Navigating at night and in poor visibility

When you can’t see

Module 5 so far has assumed you can see. At night the shapes and colours go and only the lights are left; in fog even the lights go, and you navigate by depth, compass, clock and electronics — and listen. This lesson pulls together lights (Lesson 19), vessels’ lights (21), sound signals and Rule 19 (22), pilotage (31) and electronic navigation (33).

Getting ready for the dark

Night vision. Your eyes take about 20 to 30 minutes to adapt fully to the dark — their dark adaptation — and a moment of bright white light undoes much of it. So below decks and at the chart table use dim red light, turn plotter and instrument screens to their night setting, and keep torches pointed down. Red light has a catch: red and magenta printing on a chart, including the magenta light flares, can be hard to read under it. Check before dark that you can read your chart by your chart light.

Watches. Agree a watch system before dark so everyone gets rest. A handover covers position, course, the lights expected next, vessels in sight and the weather. Standing orders say when to call the skipper: a light not seen when expected, a vessel whose bearing isn’t changing, visibility dropping — or any doubt. Keep the log hourly (Lesson 15), and check the navigation lights work before dark (Lesson 21).

A pilotage plan written in lights

For a night entry, rewrite your pilotage plan (Lesson 31) in lights: for each leg, the lights you should see, their characteristics and bearings, and roughly when each should appear. Note what is not lit: on the practice chart The Teeth and Head Ledge have only the lit buoys that guard them, and the wreck has no light at all.

When will a light appear?

A light’s nominal range depends on its brightness, and in haze you see it less far (Lesson 19). The curve of the Earth sets a second limit. The distance at which a light first appears over the horizon is its rising distance; where it drops out of sight as you leave, its dipping distance — the same figure, its geographical range. It depends on the light’s height and your eye’s. Almanacs print a table; the rule of thumb behind it is:

You see a light no further than the lesser of its range in tonight’s visibility and its rising distance.

Elevations are above MHWS, so the answer errs slightly on the cautious side. The loom of a powerful light — its glow in the sky — often shows before the light itself. A light right on the horizon may appear when you stand and vanish when you crouch: bobbing the light tells you that you are at about its rising distance, which with its bearing gives a rough position (rough, because the bending of light in the air varies).

Entering Port Kerran at night

Here is a pilotage plan for the Port Kerran approach (Lesson 31), written for the dark. Every figure comes from the chart.

On and around this approach three pairs of lights are easy to confuse. No 1 and No 3 are both green with a 5-second period — but No 1 flashes singly and No 3 in pairs. Mew Rock beacon flashes in pairs every 5 seconds like Nos 3 and 4, but it is white. And the leading lights share their 4-second period with Kerran Head light, which you meet coming round the headland (the island hides it from the approach): Kerran Head is isophase (on and off for equal times), the leading lights occulting (mostly on, blinking out). As Lesson 19 showed: colour, count, period — then check the bearing.

Background lights

Near a harbour, navigation lights sit among street lights, windows and cars — the collision rules list background light among the things that decide a safe speed at night (Rule 6). Look for what shore lights don’t do: navigation lights keep a set rhythm, buoy lights are low on the water, and leading lights stay one above the other while you hold the line (Lesson 13 showed which way to steer when they open).

Fixing your position at night

Lighthouses and lit beacons make the best bearing objects at night: bright, unmistakable once identified, exactly charted, and fixed — unlike buoys, which swing round their moorings. The hand-bearing compass works as by day (Lesson 11), and is assumed to have no deviation when used away from the boat’s steel and electronics. First check on the chart that nothing lies between you and the light: a headland or a stretch of coast hides it, and a real chart marks the arcs where a light is obscured. The practice chart gives no land heights or obscured arcs, so in this course a light counts as hidden if your line of sight passes within 0.2 M of the coast. That rules out both lighthouses from the Port Kerran approach: each stands at one end of the island, behind its shore.

