Why fog changes everything

Fog is the one condition that strips away almost everything we rely on as coastal cruisers. The shoreline disappears, the buoy you were aiming for vanishes, and the reassuring shape of a channel marker becomes a grey nothing. On a catamaran you have some advantages in these conditions — a stable platform, twin engines, and often a good turn of speed — but speed is also the problem: the faster you close a target, the less time you have to decide what to do about it. Radar is the tool that buys that time back, but only if you've set it up properly and know what you're looking at.

If your radar is mast-mounted — typical on most cruising cats — you'll have a theoretical horizon of around eight nautical miles for a small vessel at sea level. That's genuinely useful, but remember that small open boats, pot buoys and unlit kayaks sit well below that theoretical line and may not appear until you're almost on top of them.

Start with gain and sea clutter

Every session should begin with the gain. Turn it up until the whole screen is speckled white, then ease it back until the speckle just clears, leaving a light, grainy texture in the outer areas. Too little gain and you lose small targets; too much and the display washes out and you can't pick anything out of the mush. Re-check it every half hour, particularly as dusk falls or the rain arrives, because the correct setting drifts with conditions.

Sea clutter is the next control, and it's where catamarans behave a little differently from monohulls. Because your radar is high up and the boat doesn't heel, the near-field clutter ring is usually smaller than you'd see on a low-mounted set on a leaning yacht. Use the automatic sea clutter setting as a starting point, then reduce it manually until the centre of the screen is clear enough to show a small target at a quarter of a mile. That last check matters: automatic clutter suppression is very good at hiding the one thing you most need to see — a small fishing boat or a RIB just ahead.

Rain clutter is a separate control and, despite the name, it isn't much help in fog. Use it sparingly, because it also suppresses real echoes.

Tuning for range, coastline and buoys

In fog, work on a short range. One and a half to three miles is the right setting for pilotage and harbour approaches; six miles is sensible offshore, where you're mainly watching for shipping. Use the range rings and the variable range marker to judge distances rather than squinting at the display, and off-centre the picture so most of the screen shows what's ahead of you rather than behind.

Coastline paints beautifully on radar. Cliffs, headlands, harbour walls and breakwaters all return strong, solid echoes, and in thick fog this is often your best confirmation of position. Buoys are a different matter. Their radar reflectors are frequently small, badly fitted or missing altogether, so a cardinal mark may show as a faint smudge — or nothing at all. Never conclude a buoy isn't there because you can't see it on the screen. If you're using radar to find a channel entrance, work from the land, not the marks.

Tracking targets

Once the picture is clean, switch on whatever tracking you have. MARPA or ARPA will acquire a target and give you a course, speed and closest point of approach, which is far more useful than a raw echo. Set a guard zone across your bow and let the set alarm you when something enters it. If you have AIS, use it alongside radar rather than instead of it — plenty of small craft don't transmit, and radar sees what AIS misses. Watch for targets that change aspect or slow down; that's often someone altering to avoid you.

A fog routine on deck

• Engines on, sails reduced, speed down to under five knots — enough to keep steerage but slow enough to stop in half your visible distance.
• Post a lookout forward with a handbearing compass, and keep someone at the chart table plotting.
• Make the correct sound signals: one prolonged blast every two minutes when under way, and one prolonged followed by two short when stopped.
• Navigation lights on, radar reflector hoisted, and listen continuously on VHF channel 16.
• Log your position every ten to fifteen minutes and plot it on paper as a backup to the chartplotter.
• Know your escape route: which way is deep water, which way is shallowing, and where you'd anchor if it all became too much.

Practise before you need it

The skippers who cope best in fog are the ones who've spent clear days playing with their radar, matching echoes to what they can actually see over the bow. Learn your own set's blind sectors — the mast, the boom, the crew standing in the wrong place. Learn how your cat's motion affects the picture in a swell. Note down the gain and clutter settings that work in drizzle and in proper fog. Do that, and when the grey closes in you'll be working from familiarity rather than guesswork.