Practice with paper instead of plastic
By Chris Lab
During a passage along the South American coast on our Passport 40, Aquamarine II, we ran into a strong storm cell with lightning, high winds, and rain. In the hopes of preventing damage from a lightning strike, we unplugged our GPS, radar, radios, and chartplotter and put as much of the gear as we could in the oven—your basic Faraday cage.
With the navigation instruments now no longer operating, we relied on dead reckoning (DR). At night, with our rhumb-line course taking us less than 15 miles off the Colombian coast, we had to be very sure of our position. Fortunately, the data we needed to begin our DR plot was already at hand because our logbook was well maintained and up to date.
Our log contained entries for latitude and longitude, course, windspeed and weather conditions, and any observed current. As we always do, we had plotted our hourly positions on a paper chart.
To navigate by dead reckoning you should start with a fix (GPS, celestial, or visual), which you plot, along with the time, on the paper chart. From that known position you plot your course and speed on the chart and make the necessary notations on the chart (see figure, page 44). While the time between two DR positions is not critical, for accuracy I like to update my DR plot at least once every 30 minutes—more frequently if the piloting situation calls for it.
Use the chart’s compass rose to extend your course line from the fix point. Also take visual bearings on charted objects and plot them if you get a chance. To update the DR plot, use the boat’s estimated speed to calculate the number of miles you have traveled since the fix and, using the latitude scale to calculate the distance traveled in nautical miles, update your DR position on the chart. Never use the longitude scale to measure distance because lines of longitude change constantly according to your distance from the equator. Latitude lines always parallel each other, so distances between them remain unchanged.
For example, if your average speed for 30 minutes is 4.6 knots, you have theoretically traveled half the hourly speed, or 2.3 miles, down the DR track. With a pair of dividers, measure 2.3 miles on the chart’s latitude scale and use this measurement to extend the DR line along the course you’ve been steering during those 30 minutes. Ideally, the end point should be close to your actual geographic position. Note the latitude and longitude of this DR position, as well as any changes in weather and wind speed, in your logbook. To get an even more accurate DR, record average boatspeed every 10 minutes and use the average of the speeds for the 30-minute plot. If you change course, note the time in the logbook, determine the average speed run to the turning point, and plot that point on the chart. Run the new course line from that point.
Maintaining an accurate DR position can be a challenge; current set and drift, sea state, and even the helmsman can influence the course and speed sailed. If you suspect one or more of these variables may be affecting your calculations, always steer a course that will keep you well clear of any dangers.
During the four hours we spent sailing off the Colombian coast without instruments, we chose a course we knew would keep us away from land and well clear of all the obstructions detailed on the chart. When the wind gusted to over 40 knots, we ran offshore under bare poles for an hour to ease the boat’s motion. We logged this course change and, using an estimated course and speed, plotted it on the chart.
It’s always a good idea to keep your DR plotting skills current. The next time you’re sailing along a coast, turn off the chartplotter and use your paper chart to run a DR plot. At the end of an hour, turn the plotter back on and see how accurate your dead reckoning has been.