few hundred dollars.

For most cruisers, a Class B transceiver is the right answer. The visibility cuts both ways — a commercial ship is far more likely to alter course around you if they see you on AIS, vs. trying to figure out what your tiny radar return is. For an extra $400-$800 over a receive-only unit, you become visible to every ship within VHF range. That's good value.

Antenna considerations

Two options:

• Split your existing VHF antenna using a splitter (some AIS units include them). Convenient, but the splitter introduces some signal loss.
• Install a second dedicated antenna for AIS. More work but better performance. Many boats use a smaller AIS-specific antenna on the radar arch or stern rail.

For best results, the AIS antenna needs clear sky view and good separation from the VHF transmit antenna. On a sailboat with a single mast-top antenna, the split is usually fine. On a powerboat with arch space, dedicated antennas perform better.

How to actually read AIS

Most chartplotters overlay AIS targets directly on the chart. The icons mean:

• Triangle pointing in direction of travel: AIS target
• Solid filled triangle: active target (transmitting normally)
• Outlined triangle: lost target (last position shown for a few minutes after transmission lost)
• Circle around triangle: alerted target (CPA below your alarm threshold)

When you tap a target, the data panel typically shows:

• Vessel name and type
• Course over ground (COG) and speed over ground (SOG)
• CPA — how close they'll come to you on current courses
• TCPA — how long until that closest approach
• Range and bearing right now

The CPA/TCPA pair is what matters for collision avoidance. CPA under 0.5 nm or so means you'll be uncomfortably close. CPA under 0.1 nm means you should be actively talking to that vessel on the radio right now.

What AIS doesn't show you

A few important limits:

• Non-AIS-equipped vessels: fishing boats under 300 GT, most sailboats, small pleasure craft. AIS doesn't replace looking out the window or using radar.
• Manual targets without GPS: rare, but some lifeboats and SAR assets broadcast position manually.
• AIS-spoofing or AIS-off vessels: rare but real — some vessels (notably illegal fishing, contraband transport) turn off AIS deliberately.
• Static obstacles (rocks, buoys, channel markers): chart data, not AIS.
• Weather and sea state: those come from other instruments.

Don't substitute AIS for radar, visual lookout, or sound signals. AIS is one layer in collision avoidance, not the whole stack.

How to use the radio with AIS

When AIS shows a developing close-approach situation, the standard radio call is:

"[Vessel name], [vessel name], this is [your vessel name], we are showing a CPA of [X] miles in [Y] minutes on our AIS. Confirm passing arrangement."

Commercial bridges expect this. Most will respond immediately on Channel 13 or 16 and confirm intentions. This is dramatically better than the old "boat on my port side, blue hull, please respond" attempts at identification.

Installation notes

AIS installation is usually mechanically simple but rich in opportunities to do wrong:

• Antenna placement matters a lot — clear of metal, good sky view, separation from VHF transmit antenna
• Power must be on the always-on circuit (so AIS transmits while at anchor)
• NMEA 2000 network connection is the modern standard — older NMEA 0183 connections still work but limit integration
• The "MMSI" (Maritime Mobile Service Identity) for a transceiver must be programmed exactly once and can never be changed — get this right
• Static data programming (vessel name, dimensions, type) must be entered carefully

For most owners, hire a marine electronics installer to do the AIS install unless you're confident on antenna theory and NMEA networking. Bad installs are why some boats' AIS works at 5 miles when others reach 30. Find one in our marine electronics directory.

What I'd buy in 2026

For most cruisers stepping up from no AIS:

• Vesper Cortex ($1,500) — Class B SOTDMA transceiver, integrated VHF radio, NMEA 2000, smartphone app. Probably the best integrated AIS+VHF package on the market. Worth the premium for the user interface alone.
• em-trak B954 ($700-$900) — Class B SOTDMA transceiver, more traditional standalone unit. Good value if you already have a VHF you like.
• AIS500 / AIS650 type receive-only ($150-$300) — only consider if budget is genuinely tight or you only need to see, not be seen. The transceiver upgrade is worth it for everyone else.

Garmin, Raymarine, and Simrad all sell branded AIS units that integrate cleanly with their own MFDs. If you have a Garmin helm, a Garmin AIS makes the install easier — buy ecosystem.

Bottom line

AIS is one of the highest-leverage safety dollars you can spend on a cruising boat. The ability to see commercial traffic by name, course, and CPA changes how you boat in shipping lanes, at night, and in fog. The ability to be seen by them shifts the collision-avoidance burden in your favor.

For most cruisers, a Class B SOTDMA transceiver fully installed runs $1,500-$2,500 — and pays for itself the first time a commercial vessel diverts around you because they see you on their bridge display.

For the broader electronics picture, see our marine electronics buying guide and VHF radio guide.

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Photos by Unsplash contributors.