Navigating in Restricted Visibility

Prerequisites

• RYA Competent Crew or equivalent sea time, with a working knowledge of the boat's instruments and safety kit.
• Day Skipper lessons on Position Fixing, COLREGs Rules and Lights and Electronic Navigation and Radar. This lesson applies them when you cannot see far, and does not repeat them in full.
• Ability to plot an estimated position (EP), read depth contours on a chart and correct a sounding for the height of tide.
• For the advanced techniques of radar plotting, blind pilotage and fog pilotage into harbour, see the Coastal Skipper lesson Night and Fog Pilotage (cs-night-fog-pilotage). This lesson stays at Day Skipper level.

Learning Objectives

By the end of this lesson you will be able to (RYA Day Skipper shorebased syllabus: Restricted visibility, and Collision regulations; practical syllabus: Passage making and Pilotage):

• Recognise the weather conditions that produce fog and read the warning signs in a forecast (RYA syllabus: Meteorology, fog).
• Prepare the boat and crew in a set sequence when visibility starts to close in (RYA syllabus: Restricted visibility, precautions).
• Apply COLREGs Rule 19, the conduct of vessels in restricted visibility, and Rule 6, safe speed.
• Make and recognise the sound signals of Rule 35 that a yacht is most likely to hear.
• Explain why a radar reflector matters for a small boat and how to mount it.
• Organise an effective lookout by eye and by ear.
• Choose a safe route into shallow water and away from shipping lanes.
• Navigate using depth contours, a handrail and the echo sounder, and cross-check GPS against independent information.

RYA Syllabus Map

What the examiner expects. On the practical course the instructor may simulate fog by asking you to navigate below deck, or may simply ask: "Visibility has dropped to 200 metres. What do you do?" They want a calm, ordered list of actions: fix the position, slow down, put on lifejackets, post a lookout, start the sound signal, hoist the reflector, get the engine ready, and decide where to go. They also want to hear the words "shallow water, out of the shipping".

Why Fog Is Dangerous

Fog takes away the one sense a small-boat skipper relies on most. Three problems follow:

1. You cannot see other vessels. A ship at 15 knots covers 1 nautical mile in 4 minutes. In 200 metres of visibility you might get under 30 seconds of warning.
2. Other vessels cannot see you. A yacht is a weak radar target, especially in a sea, and many small craft have no AIS transmitter. A watchkeeper on a ship may not know you are there.
3. You cannot see the land or the marks. Visual fixing becomes impossible, and the risk of running onto a danger you thought was miles away rises sharply.

Fog is also disorientating. Sound behaves strangely, and many sailors find it hard to judge the direction of a fog signal. Crew may become anxious. A prepared skipper with a clear plan keeps everyone calm.

How Fog Forms

Fog is cloud at sea level. It forms when air is cooled to its dew point, and visibility drops below 1000 metres. In UK shipping forecasts, visibility is described as good (more than 5 nautical miles), moderate (2 to 5 M), poor (1000 metres to 2 M) and very poor (less than 1000 metres). "Fog" in a forecast means very poor visibility.

Sea fog (advection fog)

Warm, moist air blows over colder sea and is cooled from below. This is the fog that matters most to UK yachtsmen. It is common in spring and early summer, when the sea is still cold, especially in the western Channel and the South West. Sea fog:

• Can form at any time of day or night.
• Can persist in strong winds, unlike land fog. A force 5 with fog is quite possible.
• Can roll in quickly from seaward and clear only when the wind direction or air mass changes.

Radiation fog

On clear, calm nights the land cools and the air above it reaches its dew point. Radiation fog forms over land and drifts into estuaries and harbours in the early morning. It usually burns off by late morning as the sun warms the ground, and rarely extends more than a mile or two offshore. If you leave harbour in it, conditions are often clear a short way out.

Warning signs

• A forecast mentioning "fog patches" or "visibility occasionally very poor".
• A warm, moist southerly or south-westerly airflow in spring over a cold sea.
• Dew point close to air temperature. When the gap is 2 degrees C or less, fog is likely.
• Haze thickening, horizon disappearing, distant headlands fading.

Planning rule: if fog is forecast, ask whether the passage can wait. A Day Skipper should not set out into forecast fog on a route that crosses a shipping lane or relies on visual pilotage.

Preparing When Fog Forms

When visibility starts to close in, act early. Work through a fixed sequence so nothing is forgotten. Many skippers keep this list in the chart table.

The immediate checklist

1. Fix the position now. Take a GPS position and, if marks are still visible, a visual fix. Mark it on the chart with the time and log reading. This is your last confirmed position, and everything that follows depends on it.
2. Reduce to a safe speed (Rule 6). See below.
3. Lifejackets on for everyone on deck, clipped on if the sea warrants it. A man overboard in fog is extremely hard to find.
4. Post a lookout forward if possible, away from engine noise, listening as well as looking.
5. Start the fog signal (Rule 35).
6. Hoist or check the radar reflector.
7. Switch on navigation lights. They help a little at short range.
8. Engine running or ready to start at once, so you can manoeuvre quickly. Under sail in light wind you may have no steerage way when you need it.
9. Switch on AIS and radar if fitted. Set the VHF to channel 16 and the local VTS or port channel; listen for traffic and safety broadcasts.
10. Prepare the liferaft and grab bag, and make sure the crew know where they are. Close hatches.
11. Decide where to go (see Choosing a Safe Route).
12. Brief the crew: what you are doing, what you want them to listen and look for, and how to report it.

Safe speed

Rule 6 requires every vessel to proceed at a safe speed so that she can take proper and effective action to avoid collision and stop within a distance appropriate to the conditions. In fog, that means slow enough to stop, or to turn away, within the distance you can see. In practice, for a yacht:

• Slow enough that you can hear other vessels over your own engine noise. Many skippers run at low revs, or under sail with the engine ticking over.
• Fast enough to keep steerage way, so you can turn quickly.
• Remember the tide: speed over the ground may be very different from speed through the water. A 2 knot foul tide plus a low engine setting might leave you hardly moving.

The Rules: Safe Speed and Rule 19

COLREGs Rule 19 applies to vessels not in sight of one another when navigating in or near an area of restricted visibility. The important point is that the normal steering and sailing rules (Rules 11 to 18, such as power giving way to sail, or crossing situations) do not apply until vessels can see each other. A sailing yacht has no right of way over a ship in fog.

What Rule 19 requires, in plain terms

• (b) Proceed at a safe speed adapted to the conditions, and have the engines ready for immediate manoeuvre.
• (c) Have due regard to the conditions when complying with the rules on lookout and safe speed.
• (d) A vessel that detects another by radar alone, and finds that a close-quarters situation is developing or risk of collision exists, must take avoiding action in good time. If this involves an alteration of course, so far as possible avoid:
• an alteration to port for a vessel forward of the beam, other than one being overtaken;
• an alteration towards a vessel abeam or abaft the beam.
• (e) Except where it has been determined that risk of collision does not exist, every vessel that hears the fog signal of another apparently forward of her beam, or cannot avoid a close-quarters situation with a vessel forward of her beam, must reduce speed to the minimum at which she can be kept on course. If necessary she must take all way off, and in any event navigate with extreme caution until the danger is over.

Applying it on a yacht without radar

Most Day Skipper boats have no radar. The practical points are:

• If you hear a fog signal forward of the beam, slow right down to steerage way, or stop. Do not guess the direction and charge off; sound direction is unreliable in fog.
• Listen and keep sounding your own signal. Ask the crew to point to where they think the sound is coming from. If several people agree and the bearing stays steady while the sound grows louder, treat it as a close-quarters situation.
• Keep clear of where ships go. The best avoidance is never to be in their path.
• If a ship appears close ahead, the usual instinctive move is a bold turn to starboard or away from her. Act decisively, and make it obvious.

Sound Signals

Rule 35 sets out the fog signals. A short blast is about 1 second; a prolonged blast is 4 to 6 seconds. A vessel of less than 12 metres in length is not obliged to give the signals below, but if she does not, she must make some other efficient sound signal at intervals of not more than 2 minutes. In practice a yacht should give the proper signal using a fog horn or the electric horn.

Which signal does a yacht make? Under sail alone, one prolonged and two short. Under engine (with or without sails up), she is a power-driven vessel: one prolonged blast. Brief the crew on the difference and give one person the job of timing the signal. A phone timer set to 90 seconds works well.

Gas horns run out. A typical aerosol fog horn gives a limited number of blasts. Carry a spare canister, or use the electric horn, a mouth horn or a VHF loud hailer if fitted.

