The question comes up on every serious cruising boat once the electronics budget gets real. Radar costs money, draws power and needs a mount. AIS is cheaper, cleaner to install and gives you a name and a CPA on a plate. So do you actually need both, or is one enough for the way you sail? The honest answer is that they solve different problems, and confusing the two is how skippers end up with a false sense of safety in exactly the conditions where they need the most information.

What each system actually sees

Start with the physics, because that is what decides where each tool fails.

Radar sees anything that reflects its pulses. Ships, buoys, cliffs, squalls, rain, a fibreglass hull with a decent reflector, sometimes a wave crest, sometimes a flock of birds. It does not care whether the target has electronics. It gives you range and bearing, and with MARPA or ARPA it derives a course and speed by tracking the return over time.

AIS sees only what transmits. A Class A target (commercial vessel, most fishing boats over 15 metres in EU waters) broadcasts position, COG, SOG, heading, rate of turn, MMSI, name, destination and type. A Class B target (most leisure boats that bothered to fit one) broadcasts a subset, less frequently. If it is switched off, unpowered, or simply not fitted, you see nothing.

That single distinction sets up the rest of the argument. Radar tells you there is something there. AIS tells you who and what, if they cooperate. Neither one is a substitute for the other, and any skipper who tells you otherwise has been lucky so far.

Where AIS alone lets you down

AIS is intoxicating on a plotter. You get a triangle, a name, a CPA and TCPA, and it feels like you have solved the collision problem. Then you meet the things AIS does not show you.

  • Small craft without AIS. RIBs, day sailors, most fishing punts, kayaks, paddleboards. In summer these are the highest density of contacts in coastal waters, and they are invisible on your MFD.
  • Vessels that switch it off. Some fishing boats mask their position when they are on a good spot. Naval vessels routinely go dark. It is not your job to police it, but it is your job to know it happens.
  • Containers, logs and floating debris. No transmitter. Radar might catch a large one at close range. AIS never will.
  • Weather. A squall line at night is a serious tactical problem. Radar shows the shape and closing speed of the rain. AIS shows nothing.
  • Land and unlit marks. Approaching a rocky coast in poor visibility, radar is the tool that keeps you off the rocks. AIS is decorative.

There is also a subtler trap. AIS updates on Class B are slow, typically every 30 seconds when moving and up to 3 minutes when slow or at anchor. In a converging situation at 20 knots combined, the picture on your screen can be old enough to matter. Our full breakdown on how to read and interpret AIS goes into the update logic and how to avoid trusting stale CPAs.

Where radar alone lets you down

Radar has its own list of things it does badly, and most of them are the things AIS does easily.

  • Identification. Radar gives you a blob. Is it a 300 metre bulk carrier at 6 miles or a fishing boat at 2 miles? At night, on a bad screen, in a seaway, you can guess wrong. AIS puts a name and a length on it.
  • Intentions. Radar tells you where a target is going based on where it has been. If a ship alters course, you find out after the fact. AIS gives you rate of turn and destination, which sometimes explains what the target is about to do.
  • Small targets in clutter. Sea return, rain, and heavy chop will bury a low freeboard hull. You can tune it out, and modern solid-state sets are far better than they used to be, but a 6 metre RIB in a Force 5 is not a reliable radar contact.
  • Shadow zones. Sails, masts, dodgers and gantries all create sectors where the antenna sees nothing. On a cruising sailboat with the radar on the mast, the aft quarter can be a hole.
  • Interpretation load. Radar rewards practice. If you only turn it on when the fog rolls in, you will not read it well. AIS is close to plug and play.

How they work together in practice

The right mental model is simple. AIS is your first filter. Radar is your ground truth. Every serious traffic situation should be resolved with both, cross-checked against your eyes and, when relevant, VHF.

A typical night watch in a busy shipping lane looks like this. You see three triangles on the plotter, all AIS. You look at the radar overlay and see five returns. Two are unknown. You take a compass bearing on the nearest one, watch it, and decide whether it is closing. If it is a ship and you have doubts, you call it on VHF with its name (which AIS gave you), not "vessel off my port bow". If it is a fishing boat that is not on AIS, you treat it as if it cannot see you either, and you make the alteration early and obvious under COLREGs.

That interplay only works if you have practised it. In fog, at night, in the approach to a busy port, is not the time to learn what your radar overlay looks like. Our guide on navigating safely in fog and poor visibility walks through the watch discipline that makes the two systems reinforce each other rather than compete for your attention.

A useful habit : whenever an AIS target appears, glance at radar to confirm the same return, and vice versa. When one shows something and the other does not, that is the target that deserves a second look.

What the rules actually require

Neither radar nor AIS is required on most leisure boats in French waters. Division 240 mandates a radar reflector for boats without a radar, and requires various visual and audible signalling equipment, but it does not force you to fit either transceiver. That is a choice you make based on where and how you sail.

What the rules do require is compliance with the COLREGs, and this is where the two systems become non-negotiable in certain conditions. Rule 5 requires a proper lookout by all available means. Rule 7 requires you to use all available means appropriate to the prevailing circumstances to determine risk of collision, and specifically names radar. Rule 19, restricted visibility, requires early and substantial action based on radar observation alone if that is what you have.

Read that carefully. If you have radar aboard and you do not use it in fog, you are not complying with the rules. If you have AIS and you rely on it exclusively in restricted visibility, you are also not complying with the rules, because AIS is not radar and Rule 7 warns explicitly against making assumptions on scanty information. The COLREGs refresher for skippers unpacks how the rules interact when the two systems disagree about a target.

Which one first, if you can only afford one

Budget is real, mast real estate is real, and not every boat can carry both from day one. If you have to choose, choose based on where you actually sail.

  • Coastal cruising in busy waters, mostly good visibility. Start with a Class B AIS transceiver (not just a receiver). It costs less, installs faster, makes you visible to ships and gives you 80% of the collision picture in the conditions you actually meet. Add radar later.
  • Fog-prone areas : the Channel, Brittany, Ireland, the Baltic. Radar first, no debate. AIS will not tell you where the coast is or where the unlit fishing boat sits.
  • Offshore, night passages, poorly lit coasts. Both, and treat neither as optional. This is the profile where the two systems save each other's blind spots on every watch.
  • Charter operators. Both, plus a written watch protocol so every skipper handles targets the same way.

One more point that gets skipped in the brochures : make sure your AIS is transmitting, not just receiving. A receive-only AIS lets you see ships. A transceiver lets ships see you, which is the version that matters when a bridge officer is deciding whether to alter for a small target he cannot quite make out on his own radar.

The data you are not logging

Radar and AIS give you a live picture, but neither one, on most installations, keeps a record. When something goes wrong, whether it is a near miss, an insurance claim, or just a debrief with the crew about how a passage went, the useful information is what happened before the situation became obvious. Course, speed, engine load, heading, position, all logged at short intervals, all replayable.

That is where the Oria Box fits in for boats that already have radar and AIS. It reads the NMEA network your electronics are already talking on, logs the voyage, and lets you replay it in the Oria app afterwards. It does not replace the lookout, and it does not make collision decisions for you. It just means that when you sit down after a foggy passage across the Channel, you can see exactly what you saw at the time, and decide whether your rules of thumb held up. Ask yourself : if you had to explain your last close-quarters situation to a surveyor, what would you actually have to show them?