Module 6 - Position, Course & Speed
Speed, Distance, and Time
What you will learn
Use compatible speed, distance and time units to calculate an ETA or distance made good, then monitor the assumption before it becomes a tidal or daylight failure.
One knot is one nautical mile per hour. The arithmetic is simple; the coastal-skipper skill is choosing a speed and distance that describe the same movement and revising the result when reality differs.
The international nautical mile is exactly 1,852 metres and one knot is one nautical mile per hour. The relationships are speed = distance ÷ time, distance = speed × time and time = distance ÷ speed. When time is in minutes, convert to hours or use distance = speed × minutes ÷ 60, giving the quick form 60D = ST.
The quantities must use the same frame. Speed through water belongs with through-water distance in a DR. Speed over ground belongs with ground distance for ETA. A log, GNSS and route plan can each be correct while describing different movement.
An ETA is conditional. State the assumed speed, rounding and starting time, then choose a monitoring point. A precise answer that is not updated after adverse tide, reduced speed, avoidance action or sea state is less useful than a rounded estimate with an early review.
Worked example
A planned ground-track leg is 7.8 M. Conservative planned SOG is 5.2 kn and the leg starts at 1340.
- 1Time = 7.8 ÷ 5.2 = 1.5 hours.
- 20.5 hour × 60 = 30 minutes, so passage time is 1 hour 30 minutes.
- 31340 + 1 hour 30 minutes = 1510 ETA.
- 4Set an early fix or distance-made-good review; revise 1510 if 5.2 kn SOG is not being achieved.
Sense check: At about 5 kn, a little under 8 M should take about an hour and a half, not 15 minutes or three hours.
| Need | Relationship | Unit check |
|---|---|---|
| Speed | S = D ÷ T | M divided by hours gives kn |
| Distance | D = S × T | kn multiplied by hours gives M |
| Time | T = D ÷ S | M divided by kn gives hours |
| Distance with minutes | D = S × minutes ÷ 60 | Convert the clock interval once |
Protect a tidal gate from false precision
The first calculation gives 1510, only 25 minutes before a conservative latest-arrival time, but the first monitored SOG is 4.4 kn.
1. Recalculate honestly
Use the remaining ground distance and the observed conservative SOG rather than preserving the original ETA.
2. Compare with the gate
Include the required contingency, not just the mathematical arrival time.
3. Act early
Use the planned alternate, delay or turn-away before the narrowing margin removes choice.
Sense check: An ETA is a decision input; if it no longer protects the gate, changing only the displayed time is not enough.
Common mistake or limitation
- Putting minutes directly into a formula that expects hours.
- Using speed through water with a distance over the ground and silently ignoring current.
- Reporting an exact ETA without a review point or contingency margin.
Recap
- Use compatible units and movement frames.
- Show the conversion when time is in minutes.
- State the speed assumption and sensible rounding.
- Monitor and revise before a gate or margin becomes critical.
Optional quick check
Section 3 of 10
At 6 kn, how far should a vessel travel in 45 minutes?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
- RYA Yachtmaster Offshore examination scope
- RYA Coastal Skipper/Yachtmaster Offshore Theory Course
- MCA MGN 610 Amendment 1 — navigation and SOLAS Chapter V
- MCA MGN 379 Amendment 1 — electronic navigational aids
- IHO S-4 chart specifications, edition 4.10.0
- BIPM SI Brochure, ninth edition
- USCG Boat Crew Handbook — Navigation and Piloting
- GPS Standard Positioning Service Performance Standard, fifth edition
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.