Echo Sounders and Speed Logs

Echo sounder: how it measures depth

In short: Read the bridge equipment carriage rules first. An echo sounder sends a sound pulse down from the hull and times the echo from the seabed. Depth = speed of sound × time ÷ 2.

The echo ranging principle

It is the same idea as radar, but with sound in water instead of radio waves in air.

1. The transmitter sends a short pulse. Radar uses radio waves at the speed of light (300,000,000 m/s); the echo sounder uses sound at about 1,500 m/s in sea water.
2. The pulse weakens (attenuates) as it travels.
3. It strikes a target: the seabed (or a fish shoal, a wreck, a layer of different water).
4. The echo comes back to the transducer, which now works as the receiver.
5. The echo is shown on the display.
6. The pulse went down and back, so: range = speed × time ÷ 2.

• Example: the echo returns after 0.02 s. 1,500 × 0.02 ÷ 2 = 15 m below the transducer.

Key components and where the transducer goes

• Transmitter: makes the electrical pulse, at a set pulse repetition rate.
• Transducer: in the hull. It turns the electrical pulse into sound, then turns the returning echo back into a tiny electrical signal.
• Receiver / amplifier: boosts the weak echo. The gain control sets how much.
• Display / recorder: shows the depth now and a record of past soundings, so you see the seabed profile.
• Timing / control unit: measures the time between pulse and echo and converts it to depth.

Transducer location:

• In the forward part of the ship, about one third of the length from the bow, close to the centreline.
• Away from interference: turbulence and aerated water, electrical noise, other hull openings and projections.
• MCA guidance: echo sounder and log transducers should be sited away from underwater openings and hull projections where practicable (MGN 610 (M+F), guidance to SOLAS V/19, para 25.6).

Carriage and the IMO standard

• Carriage: an echo-sounding device on ships of 300 GT and over and on all passenger ships (SOLAS V/19.2.3).
• It is also a standard ECDIS input (depth), with gyro heading, log speed and GNSS position.
• The original IMO performance standard, IMO A.224(VII) (1971), asks for:
  • depths from 2 m to 400 m under the transducer;
  • at least two range scales: a deep scale covering the whole range and a shallow scale of one tenth of it;
  • a graphical display giving the depth now and a visible record: at least 15 min of soundings on the deep scale;
  • a pulse repetition rate of at least 12 pulses per minute;
  • accuracy, based on 1,500 m/s: ±1 m (shallow scale) or ±5 m (deep scale), or ±5 % of the depth, whichever is greater;
  • full performance while rolling ±10° and pitching ±5°.
• Echo sounders installed on or after 1 January 2001 must meet the amended standard in IMO MSC.74(69) Annex 4.

Setting up the echo sounder

In short: Pick the frequency for the depth (200 kHz shallow, 50 kHz deep), set the datum (below keel or below surface), the range scale, the gain and a shallow-depth alarm.

Frequency: shallow or deep water

• Memory hook: high frequency = high detail, short reach; low frequency = long reach.

Datum, gain, range and alarms

• Datum: the transducer sits some way below the waterline, so the raw reading is depth below the transducer. Enter the offsets so the display shows what you want:
  • depth below keel = under-keel clearance (UKC), what the OOW usually watches;
  • depth below surface = transducer depth + draught offset, the one to compare with the chart.
  • Always check which datum is on the screen before you use the number.
• Range scale: choose one that keeps the bottom on the display. Use the shallow scale in pilotage waters.
• Gain: just enough for a clear, steady bottom trace.
  • Too much gain → noise, multiple echoes, false echoes from fish or water layers.
  • Too little → a weak echo (deep water, soft mud) is lost.
• Shallow-depth (range) alarm: set it to the minimum UKC in the passage plan, so the sounder warns you before the ship gets there.
• On the bridge: switch the sounder on and check it before departure and before entering shallow or confined waters.

Echo sounder errors and checks

In short: The sounder assumes sound travels at 1,500 m/s and that the first good echo is the seabed. Water density, bubbles, shallow water, multiple and second-trace echoes break those assumptions. Always cross-check with the chart and the tide.

The errors

• Squat: in shallow water at speed the ship sinks and trims, so UKC shrinks. The sounder shows clearance only under the transducer; the lowest point of the hull may be elsewhere.

Checking the sounder on the bridge

• Lead line: alongside or at anchor, compare the sounder with a hand lead. You must know the sounder is right before you use depth for navigation.
• Chart + tide: expected depth below the surface = charted depth + height of tide (see tidal calculations); expected UKC = that minus the draught. A big difference means a sounder fault, a wrong datum, or a wrong position.
• Position line from a depth contour: a bearing or transit crossed with a charted depth contour gives a position line fix. It only works with a checked sounder and the height of tide worked out.
• Standing by a vessel aground: switch the echo sounder on and note the UKC before you close her.

Speed logs: through the water or over the ground

In short: A log measures speed and distance. STW (speed through the water) is the ship’s speed relative to the water around it; SOG (speed over the ground) includes the tidal stream and current. SOLAS wants STW from 300 GT, and SOG fore-and-aft and athwartships too from 50,000 GT.