Being Seen: The Radar Reflector

A ship's radar picks up a small yacht poorly. Glass fibre hulls are almost transparent to radar, and a target low in the water is lost in sea clutter. SOLAS Chapter V, Regulation 19, requires vessels under 150 gross tonnage, where practicable, to carry a radar reflector or other means to enable detection by ships navigating by radar. For a yacht, that is effectively every passage.

Choosing and mounting

• Passive reflectors use metal plates set at right angles. The cheap octahedral type works well only when mounted in the "catch rain" position (one cavity pointing straight up), and has blind spots. Tube reflectors and lens types give a more even response. Look for a reflector tested to ISO 8729.
• Active reflectors (radar target enhancers) receive a ship's radar pulse and return an amplified signal. They give a much stronger echo but need power.
• Height matters. Mount the reflector as high as practicable, ideally at least 4 metres above the sea. The higher it is, the further away it shows and the further it is above wave clutter.
• Heel matters. Many reflectors lose much of their performance when heeled. Check the manufacturer's data.

AIS

An AIS transmitter (Class B is common on yachts) puts your position, course and speed on ships' displays. It is a strong aid in fog, but not every vessel watches AIS closely, and Class B transmissions can be filtered out in busy areas. Use it together with a reflector, not instead of one.

Keeping a Lookout

Rule 5 requires a proper lookout by sight and hearing at all times. In fog, hearing becomes as important as sight.

• Post the lookout forward, at the bow if it is safe, away from the engine and the cockpit chatter. Clip them on.
• Stop the engine briefly every few minutes, if conditions allow, to listen. You may hear engines, fog signals, breaking water on rocks or a bell buoy that the engine would mask.
• Rotate the lookout often. Concentration in fog fades quickly; 30 minutes is plenty for one person.
• Agree on reporting: "Ship's horn, fine on the starboard bow, getting louder." Clock bearings or relative bearings are clear and fast.
• Use every sense. The smell of diesel or seaweed, the sound of surf, or a sudden change in the sea surface can all tell you something.
• Keep the radio on. Coastguard, VTS and other vessels may broadcast useful traffic information, and you can call a ship by name if her AIS shows she is heading for you.

Choosing a Safe Route

The single most important decision in fog is where to go. The standard Day Skipper answer is to get out of the shipping into shallow water, where large vessels cannot follow, and then to reach a safe harbour or anchorage using a route that does not need you to see.

Options

Where shipping goes

Large ships stick to deep water, traffic separation schemes, port approach channels and recommended routes. The chart shows these. Ferries follow predictable lines between ports. In fog, these are the places to avoid or cross quickly at right angles. If you must cross a traffic separation scheme, Rule 10 still applies: cross on a heading as nearly as practicable at right angles to the general direction of traffic flow.

Why shallow water works

A small yacht with a 1.8 metre draught can sail safely in 5 metres of water, where a ship drawing 10 metres cannot go. Choosing a route along, for example, the 5 or 10 metre contour, inshore of the main channel, puts a physical barrier between you and large ships. The risk shifts to dangers you can manage: rocks, pot buoys and small craft moving slowly.

But not too shallow. Know the height of tide, and allow a margin. A rocky coast in fog is not where you want to be. Choose a stretch of coast with a gently shelving bottom and no off-lying dangers.

Navigating Without Seeing

With visual fixes gone, a Day Skipper navigates on an accurate EP, the echo sounder and electronic aids, each checking the others.

Keep the plot going

• Log the position every 15 to 30 minutes in fog, more often near danger. Record time, log reading, course steered, GPS position and depth.
• Work up an EP from your last fix, allowing for tidal stream and leeway. If GPS and EP disagree by more than you expect, find out why.
• Steer accurate courses. In fog the helm has no visual reference, so the compass is everything. Give the helmsman a course to steer, not a heading to "something".

Depth contours

The echo sounder gives an independent position line. Correct the chart depth for height of tide (depth on the sounder = charted depth + height of tide, minus any offset in the sounder setting).

