Navigating in Fog

Modern electronics and time-tested seamanship skills will keep you going when visibility diminishes.

fog navigation
Iron Bark II shortens the chain while getting ready to depart as soon as the rising sun disperses the fog in the anchorage.Tom Zydler

At first on that early mid-July morning in Old Saybrook, Connecticut, on Long Island Sound, we could still see the end of the outer dock. We got underway. Half an hour later, a blanket of wet cotton spread over the channel, our ketch’s bow only vaguely looming ahead. Dots, thick as measles, pulsed on the radar screen — the start of weekend boat traffic on one of the most congested bodies of water in the nation. Unseen runabouts buzzed around like mosquitoes. The Automatic Identification System display didn’t help much in such close quarters, as scores of signals came from boats on moorings or tied up, their units still transmitting. We could breathe easier once we crawled out into the wider waters of the sound. With the motor off as we ghosted along under sail, engine noises from other boats resounded through the fog, and most of the time we could relate them to the radar returns. We hoped the powerful electric horn on the mast screeched loud enough to announce our presence to the speed maniacs.

Electronics Make It Safer

Summer fog on Long Island Sound — or any other body of water where visibility can be reduced to a matter of feet — can be a real curse to navigation, although radar, AIS and chart plotters have restored some sanity to dealing with whiteout conditions. Precise, wide-awake navigation is essential. When a new target pops up on the radar, check the chart to see the bearing and the distance to the closest navigational aid, and compare it with the new target’s range and bearing on the radar display to decide whether the new target is a buoy, beacon or vessel. More sophisticated radars offer an automatic radar plotting aid (ARPA) function, which illustrates a target’s position data. Small pleasure-craft radars offer the mini version, MARPA, which requires manual selection of the target. These small radars typically receive speed and latitude/longitude information from a GPS at certain preset time intervals. As a result, what MARPA delivers isn’t superbly precise but is good enough to tell the navigator whether the target is a moving vessel or a fixed object. The small-yacht navigator should treat the MARPA function — as well as chart-plotter and AIS data — as aids to cautious navigation. Holding on to a grain of skepticism regarding the accuracy of technology will make your life on the water more secure.

While the AIS display delivers very reliable data, hundreds of pleasure boats and fishing vessels operate without transponders. A special risk comes from encounters with tugboats. They usually carry AIS transmitters, and assuming you have at least an AIS receiver, you can hail them by name on VHF channel 13 or 16. The tugs either push their loads or tow them behind. A tugboat towing astern paints an unmistakable radar return that looks a bit like a catamaran. Be especially careful to keep your distance when steering astern of such a tow, as getting caught in the towline can result in dismasting or sinking. Remember that a working towboat has the right of way over any other vessels, including sailboats under sail. And when motorsailing, a sailboat is legally a vessel under power.

fog navigation
Radar is an invaluable tool when visibility is low. When radar is overlaid on a chart, as can be done on many modern chart plotters, it is ­easier to distinguish land from potentially moving hazards, such as other boats.Courtesy of Furuno

Practice with Electronics

If your boat is equipped with working radar but does not have an AIS receiver, maneuver the radar cursor to put a bearing line on a target of interest. If the bearing remains constant and the target stays on the line, you and the target are on a converging course that will lead to a collision. Put out a VHF sécurité call (channel 13 and 16) with your latitude/longitude position and the distance to the target.

Find the optimal range that delivers the clearest information on your radar. That’s 6 miles in the case of our Raytheon Pathfinder model. Often this range gives the first warning for weak targets at close quarters, whereas a setting of 3 miles fails to reveal the same target. When this particular radar is set on the 3-mile range, the target may be as close as 1.5 miles before it appears on the screen. On other models, such spotty performance may occur at different distances, so practice using your radar in good visibility to learn its idiosyncrasies. When at sea in fog, vary the range settings regularly and you may be surprised by what’s there: Runabouts and even fairly large wooden vessels do not paint good returns. Rough water masks the presence of smaller vessels, and the sea-clutter control at high settings will actually hide small targets. Rain can also diminish radar performance. Anything more than gentle drizzle calls for extra caution. In heavy rain, the AIS unit will be your best friend to detect larger vessels.

AIS delivers the name, bearing, range and speed of the target, as well as the closest point of approach (CPA) and time to CPA (TCPA). Because of a small yacht’s erratic speed and course in rough seaways (erratic at least in electronic terms), treat AIS data with some caution. And even with AIS aboard, radar is a much-appreciated piece of equipment when sailing in the coastal waters of North America or anywhere with shipping and frequent fog.

Fog Dynamics

Anywhere from New England to Nova Scotia, and farther north during calm, clear-sky weather, the night breeze will push warm air offshore. This will turn to fog over the cold sea, forming a prominent belt. Savvy locals often move along the coast during the morning hours of good visibility before the afternoon onshore breeze rolls the wisps back to shore. Very close inshore or between islands, foggy air heated by the sun loses moisture, causing visibility to greatly improve. This is convenient for entering island anchorages in places like Maine, Nova Scotia or Newfoundland. Fresh winds may disperse some fog. However, strong southeasterlies or easterlies that begin to howl before the arrival of a depression drive ever-thicker haze across the seas. This is when you put in some deep reefs in order to slow down and have time for collision avoidance.

Fog can assume a bewildering variety of shapes and densities, from a disorienting whiteout to steam ghosts waving in the breeze or even multicolored “fogbows.” There is black fog and gray fog, and when backlit by low sun, fog can turn pink. Preparing yourself and your vessel with the right tools to navigate can make the difference between a stressful passage and a peaceful sail, ghosting through fog in light air.

Quick Tip: In reduced visibility, all vessels must turn on their running lights; in low-lying fog, a masthead light may actually be spotted from the bridge of a ship. Hoist the largest radar reflector the rig can tolerate, as it will magnify the signature of the boat on a receiving radar. At close quarters, a strong spotlight may throw a bright circle in the fog as a last-minute warning.

* Tom and Nancy Zydler spent the summer dodging the fog as they sailed their Mason 44,* Frances B, through Baffin Bay and up the west coast of Greenland.