Fog: stay in, stay out or press on?

Fog at sea is most often sea fog (advection fog): warm, moist air moving over colder sea. Around the UK it is commonest in spring and early summer, and it can persist for days. Radiation fog forms over land on clear, calm nights and can drift out over harbours and estuaries, usually clearing during the morning. Lesson 38 covers forecasts; here the question is what to do.

If fog is forecast, the easy decision is to stay in harbour. At sea it turns on how sure you are of your position, how hard the way in is, and the crew and boat. A short, deep, well-marked approach may be worth pressing on with, slowly; a rocky entrance or a route across a shipping lane usually isn’t.

The safest place in fog is in harbour — or in safe water you know, out of the way of ships.

Safe speed, lookout and being seen

Lesson 22 covered the fog signals, Rule 19 and a fog routine; they all still apply. The rules also require every vessel to keep a proper lookout by sight and hearing and by all available means (Rule 5), and to go at a safe speed (Rule 6): one at which you can take proper and effective action to avoid a collision and stop within a distance appropriate to the conditions. The rules give no number. In fog that means being able to stop or turn away within the distance you can see — while keeping enough way to steer and to stop the tide setting you onto danger.

Being seen matters as much as seeing. A yacht, especially one of glass-reinforced plastic or wood, is a poor radar target, so hoist a radar reflector as high as you can (an octahedral reflector is hung in the “catch-rain” position, with one hollow facing up), or use an active radar target enhancer. An AIS transmitter (Lesson 33) helps ships with AIS find you — but not every vessel carries AIS, so it never replaces the lookout.

Navigating in fog

• Plot regularly. Write the GPS position, time and log reading into the log at short, regular intervals, plot it, and keep an EP going (Lesson 14), so you have a position if the GPS fails.
• Keep clear of shipping lanes and headlands. Traffic converges off headlands, which often have races and ledges too. If you must cross a lane, cross as quickly as you can, heading at right angles to the traffic.
• Prefer shallower water, where big ships can’t go (Lesson 22) — as long as it is free of dangers.
• Aim off. To find an entrance on a coast, steer deliberately to one side of it. When you reach the coast or a contour, you know which way to turn.

Following a depth contour

A depth contour is a position line that needs no visibility. Choose one that runs where you want to go with no dangers on or near it, get onto it, and follow it: alter towards deeper water when the depth falls, back towards the shallows when it rises. The sounder shows the actual depth — charted depth plus height of tide (Lesson 8) — measured below the surface, the transducer or the keel, depending on its setting.

Off Kerran Head the 10 m contour is a trap: Head Ledge rises right on it. So start on the 10 m line south-east of the ledge, about 1.0 M from it, and follow it east with the island to port. It passes about 0.7 M inshore of Mew Rock — which sits on the 20 m line, so a boat following 20 m would hit it — and leads to the entrance of Kerran Bay near No 4 buoy.

Radar: the basics

Radar sends out pulses of microwave energy from a rotating aerial and shows the echoes: the time an echo takes to return gives its range, and the direction of the aerial its bearing, with your boat at the centre. Coasts, ships, buoys and rain show well. Small yachts may show poorly or not at all, especially among echoes from waves (sea clutter) or rain; a low shore may show further off than the water’s edge, because the echo comes from higher ground behind; and anything behind a headland or ship is hidden in its shadow.

Range rings are circles at set intervals (the screen shows the spacing, for example 0.5 M on the 3 M range) for judging distance at a glance. The variable range marker (VRM) is a ring you move onto an echo to read its range, and the electronic bearing line (EBL) a line you swing onto it for its bearing. Radar ranges are more accurate than radar bearings, so a fix from the ranges of two or three charted objects is usually better than one from radar bearings.

Module 5 complete

That completes planning and pilotage. Whatever the conditions, the method is the same: plan in advance, keep a plot, and check one source against another. Module 6 takes these skills to Yachtmaster level.