Carriage (SOLAS V/19)

• Ships of 300 GT and over and all passenger ships: a speed and distance measuring device for speed through the water (SOLAS V/19.2.3.4).
• Ships of 50,000 GT and over: in addition, a device for speed over the ground in the forward and athwartships directions (SOLAS V/19.2.9.2).
• These must be two separate devices, one for STW and one for SOG (IMO MSC.1/Circ.1429). The performance standard was amended to match by IMO MSC.334(90), for ships constructed on or after 1 July 2014.
• Which log reads what:
  • Impeller and electromagnetic logs: STW only.
  • Doppler log: SOG (bottom track) or STW (water track).
  • GNSS also gives SOG, but it is a position sensor, not a log: see GNSS errors.

What the IMO standard asks

From IMO A.824(19) (devices installed from 1 January 1997; amended by IMO MSC.96(72) for devices installed from 1 July 2002):

• Full performance up to the ship’s maximum speed and in water more than 3 m under the keel.
• Speed error: not more than 2 % of the ship’s speed or 0.2 kn, whichever is greater.
• Distance error: not more than 2 % of the distance run in 1 h or 0.2 NM per hour, whichever is greater.
• Digital speed in steps of 0.1 kn; distance from 0 to at least 9,999.9 NM, in steps of 0.1 NM.
• If it can work in STW or SOG mode, the mode must be shown. Any non-forward speed must show its direction clearly.
• Works while rolling ±10° and pitching ±5°.
• Hull fittings must not let water in, even if damaged. A part that extends from the hull must show on the display whether it is extended or retracted.
• The handbook must explain effects of sea state, water temperature, salinity, sound velocity, depth under the keel, heel and trim.

Impeller and electromagnetic logs

In short: Both measure speed through the water at the hull. The impeller counts the turns of a small wheel; the electromagnetic (EM) log measures the voltage made by water moving through a magnetic field.

Impeller log

• A small impeller (paddle wheel) sticks out from the hull in a retractable housing and turns with the water flow.
• A pick-up sensor counts each turn, using a magnet on the impeller.
• Each turn equals a known distance (from the blade pitch). Turns counted = distance; distance ÷ time = speed.

Electromagnetic (EM) log

• Works on Faraday’s law: a coil in the sensor makes a magnetic field in the water. Sea water is a conductor; as it flows past, it makes a small voltage.
• The voltage is directly proportional (linear) to the speed of the water past the sensor → STW.
• No moving parts.

Doppler log

In short: The Doppler log sends sound at a known frequency and measures the frequency shift of the echo. The shift is proportional to speed. Locked on the seabed it gives SOG; locked on the water it gives STW.

The Doppler shift principle

• The transducer sends an acoustic pulse at a known frequency, angled forward and down.
• The echo comes back from the seabed or from particles in the water.
• Ship moving towards the reflector → echo at a higher frequency; moving away → lower.
• The change of frequency is proportional to speed: shift ≈ 2 × transmitted frequency × speed × cos(beam angle) ÷ speed of sound.
• Bottom track (shallow water): echo from the seabed → SOG.
• Water track (deep water, seabed out of reach): echo from a layer of water below the hull → STW.
• The display tells you which mode it is in. Always check it.

Janus configuration

• Named after the Roman god with two faces: beams point forward and aft (and on dual-axis sets, port and starboard too).
• Comparing the forward and aft beams cancels most of the error from the ship pitching (vertical movement) and from trim.
• The side beams measure sideways (athwartships) movement: useful when berthing a big ship, and required for SOG at 50,000 GT.

Advantages, disadvantages and errors

Logs, radar and ARPA

In short: The radar’s true vectors and target aspect depend on the speed input. For collision avoidance in open water use sea stabilisation (STW); ground stabilisation (SOG) shows a target’s aspect differently from what you see out of the window (MGN 379 (M+F), para 3.6.1).

Sea or ground stabilised

• A sea-stabilised radar takes STW from the water log. UK guidance: sea stabilisation is the preferred mode in the open sea and for anti-collision decisions (MGN 379 (M+F), para 3.6.1).
• In ground stabilisation (SOG from a Doppler bottom track or GNSS), the set and drift of the tide or current is applied to targets too. Their aspect on the radar is then different from the visual aspect, which can lead to the wrong action under the COLREG (MGN 379 (M+F), para 3.6.1). Ground stabilisation is useful for navigation: it shows your own set and drift clearly.
• Inaccurate heading or speed input reduces the accuracy of targets’ true vectors (MGN 379 (M+F), para 3.3). A wrong manual speed setting spoils all ARPA/ATA data (para 3.2).
• Log failure: the radar can then only use SOG (para 3.9).
• Know which sensor feeds the radar (log, gyro) before you rely on it (para 3.4.1).
• More in collision avoidance with radar.

Checking the log

• Compare log speed with GNSS SOG in slack water, or allow for the tidal stream you know.
• Measured mile: run a known distance at constant power, in both directions, and average the results to cancel the tide. The log error = (log distance − true distance) ÷ true distance × 100 %.
• Impeller and EM logs need regular calibration; an impeller that suddenly under-reads is often fouled.
• Never rely on one sensor: cross-check (MGN 379 (M+F)).