• Follow a contour. Choose a contour, say the 10 metre line, that runs towards your destination clear of dangers. Steer to keep the corrected sounding near 10 metres: if it shoals, turn a little to seaward; if it deepens, turn a little inshore. This is a depth "handrail".
• Use a line of soundings. Take soundings at regular intervals, say every 0.2 M, and mark them on tracing paper along your course line. Slide the paper over the chart along the course until the soundings match the charted depths. This gives an approximate position.
• Set the shallow alarm at a safe depth, so you are warned before you cross into danger.
• Look for features. A sudden step in depth, a channel or a bank shown on the chart can confirm your position as you cross it.

Handrails

A handrail is any feature you can follow continuously without seeing far: a depth contour, a line of buoys, the edge of a channel, or a steep shore in the case of very close inshore work. Plan to reach a handrail some way up-tide or to one side of your destination, then follow it. This is the principle of aiming off: deliberately aim to one side of the target so that when you reach the handrail you know which way to turn.

For example, if the harbour entrance is on a coast running north to south and you approach on an EP, aim to reach the coast clearly north of the entrance. When you pick up the 5 metre contour you know you must turn south, rather than wondering which way you missed.

Buoy hopping

In moderate fog you may be able to go from buoy to buoy. Plan each leg on the chart with course, distance and expected time. Steer the course carefully, allow for tide, and be ready to stop at the expected time if the buoy is not seen. Do not go searching around blindly; return to the last known mark or anchor if a mark does not appear.

GPS Cross-Checks

GPS and chartplotters are the main aid in fog for most yachts, and they are very accurate. But a single source of position is risky when you cannot see. Cross-check it every time you log:

• GPS against EP. Is the GPS position where your EP says you should be? A large jump suggests an error somewhere.
• GPS against depth. Does the corrected sounding match the charted depth at the GPS position? This is the simplest and best independent check.
• GPS against chartplotter. Is the plotter using the right chart datum (WGS84) and an up-to-date chart? Is it zoomed in enough to show small dangers?
• GPS against radar if fitted: does the radar range of the coast agree with the plotter?
• Plot the GPS position on the paper chart regularly, so you can carry on if the electronics fail.

Remember that a chartplotter may show a confident, detailed picture that is wrong, if the chart is old, the datum mismatched, or the receiver has lost its fix and is showing a stale position. Most receivers show a warning when the fix is lost. Know where yours shows it.

For radar plotting, parallel indexing and detailed blind pilotage into harbour, see Night and Fog Pilotage in the Coastal Skipper course.

Arriving and Anchoring in Fog

• Approach slowly, with crew forward listening and looking.
• Use the sounder continuously and stop if depth does not match the plan.
• Expect moorings, pot buoys and small boats close inshore, often unlit and silent.
• If you anchor, sound the anchor signal (rapid bell for about 5 seconds every minute) and keep a listening watch. Show anchor lights.
• Harbour authorities may broadcast traffic information on VHF; call the harbour master for advice before entering.

Worked Example

You are on a 10 metre yacht (draught 1.8 m) heading east along a south-facing coast towards a harbour 8 M away. The coast shelves gently. A traffic route used by ferries runs 3 M offshore. At 1100, with the harbour bearing 080 degrees T, sea fog rolls in and visibility drops to about 200 metres. The tidal stream is flowing east at 1.5 knots. Height of tide is 2.0 metres.

Step 1: Fix. The GPS gives a position 2 M south of a headland, on the 20 metre contour. You mark it on the chart at 1100 and log 23.4.

Step 2: Immediate actions. Engine on at low revs, sails reduced to the main for stability. Lifejackets on and clipped on. Navigation lights on. Fog horn: you are motor-sailing, so one prolonged blast at least every 2 minutes. Radar reflector checked at the spreaders. AIS on. VHF on channel 16 and the port channel. Crew briefed; one lookout at the bow.

Step 3: Route. You are between the coast and the ferry route, but only 1 M inshore of it. You decide to head north-east into shallower water and pick up the 10 metre contour, which runs east towards the harbour clear of dangers. With 2.0 metres of tide, the 10 metre contour should read about 12 metres on a sounder set to show depth below the surface. You set the shallow alarm at 6 metres.

Step 4: Aim off. The harbour entrance lies just inshore of the 10 metre line. You plan to follow the contour until the log shows you are 1 M short of the entrance, then turn in on a bearing using the GPS and the sounder, with the shallow alarm ready.

Step 5: Cross-check. Every 15 minutes you log the GPS position and the sounding. At 1130 the GPS shows you on the 10 metre line and the sounder reads 12.2 metres: the checks agree.

