Module 6 - Position, Course & Speed
Echo Sounders and Speed Logs
What you will learn
Translate sounder and log displays into the correct physical reference, compare them with an independent expectation and respond to disagreement before safety margin is consumed.
An instrument number is useful only when the skipper knows where it is measured from, how it is calibrated and which environmental effects sit outside it.
An echo sounder times a pulse to the seabed and back. The displayed value may be configured as depth below the transducer, below the keel or below the waterline. Record the transducer position and any offset before comparing the reading with charted depth plus height of tide. The charted sounding, tide estimate and sounder may also use different reference levels.
Aeration, heel, weed, soft mud, a steep seabed, transducer installation and inappropriate gain or range can degrade the return. A shallow-water alarm is a warning layer, not evidence that water is safe until it sounds. Compare trend and expected profile, retain margin and act on an unexplained discrepancy.
A yacht log commonly measures speed through water. Fouling, turbulence, heel and calibration affect it. GNSS SOG measures movement over the seabed and includes current. The difference can help expose set and drift, but only after heading, leeway, instrument health and time basis are checked.
Worked example
The charted depth is 2.4 m, estimated height of tide is 1.7 m and the sounder transducer is 0.4 m below the waterline. The display is configured below transducer.
- 1Estimated waterline depth = 2.4 + 1.7 = 4.1 m.
- 2Expected below-transducer reading = 4.1 − 0.4 = 3.7 m.
- 3The instrument shows 3.0 m, a 0.7 m disagreement with that expectation.
- 4Do not manufacture a clearance answer: verify tide/source/time, offset, location and return quality, slow or stop as appropriate and seek independent position/depth evidence.
Sense check: A below-transducer value should be smaller than waterline depth by the transducer depth; adding 0.4 m would use the offset in the wrong direction.
| Display | Reference | Before comparison |
|---|---|---|
| Below transducer | Transducer face | Add transducer depth for waterline reference |
| Below keel | Keel bottom | Confirm configured keel offset |
| Below waterline | Water surface | Confirm no duplicate manual offset |
| Log STW | Movement through water | Check calibration, fouling and heel |
| GNSS SOG | Movement over seabed | Check position integrity and time window |
Use a disagreement as an early warning
The planned depth profile should deepen steadily, but the sounder trend is flattening while GNSS still places the yacht on track.
1. Protect the vessel
Reduce risk immediately according to sea room, speed and the passage plan rather than waiting for one instrument to win the argument.
2. Check the reference chain
Review sounder offset and quality, height-of-tide source/time, chart position, charted depth and GNSS integrity.
3. Find independent evidence
Use a visual or radar position, transit, lead line where appropriate, or a controlled move back towards known safe water.
Sense check: Depth disagreement beside danger is a navigation decision, not a reason to average two numbers.
Common mistake or limitation
- Comparing below-keel or below-transducer display with waterline-based chart depth without converting references.
- Treating a smooth digital reading as proof that the transducer return is valid.
- Calling log speed inaccurate merely because tidal current makes SOG different.
Recap
- Reference level comes before interpreting depth.
- Expected depth is a chain of chart, tide, position and instrument assumptions.
- Log STW and GNSS SOG answer different questions.
- Unexplained disagreement should increase caution and trigger another check.
Optional quick check
Section 4 of 10
Expected below-transducer depth is 3.7 m but the display holds at 3.0 m beside danger. What is the safest response?
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.