Speed, time and distance

How it works

The three quantities are tied by one formula: distance = speed × time. Distance is in nautical miles (NM), speed in knots (one knot is one nautical mile per hour), time in hours. Knowing two, you find the third: speed = distance / time, time = distance / speed. We never say 'knots per hour': the knot already contains the hour.

The step that causes mistakes is time. An hour has 60 minutes, not 100. To use the formula, time must be in decimal hours: divide the minutes by 60. 1 h 45 min = 1 + 45/60 = 1.75 h (not 1.45). To convert back, multiply the decimal part by 60: 2.8 h = 2 h and 0.8 × 60 = 48 min, that is 2 h 48 min. Example: 14 NM at 5 knots takes 14/5 = 2.8 h, that is 2 h 48 min.

For short times there is the '60 rule': miles = knots × minutes / 60. It is the same formula with minutes instead of hours, without going through decimals. At 6 knots for 20 minutes: 6 × 20 / 60 = 2 NM. At 7 knots for 25 minutes: 7 × 25 / 60 = 2.9 NM. Conversely, the minutes needed for one mile are 60 / knots: at 6 knots it takes 10 minutes, at 4 knots 15.

A handy case is the six-minute rule: in 6 minutes the distance in tenths of a mile equals the speed in knots. At 5.5 knots, in 6 minutes you cover 0.55 NM. It helps when you plot your position at short intervals (every 6, 12, 30 minutes) without a calculator.

For passage planning and the arrival time (ETA): time = distance / speed, then add it to the departure time. From a position 18 NM from port at 6 knots: 18/6 = 3 h. If it is noon, you arrive at 15:00. Remember that the log measures speed and distance through the water: if there is a tidal stream, the distance over the ground (and so the arrival time) is different. You solve that case with the course-to-steer procedure.