Bye, Bye Birdie: Dead Reckoning

By Robert Reeder

September 12, 2017

Successful navigation is really just answering three simple questions:

Where am I?

Where am I going?

How do I get there from here?

Even when all of our navigation tools are operational, it is prudent to have three independent sources of navigation. For coastal navigation on a clear day, I would probably use Dead reckoning, visual and radar navigation; if fog took away my visuals, I’d probably add in GPS and a chart plotter. Dead reckoning will always be the cornerstone of my navigation under any condition because removed of all other navigational tools, dead reckoning can never be taken away from me.

Dead reckoning is the cornerstone of all European-style navigation, and there are many different methods to go about it. All of the methods extrapolate our current position from a known position in the past, or else predict our future position from a known position in the present. All of it is based on the course and speed we have made or intend to make since the known position.

This preliminary known position, based on information external to our boat, is called a “fix.” A fix may be derived from visual bearings, radar ranges, bathymetry (as discussed in previous articles), GPS, and altitudes of celestial bodies; basically any source of position information that does not originate inside our vessel. For this exercise, our fix is a range and bearing to a fixed object close aboard. I use the Chesapeake Light for this example, laying one-tenth of a nautical mile abeam to port, while on course 015° True.

We begin our dead reckoning by plotting the fix directly on our chart. It is critical that we label the time of this fix. We must choose a workable interval with which to plot our fixes, whether on paper or on an electronic chart plotter. In open ocean, the interval may be only once per hour, but in pilotage waters it should be every three or six minutes. A good rule of thumb when in close proximity to shoal water is for your interval between external fixes to be less than half the distance to shoal water. For this exercise, we’ll use a 30-minute interval.

From our fix we draw the course line on our chart, using the compass rose and parallel rulers. Whatever my heading source, if needed, I convert it from Magnetic to True before plotting it, and extend it out to some amount of time into the future. For this, we continue on course 015° True at a speed of 12.2 knots, so our dead reckoning intervals of 30 minutes will be 6.1 nautical miles. We will extend out twice that distance in order to see our dead reckoning position in one hour.

We want to use Speed Through the Water for our dead reckoning rather than GPS-derived Speed Over Ground, because the latter is a constantly changing variable.

Note that the standard symbol for our visual fix is a dot with a circle around it; the standard symbol for a dead reckoning position is a dot with a semicircle, or bird’s-eye; the idea being that a dead reckoning is about half as reliable as a visual fix (see reference above).

At time 1500 we obtain an external fix of 37° 04.8’N, 075° 38.1’W. For simplification, I’m using GPS. We plot this on the chart and compare to our dead reckoning. Note that I do not know or care what the latitude or longitude of my dead reckoning is, I only know where they happen to be on the chart. For the past hour the currents have pushed me in a direction of 155° True, at a speed of 1.3 knots. This is my “set and drift,” with set being the direction and drift being the speed at which I was pushed. In this case, I was pushed 1.3 nautical miles in exactly one hour, so my drift is 1.3 knots; however, for any time interval other than one hour, you will need to correct for the actual time run. For example, if I had run 30 minutes and had been pushed 0.7 nautical miles over that time, my drift would be 1.4 knots.

Some older books and newer books written for sailors, will separate out set and drift as applying to currents and leeway as applying to wind, into two separate categories. For powerboats, we really don’t care what is causing our offset from our dead reckoning, we just have to know what it is and adjust accordingly.