A primer on basic Dead Reckoning Calculations

Who should read this and do the exercises?

This short article is aimed at folks who want to excel at DR but thought they had left the dreaded Algebra far behind in their lives.

Engineers, draftsman, programmers and the mathematically inclined need not go farther. This is, however, essential reading for most English majors and others who have studied the intellectually challenging (for engineers, draftsman, programmers and the mathematically inclined ) Liberal Arts.

Distance equals Speed times Time

Fair enough: We're assuming that everyone can figure out that if you travel 10 mph for 2 hours you'll cover 20 miles. If you're on a boat, it's knots instead of mph, but the calculation is the same.

The problems start to arise among English majors when the numbers are more like traveling 5.4 knots for an hour and 34 minutes.

You know where this is going: we've got to convert, get out the calculator, and ... oh, this is going to get messy.

Fear not.

We have a very simple tool for taking care of most of the nasty algebra stuff by converting the problem to minutes. All you have to remember is this address: 60 D Street

60 X Distance (Nautical Miles) = Speed (knots) X Time (minutes)

You cruise for an hour and 34 minutes (1+34) at 5.4 knots. How far will you go?

Convert the time to minutes: 1 hour (60 min) + 34 = 94 minutes

60D = Speed (5.4) X Time (94)
60D = 507.6
D =507.6/60 = 8.46nm

After 2+15 underway, you get a fix and find you have traveled 12.7 nm. What was your speed over the ground (SOG)?

Convert the time to minutes: 2 hours (120 min) + 15 = 135 minutes

60D = Speed X Time (135)
60 X12.7 = Speed X 135
762 = Speed X 135
762/135 = Speed
5.64 knots = Speed

Your GPS shows 5.7 SOG, how long will it take to go 24 miles to Prisoners Harbor?

60D = Speed X Time
60 X24 = Speed X Time
1440 = Speed X Time
1440 = 5.7 X Time
1440/ 5.7 = Time
266 min = Time

Convert the time to hours: 266 minutes = 240 (4 hours) + 26 min = 4+26

This calculation facilitates figuring out ETA - estimated time of arrival for instance, if you'd left at 0800, you'd expect to arrive at 1226.

DR Exercises

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