Module 6 - Position, Course & Speed
Echo Sounders and Speed Logs
An echo sounder measures the time taken for a sound pulse to travel to the seabed and return. The display may be set to show depth below the transducer, depth below the keel, or depth below the waterline. The navigator must know the offset before comparing the reading with the chart or using it as a clearing check.
Depth soundings are an excellent independent cross-check on position, especially when compared with charted depth plus height of tide. They have limitations: aerated water, weed, soft mud, steeply shelving seabed, heel, transducer position, and incorrect sounder offset can all mislead. In shallow pilotage, build in margin and do not wait for the alarm before acting.
A speed or distance log usually measures speed through the water, not speed over the ground. Paddlewheel, electromagnetic, and ultrasonic logs can be affected by fouling, turbulence, calibration error, and heel. The distance log is valuable for DR, but tidal stream and current mean that GPS SOG/COG may be very different from log speed and heading.
Key points
- Echo sounders measure depth by timing a sound pulse to the seabed and back
- Know whether the display is below transducer, below keel, or below waterline
- Compare observed depth with charted depth plus height of tide as an independent position check
- Sounders can be affected by offset errors, aeration, weed, mud, heel, and seabed slope
- Speed logs usually measure speed through the water, not speed over the ground
- Calibrate logs and remember tide/current can make SOG differ from log speed
Continue studying Position, Course & Speed
This topic is part of Module 6. Open the full module for lessons, quizzes, flashcards, and revision tools.