Why AIS earns its place on a catamaran

AIS is one of those bits of kit that quietly changes how a coastal passage feels. On a catamaran you have a stable platform, good visibility from the raised helm, and often a small crew — which means the off-watch is genuinely asleep, not hovering by the companionway. AIS fills that gap. It tells you the name, position, course and speed of vessels around you, and it does it in fog, at night, and when a cross-Channel ferry is closing at 20 knots from a bearing you haven't looked at yet.

It is also, emphatically, an aid. AIS shows you the boats that transmit; radar shows you the steel; your eyes show you everything else. Use it as an extra layer, not a substitute for keeping watch.

Choosing and installing the kit

For coastal cruising in UK waters, a Class B transceiver is the sensible choice. A receiver-only set lets you watch traffic, but the point of AIS in a shipping lane is that other people can see you. Class B transmits at 2 watts, giving a realistic range of 5–10 nautical miles to a ship's antenna, more in good conditions. That is plenty for a coastal passage if you start looking early.

• Aerial: a dedicated VHF antenna with at least 0.5 m vertical separation from your main VHF whip is cleaner than a splitter, and keeps your DSC radio independent if one fails.
• Data: NMEA 2000 is the tidy route on a modern boat; NMEA 0183 still works for older plotters, but check which sentences your display expects.
• GPS antenna: give it a clear view of the sky, away from the radar beam and the VHF aerial. On a cat the radar arch or coachroof is usually ideal.
• MMSI: issued with your ship radio licence. Program it once, correctly, and never borrow another vessel's number.
• Power: typically 1–3 amps at 12 V, so run it on a fused circuit you actually leave on — ideally the same one as the plotter.

Mount the display where you genuinely glance at it: at the helm, not buried at the chart table.

Reading targets like a navigator

A target on screen is a triangle with a course vector. The pointed end faces its heading; the vector shows where it is going. A Class A target from a ship usually carries static data — name, dimensions, draught, destination, navigational status. A Class B target from a yacht or small fishing boat may show nothing but position and speed, and its update rate is slower, so treat a Class B track with a little more caution.

• CPA and TCPA: closest point of approach and time to it. These are the two numbers that matter.
• Lost and sleeping targets: a target that goes quiet or drops off may be behind an island, or may have switched off. Either way, keep looking.
• Not everyone transmits: small craft, some fishing vessels and some naval ships may be invisible. AIS silence proves nothing.
• Data can be wrong: antennas are mounted badly, MMSIs get mistyped. Cross-check with what you can see.

If a ship's CPA is under a mile and closing, call it on VHF. Use the vessel's name and MMSI, state your position and intention clearly, and remember the watchkeeper on the bridge may never have noticed you on a screen full of targets.

Setting alarms you will actually trust

The default alarm settings on most sets are far too sensitive, and an alarm that cries wolf gets muted within a day. Tune them to the passage.

• Offshore: CPA 0.5 nm, TCPA 20 minutes.
• Inshore and in harbours: CPA 0.25 nm, TCPA 10 minutes.
• Guard zone: a ring around your own vessel — often 1 nm, biased ahead of you, is more useful than a circle.
• Anchoring: AIS is not an anchor watch. Use a GPS anchor alarm, but a guard zone is handy in a busy anchorage.

Test the alarm during commissioning, make sure it is loud enough to wake the off watch, and only mute it deliberately, never out of irritation. A laminated card at the chart table with your chosen settings saves an argument at 0300.

Integrating with chartplotter, radio and tablet

When AIS is overlaid on the chartplotter, targets sit in their real geographic place, which is far easier to interpret than a list of bearings and ranges. Filter by range and by class, and keep the range at about 6–12 nm in coastal waters so you spot the fast traffic early without cluttering the screen.

Radar and AIS complement each other beautifully: radar sees the hull, AIS sees the transponder. A DSC VHF linked to the plotter lets you select a target and call it directly, and a second display or a tablet on the boat's wifi lets the off watch keep an eye on things from below. AIS MOB and AIS SART beacons appear as distinctive icons — know what they look like before you need them. Update your firmware when the manufacturer releases it, and now and then ask a nearby yacht to confirm you are transmitting the right name and details.

Passage habits that make it work

• Cross shipping lanes at right angles, aiming ahead of the ship, and back the screen up with a visual bearing — if the bearing stays constant, you are on a collision course.
• Zoom out regularly. AIS range far exceeds what the screen shows at close scale.
• Note the names and MMSIs of vessels you intend to call, and log anything that made you alter course.
• Brief the crew on the alarms, and remind them that AIS is a supplement to a proper lookout, not a replacement for one.

Do that, and AIS stops being a gadget and becomes a quiet, reliable member of the crew.