10 min read

Automatic Identification System (AIS) is one of the most powerful collision avoidance technologies available to recreational sailors. When it works, it’s transformative. But AIS has a fundamental problem that nobody wants to talk about: it only works if everyone uses it. And they don’t.

How AIS Actually Works

AIS is beautifully simple in concept. Every equipped vessel broadcasts its position, course, speed, and identity via VHF radio every few seconds. Your AIS receiver picks up these broadcasts and displays all nearby vessels on your chart plotter or dedicated AIS display.

Unlike radar, which requires interpretation and can miss targets, AIS tells you:

• Vessel name and identification
• Precise GPS position
• Course and speed
• Rate of turn
• Destination and ETA (for commercial vessels)
• Vessel dimensions

From this data, your system can calculate Closest Point of Approach (CPA) and Time to CPA (TCPA), giving you advance warning of collision risks.

The Power Against Collisions

When both vessels have AIS, collision avoidance becomes vastly simpler:

For collision avoidance, AIS is transformative. It removes uncertainty, provides early warning, and works in all conditions.

The Requirement Reality

Here’s where it gets complicated. AIS carriage requirements vary dramatically:

Required to have AIS:

• Commercial vessels over 300 gross tons on international voyages
• Commercial vessels over 500 gross tons not on international voyages
• All passenger vessels regardless of size

Not required to have AIS:

• Recreational vessels (sailboats and powerboats used for pleasure)
• Most fishing vessels under 300 gross tons
• Small commercial vessels under the tonnage thresholds

This means the vast majority of boats you’ll encounter while sailing are not required to broadcast AIS – and most don’t.

The Invisible Majority

Walk through any anchorage and you’ll see the problem immediately. Dozens of recreational boats, fishing vessels, small charter boats, sailing dinghies, kayaks, and small motorboats – all invisible to AIS.

If you’re relying on AIS for collision avoidance, here’s what you won’t see:

• The recreational fishing boat running without lights at dawn
• The sailing dinghy practicing in the harbor
• The kayaker crossing the channel
• The small fishing boat tending nets
• The fellow cruiser who hasn’t installed AIS
• The charter boat operating under tonnage limits
• The inflatable tender running between boats

In coastal waters, these invisible vessels can outnumber AIS-equipped vessels 10 to 1 or more. So if you’re watching your AIS display confidently, you’re only seeing a small fraction of the traffic around you.

The Fishing Boat Problem

Here’s where it gets more complicated: even among vessels that have AIS, many fishing boats choose not to use it – or deliberately turn it off.

Why? Privacy and competition.

Commercial fishing operations guard their fishing grounds as intellectual property developed over years of experience. Broadcasting their position tells competitors exactly where the fishing is good. For a fishing boat, turning off AIS means protecting their business.

The result: fishing boats – precisely the vessels recreational sailors frequently encounter in coastal areas – are often invisible to AIS even when they have the equipment installed.

Studies have documented fishing vessels deliberately “going dark” by turning off AIS transponders, particularly when operating in areas where they want to avoid detection or competition.

The Vaccination Paradox

AIS faces the same problem as vaccines: it only works if enough people participate.

If 100% of vessels had AIS and kept it on, collision avoidance would be dramatically simpler. Everyone would see everyone else. Collision risks would be obvious. Night passages would be safer.

But if only 30-40% of vessels have AIS (a generous estimate for recreational boating areas), and some percentage of those turn it off for privacy or competitive reasons, the system develops dangerous gaps.

The false security problem: Sailors with AIS start to rely on it. They watch their AIS display and feel confident. They see clear screens and assume no traffic. But they’re only seeing the vessels that choose to participate – potentially a minority of actual traffic in their area.

This is more dangerous than not having AIS at all, because it creates false confidence while leaving invisible threats.

The Cost Barrier That Doesn’t Exist Anymore

Here’s the frustrating part: AIS equipment is no longer expensive.

AIS Receiver (receive only): $150-$300
• See all AIS-equipped vessels
• No broadcasting from your boat
• Minimal power consumption

Class B AIS Transponder (transmit and receive): $400-$800
• Other vessels see you AND you see them
• Required for the system to work collectively
• Still modest power consumption (2-3W)

For the cost of a few tanks of fuel or a sail repair, you could dramatically improve collision avoidance capability – both for yourself and for everyone around you.

