Chapter 22: Calculations and Conversions
Most navigational blunders are arithmetic or unit blunders. Mastering the speed-time-distance family and the conversion factors eliminates a large class of errors.
Speed, time and distance
D = S × T with speed in knots and time in hours. The practical form, 60 D = S × T with time in minutes, is the '60 D Street' rule and is the fastest mental tool on the bridge. ETA problems are simply the same relation rearranged; always carry current and expected speed loss in the estimate rather than arriving at a precise but wrong ETA.
Units and conversions
One international nautical mile is 1852 m exactly, equal to 6076.1 ft, while a statute mile is 5280 ft; a knot is one nautical mile per hour. A cable is one tenth of a nautical mile, a fathom six feet. Useful approximations: 1 m/s ≈ 1.94 kn, 1 kn ≈ 1.15 mph, and 3 minutes of run at any speed in knots gives distance in hundreds of yards equal to speed (the three-minute rule: distance in yards = speed × 100).
- Three-minute rule: yards run = knots × 100
- Six-minute rule: NM run = knots ÷ 10
- 1 NM = 1852 m = 6076 ft = 1.15 statute miles
Formula cards
Speed-time-distance
D = S × T | 60 D = S × T(minutes)
Consistent units are everything.
Six-minute rule
Distance (NM) in 6 min = speed (kn) ÷ 10
12 knots gives 1.2 NM in six minutes.
Key takeaways
- • 60 D = S × T when time is in minutes.
- • Three-minute rule and six-minute rule for instant mental estimates.
- • 1 nautical mile = 1852 m, by definition, not by measurement.
Knowledge check
Immediate feedback with rationales. Answer each question to see the explanation.
Chapter 22 quiz
0 of 3 answered
1.At 18 knots, how far does the vessel run in 20 minutes?
2.A vessel makes 12 knots. Using the three-minute rule, distance run in three minutes is:
3.One nautical mile equals: