Marine Navigation & Compass

Navigating on the water uses the same compass skills as on land, plus a few of its own: there are no trails, the chart replaces the map, current and wind move you sideways while you sail, and the boat itself can throw your compass off. A chartplotter makes it easy until the power fails or the screen goes dark, so every sailor should be able to find their position the traditional way.

The Compasses Aboard

  • Steering compass: mounted where the helm can see it, with a lubber line marking the boat's centreline. You steer by keeping the course under the lubber line.
  • Hand-bearing compass: a small handheld compass for taking bearings on lights, buoys, headlands and other boats.
  • Electronic compasses in chartplotters, autopilots and phones are useful backups, but they need power.
  • It's the law: Transport Canada requires a magnetic compass on most pleasure craft, except boats 8 m or less that stay within sight of navigation marks. See the required equipment list.

Variation and Deviation

Two errors separate the course on your chart from the number on your compass:

  • Variation is the difference between true north and magnetic north where you are. It's printed on the chart's compass rose, with the year and its annual change. (On land it's called declination.)
  • Deviation is the error caused by your own boat: the engine, steel fittings, speakers and electronics near the compass. It changes with the boat's heading. Have the compass "swung" and keep a deviation card beside it, and keep phones, radios and tools away from it.

Sailors remember the order as T-V-M-D-C: True, Variation, Magnetic, Deviation, Compass. Going from compass to true ("correcting"), add easterly errors and subtract westerly ones. Going from true to compass, do the opposite: subtract east, add west.

Charts

  • Use current charts for your waters: Canadian Hydrographic Service charts in Canada, NOAA charts in the U.S. Keep them up to date with Notices to Mariners.
  • Read the title block before you rely on a chart: are depths in metres or feet, and measured from what datum?
  • Measure distance on the latitude scale, the scale up the side of the chart. One minute of latitude is one nautical mile (1,852 m). Never use the longitude scale along the top and bottom.
  • Carry paper charts or a printed chart book for the area as your backup.
  • Plotting tools: a pencil and eraser, dividers, and parallel rules or a course plotter.

Fixing Your Position

  1. Cross bearings. Take hand-bearing compass bearings on two or three charted objects, such as a lighthouse, a headland or a buoy, ideally 60 to 90 degrees apart. Convert them to true, and plot each line from the object back toward you. You're where they cross. Three lines usually form a small triangle (a "cocked hat"); assume you're at the corner closest to danger.
  2. Ranges (transits). Two charted objects in line give an exact line of position with no compass error at all.
  3. Depth. Compare your depth sounder with the depth contours on the chart as a check.
  4. Electronics. GPS, radar ranges and AIS are excellent, but cross-check them against what you can see.

Dead Reckoning

Dead reckoning works out where you are from where you were, using your course, speed and the time that has passed. It's what you fall back on in fog, at night, or when the electronics fail.

  • Distance = speed × time. The six-minute rule makes it quick: in six minutes you cover one tenth of your speed in nautical miles. At 6 knots, that's 0.6 nm.
  • Keep a log. Every hour, and at every course change, write down the time, position, course, speed and log reading.
  • Adjust for set, drift and leeway (below) to turn a dead-reckoning position into an estimated position.

Current, Tide and Leeway

  • Current carries the whole boat. Its direction is called set and its speed is called drift. Check tide and current tables before you leave, and steer a course that allows for it.
  • Tide changes the depth under you, sometimes by many metres. Work out the height of tide before crossing a shallow bar or anchoring.
  • Leeway is the sideways slip of a sailboat pushed by the wind, greatest when beating to windward. Look back at your wake to judge it.

Buoys and Lights

  • Red, right, returning. Canada and the U.S. use the IALA Region B system: coming in from sea, keep red marks on your right (starboard) and green on your left.
  • Cardinal buoys, common in Canada, show which side of a hazard the safe water is on: north, east, south or west.
  • Learn the light characteristics printed on the chart, such as "Fl G 4s," so you can identify a mark at night by its colour and timing.

Avoiding Collisions

  • Constant bearing, decreasing range means collision. Take repeated hand-bearing compass bearings on an approaching vessel. If the bearing doesn't change while it gets closer, you're on a collision course: act early and obviously.
  • Keep a proper lookout at all times, using eyes, ears, AIS and radar if you have them.
  • Know the give-way rules and the light and sound signals for night and fog.

Electronics Without Over-Reliance

  • Plot your position on the paper chart at regular intervals, even when the chartplotter is working.
  • Carry a handheld GPS and spare batteries, and a phone chart app with offline charts downloaded.
  • Know how to steer a compass course and find your position if every screen goes dark.

Offshore, or just curious? Celestial navigation works anywhere on earth with no power at all. See How to Use a Sextant.

See also: Top 10 Sailing Safety Tips, How to Use a Compass (on land), Sailboat Upgrades to Consider and Scan Your Plan for float plans.