Speed, Distance, and Time

The international nautical mile is exactly 1,852 metres, and one knot is one nautical mile per hour. Distance is commonly written M or NM and speed kn or kt; follow the notation used by the chart, publication or exercise and do not say 'knots per hour'.

The three relationships are speed = distance ÷ time, distance = speed × time and time = distance ÷ speed. If time is in minutes, convert it to hours before using knots, or use distance = speed × minutes ÷ 60. This is the origin of the quick form 60D = ST when D is nautical miles, S knots and T minutes.

Check that the quantities describe the same movement. Speed through the water and distance through the water belong in a DR calculation; speed over ground and distance over the ground belong in an ETA calculation. Mixing them silently can produce a tidy answer to the wrong problem.

An ETA is a forecast, not a promise. Compare actual fixes, log readings, ground speed and conditions with the calculation, then update the ETA before a tidal gate, daylight limit or harbour closing time becomes critical.