Step 6: Hazard. At 1140 the lookout hears a single prolonged blast, fine on the port bow, getting louder. Under Rule 19(e), you reduce to the minimum speed to keep steerage way and keep sounding your own signal. The sound passes down the port side and fades. You resume your plan.

Step 7: Arrival. You enter at 1250, slowly, with a crew member forward. The harbour master has warned on VHF of a fishing vessel leaving; you hold position outside the entrance until she has passed.

Common Mistakes

• Waiting too long to act. The best time to fix your position is before the fog arrives.
• Assuming sail has right of way. In fog, Rules 11 to 18 do not apply until vessels are in sight of one another.
• Wrong fog signal. A yacht motor-sailing is a power-driven vessel and should give one prolonged blast.
• Charging towards a fog signal. Slow down and listen. Sound direction is unreliable.
• Altering to port for a vessel ahead. Rule 19(d) says avoid this where possible.
• Trusting the plotter alone. Cross-check with depth and EP.
• Forgetting height of tide when comparing soundings with charted depths.
• Staying in the shipping lane because it is the direct route.
• Crew without lifejackets or not clipped on.
• Running out of horn gas halfway through a long fog.

Summary

Fog removes sight, so the skipper must replace it with preparation, sound and careful navigation. Fix the position early, slow to a safe speed, put lifejackets on, post a lookout, start the correct fog signal, hoist the radar reflector, and get the engine ready. Remember that Rule 19 replaces the normal steering rules: no vessel has right of way, and hearing a fog signal forward of the beam means slowing to steerage way. The safest place for a yacht is usually in shallow water, away from shipping, following a depth contour or other handrail towards a harbour with an easy approach. Keep a regular log, and cross-check GPS against EP and soundings at every entry.

Check Your Understanding

1. Visibility suddenly drops to 300 metres. List the first six things you would do.

Answer: Fix and record the position; reduce to a safe speed; lifejackets on (and clip on); post a lookout; start the fog signal; hoist or check the radar reflector. Then lights, engine ready, AIS and VHF, prepare the liferaft and decide where to go.

1. What fog signal does a sailing yacht make under sail alone, and under engine?

Answer: Under sail: one prolonged blast followed by two short, at intervals of not more than 2 minutes. Under engine (even with sails set): one prolonged blast at intervals of not more than 2 minutes.

1. You hear a fog signal apparently forward of your beam. What does Rule 19 require?

Answer: Unless it has been determined that risk of collision does not exist, reduce speed to the minimum at which you can keep your course, take all way off if necessary, and navigate with extreme caution until the danger is over.

1. Why is shallow water usually the safest place for a yacht in fog?

Answer: Large ships cannot enter it because of their draught, so it removes the greatest collision risk. A yacht drawing about 2 metres can navigate safely in water a ship cannot, provided the coast is free of off-lying dangers and you allow for height of tide.

1. Explain how you would use a depth contour as a handrail.

Answer: Choose a contour that leads towards your destination clear of dangers. Correct it for the height of tide to find the expected sounder reading. Steer to keep that reading: if it shoals, turn to seaward; if it deepens, turn inshore. Set the shallow alarm as a backstop.

1. What is aiming off, and why is it useful?

Answer: Deliberately steering to one side of your target so that when you reach a handrail (the coast or a contour) you know which way to turn to reach it. Without aiming off, you may not know which side of the target you have arrived on.

1. Give three ways to cross-check a GPS position in fog.

Answer: Compare it with your EP; compare the corrected sounding with the charted depth at the GPS position; check the chart datum and chart currency on the plotter; compare with radar ranges if fitted.

1. Why should a radar reflector be mounted high?

Answer: Height increases the range at which a ship's radar can detect it and lifts it above wave clutter. A height of at least 4 metres above sea level is a common guideline.

Further Reading

• Night and Fog Pilotage (Coastal Skipper): radar plotting, parallel indexing and blind pilotage.
• COLREGs Rules and Lights: the full text of Rules 5, 6, 19 and 35.
• Electronic Navigation and Radar: GPS, chartplotters, AIS and radar basics.
• Position Fixing: EP, depth as a position line and running fixes.
• Weather and Forecasting Tools: forecasts and fog warnings.
• MCA guidance on radar reflectors for small craft, and the International Regulations for Preventing Collisions at Sea (COLREGs).