Yet adoption among recreational sailors remains relatively low. Industry estimates suggest only 20-40% of cruising sailboats over 35 feet have AIS transponders installed, with receiver-only installations being somewhat more common.

The Coastal vs. Offshore Reality

The collision risk – and AIS value – varies dramatically by location:

The irony: AIS is most valuable offshore, but that’s where small boats face another problem.

The Sea Mine Problem

Here’s the offshore paradox: if you’re sailing a small boat offshore without AIS, you’ve become an invisible hazard to everyone else.

That container ship 10 miles away? Their watch officer is monitoring AIS, not straining to spot your 40-foot sailboat on a dark night. Their radar might pick you up eventually, or it might not – metal sailboats show up better than fiberglass, and radar returns from small vessels can be lost in sea clutter.

If you don’t have AIS broadcasting your position, you’re essentially a sea mine from their perspective – an invisible hazard they might not detect until it’s too late to avoid you.

The power consumption excuse: Some small boat sailors avoid AIS transponders citing power consumption concerns. Class B AIS draws 2-3 watts continuously. On a modest cruising boat with 200Ah of battery capacity, that’s about 1.5Ah per day – roughly 0.75% of your battery bank.

For less than 1% of your electrical budget, you make yourself visible to every commercial ship, fishing vessel, and fellow cruiser in range. The power consumption argument doesn’t hold up against the safety benefit.

The Privacy Concern

Some recreational sailors resist AIS citing privacy – they don’t want their position broadcast publicly.

This is a legitimate concern. Your AIS signal can be picked up by shore stations and vessel tracking websites, making your movements public. Some boaters find this uncomfortable.

But consider the tradeoff:

• Privacy: your location isn’t publicly broadcast
• Safety: commercial vessels can’t see you, other recreational boats can’t see you, and in many collision scenarios you’re the invisible party

BoatUS and other organizations have raised concerns about mandatory AIS for recreational vessels specifically because of these privacy implications.

The question each sailor must answer: is privacy worth being invisible to vessels that could collide with you?

The False Security Warning

Even with AIS, you must never assume:

• That you’ve seen all vessels (many won’t have AIS)
• That other vessels see your AIS signal
• That they’ll take action to avoid you
• That AIS replaces vigilant lookout

Critical limitation: As with radar, AIS should supplement – not replace – visual and radar lookout. A prudent navigator practices “see and avoid” whether or not AIS targets appear.

The most dangerous outcome is sailors who watch clear AIS screens and assume they’re safe, while invisible fishing boats, recreational vessels, and kayaks operate all around them.

What We Actually Need

For AIS to deliver on its collision avoidance promise, we need:

• Near-universal adoption: If 95% of vessels had AIS transponders (not just receivers), collision avoidance would transform. The system works when everyone participates.
• Reliable use: Fishing boats and others keeping transponders on, not turning them off for competitive advantage or privacy.
• Realistic expectations: Understanding that AIS shows only participating vessels, not all traffic.
• Integration with other methods: AIS + radar + visual lookout + common sense, not AIS alone.
• Affordable equipment: The technology is already inexpensive – the barrier is awareness and installation, not cost.

The Coastal Collision Prevention We’re Missing

Here’s what frustrates from a safety perspective: coastal and harbor collisions are common, predictable, and largely preventable.

If recreational vessels in high-traffic coastal areas had AIS transponders:

• Vessels would see each other in crowded harbors
• Commercial traffic would see recreational boats
• Night navigation near shore would be dramatically safer
• Collision risk in shipping channels would decrease

The technology exists. It’s not expensive. Power consumption is minimal. But adoption remains incomplete, and we continue to have collisions that AIS could have prevented.

The Bottom Line

AIS is powerful technology for collision avoidance – when it works. Large commercial vessels are nearly all equipped and broadcasting. For offshore passages, AIS dramatically improves safety.

But the vaccination paradox remains: the system only works if most vessels participate. As long as fishing boats turn transponders off, recreational sailors skip AIS installation, and small vessels remain invisible, we’ll have dangerous gaps in coverage.

If you’re sailing with AIS, you’re safer – but only if you remember that what you don’t see on AIS might be more dangerous than what you do see.

And if you’re sailing without AIS in offshore waters, you’ve become the invisible hazard everyone else is worried about – a sea mine they can’t detect until it’s too late.

The technology works. It’s affordable. It could prevent collisions. The question is whether enough sailors will choose to participate to make the system work for everyone.

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