Module 6 - Position, Course & Speed
Speed, Distance, and Time
What you will learn
Choose the correct speed-distance-time relationship, convert minutes to hours, and turn the answer into a usable arrival or monitoring decision.
One knot is one nautical mile per hour. Navigation calculations work only when distance, speed and time refer to compatible quantities and the time unit is converted correctly.
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.
Worked example
A measured chart leg is 4.3 M and the planned speed over ground is 5 kn.
- 1Time in hours = 4.3 ÷ 5 = 0.86 hours.
- 2Time in minutes = 0.86 × 60 = 51.6 minutes.
- 3Round suitably for the plan: about 52 minutes, then monitor actual progress.
Sense check: At 5 kn the vessel covers 5 M in one hour, so 4.3 M must take a little less than an hour.
Predict and monitor a short leg
A plotted ground-track leg is 7.2 M. Your conservative planned speed over ground is 4.8 kn and the leg begins at 1315.
1. Calculate hours
Time = distance ÷ speed = 7.2 ÷ 4.8 = 1.5 hours.
2. Convert the fraction
0.5 hour × 60 = 30 minutes, so the leg should take 1 hour 30 minutes.
3. Create an ETA
1315 + 1 hour 30 minutes = 1445, subject to the assumed 4.8 kn SOG remaining realistic.
4. Set a review
At an early fix, compare distance made good and conditions with the plan and revise the ETA if the assumption is wrong.
Sense check: At just under 5 kn, travelling a little over 7 M should take about an hour and a half, not 15 minutes or three hours.
Common mistake or limitation
- Putting minutes directly into a formula that expects hours, so a 30-minute interval is treated as 30 hours instead of 0.5 hours.
- Mixing speed through the water with a ground-track distance without accounting for current.
- Reporting an ETA to the minute without monitoring whether the speed assumption remains valid.
Recap
- One knot is one nautical mile per hour.
- Use S = D ÷ T, D = S × T or T = D ÷ S with compatible units.
- For minutes, D = S × T ÷ 60.
- State whether speed and distance are through water or over ground, then monitor the assumption.
Optional quick check
Section 3 of 6
At 6 kn, how far should a vessel travel in 45 minutes?
Sources and factual review
Continue studying Position, Course & Speed
This public lesson is part of Revision Module 6. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.