2.8 Dead Reckoning: Speed-Time-Distance

Dead Reckoning: Speed-Time-Distance

Quick Answer: Dead reckoning (DR) projects where you will be using only your course and speed over time, starting from a known fix. The math is the 60D = ST relationship, where distance is in nautical miles, speed in knots, and time in minutes. DR ignores wind and current, so you correct it with fixes as landmarks allow.

Dead reckoning is the skeleton of every plot. Fixes tell you where you were; DR tells you where you are heading between them, and the Chart Plot module is built on it.

What Dead Reckoning Is

Dead reckoning is the process of advancing a known position (a fix) forward using only the course you steered and the speed you ran, for a measured time. It uses no GPS, no bearings, no depth — just course and speed. Because it takes no account of set, drift, or leeway, a DR position is an estimate that drifts from reality until you correct it with a fix. That is exactly why you keep a DR plot going: it lets you predict arrival times, anticipate hazards, and know roughly where you are the moment your electronics fail. Note that the speed used is speed through the water from your log or engine RPM, which a current can make different from your true speed over the ground.

The 60D = ST Formula

The relationship among speed, time, and distance is captured in one equation, often called "the 60 D Street":

60 x D = S x T

where D is distance in nautical miles, S is speed in knots, and T is time in minutes. Rearranged for whichever value you need:

• Distance: D = (S x T) / 60
• Speed: S = (60 x D) / T
• Time: T = (60 x D) / S

The 60 appears because speed is in nautical miles per hour but time is in minutes, and there are 60 minutes in an hour. Keep time in minutes and the formula stays simple.

Worked Examples

Finding distance: You run 12 knots for 25 minutes. D = (12 x 25) / 60 = 300 / 60 = 5.0 nautical miles.

Finding time: You must cover 8 nautical miles at 16 knots. T = (60 x 8) / 16 = 480 / 16 = 30 minutes.

Finding speed: You covered 3 nautical miles in 20 minutes. S = (60 x 3) / 20 = 180 / 20 = 9 knots.

Fast Shortcuts

Two rules of thumb save time on the water and on the exam:

• The three-minute rule: the distance in yards traveled in 3 minutes equals the speed in knots times 100. At 10 knots you cover about 1,000 yards in three minutes; at 20 knots, about 2,000 yards. (This works because one knot is roughly 2,000 yards per hour, and three minutes is one-twentieth of an hour.)
• The six-minute rule: the distance in nautical miles in 6 minutes equals the speed in knots divided by 10. At 15 knots you cover 1.5 nautical miles in six minutes.

Both come straight out of 60D = ST for convenient time intervals, and both are handy for a quick "how far have I gone since that last mark" check without a calculator.

Plotting and Labeling a DR Track

A DR plot follows strict labeling conventions so anyone can read it:

• Start from a fix, marked with a dot inside a circle and the time.
• Draw the intended course as a straight line and label it above the line with a "C" (for example, C 090). Label the speed below the line with an "S" (S 10).
• Mark each DR position with a semicircle (half-circle) on the line and the time beside it (a four-digit time such as 1015).
• An estimated position (EP) — a DR position corrected for current or leeway — is marked with a square and the time.

Advance the DR position at every course change, every speed change, at each fix, and at least every hour.

DR Versus Estimated Position

Because DR ignores current, the set and drift of a current will carry you off the DR track. When you apply the current to the DR, you get an estimated position (EP) — still not a true fix, but closer to reality. When a landmark comes into view, you take a fix, then start a fresh DR from it. This cycle — fix, DR ahead, correct to EP, fix again — is the rhythm of coastal navigation.

Speed over ground versus through the water: suppose your log reads 10 knots but a 2-knot current is flooding directly behind you. Your DR, based on 10 knots through the water, will place you behind your real position, because your true speed over the ground is 12 knots. That gap is exactly the error an estimated position corrects, and it is why you re-fix whenever landmarks allow rather than running on DR indefinitely.

Common traps: mixing hours and minutes (keep T in minutes for 60D = ST), confusing yards with nautical miles between the three- and six-minute rules, and forgetting that DR alone cannot account for a current — that is what the set-and-drift problems in the Chart Plot module exist to solve.