Speed Log

What sets a speed log apart from the rest of the navigation suite

A speed log measures the ship's own speed and, on some types, distance run, feeding that into ECDIS, radar for true motion and the automatic identification system for calculated course over ground. It is a different job from the echo sounder, which measures water depth under the keel, and from the voyage data recorder, which only records what other instruments already report. Within speed logs there are two working principles that give genuinely different information: an electromagnetic, or EM, log measures speed through the water only, by sensing the voltage induced as seawater flows past an electrode; a Doppler log measures speed relative to the seabed when in bottom-track range, switching to water-track, effectively the same measurement as an EM log, once the seabed is too deep to return a usable signal.

Main components

EM log sensor

A retractable probe fitted through the hull carries a pair of electrodes exposed to the flow of water past the hull. A hull valve lets the probe be withdrawn for cleaning or before dry-docking without flooding the compartment.

Doppler log transducer

A transducer, usually with two or four angled beams, is mounted flush in the hull and transmits acoustic pulses toward the seabed. The frequency shift in the returned echo gives speed over ground; a second, near-vertical beam pair can add athwartship speed for docking use.

Processing and display unit

The electronics unit converts the raw sensor signal into speed and, where fitted, integrates it over time to give distance run, then distributes the data over serial or network interfaces to ECDIS, radar, the VDR and the engine control system.

Selection and sizing

• Bottom-track depth range for a Doppler log, since a unit with a shallow maximum range reverts to water-track far from what the officer expects
• Single-axis versus dual-axis output, dual-axis units add athwartship speed and are increasingly specified for docking assistance on larger ships
• Sensor draft and hull location, chosen to stay clear of bow thruster wash and hull vibration sources that degrade the signal

Regulations and class

SOLAS Chapter V regulation 19 requires ships of a given gross tonnage to carry a speed and distance measuring device, and the equipment must meet the IMO performance standard for such devices, which sets a maximum permissible error, commonly summarised as within about two per cent of speed or 0.2 knots, whichever is greater. Approved units carry type approval certification referencing that performance standard, checked at survey alongside the rest of the navigation equipment fit.

Typical faults

• EM sensor fouled by marine growth or paint overspray, producing a speed reading that drifts low without an obvious alarm
• Doppler log losing bottom lock in deep water and silently switching to water-track, so speed over ground readings quietly become speed through water
• Retractable EM probe seized in the extended position, forcing a diver or dry-dock intervention before the hull valve can be closed safely
• Calibration drift after hull cleaning or propeller change, since the log's speed correction factor is set against the ship's actual hull condition

What to look for in a supplier

• Type approval certificate referencing the current IMO performance standard for speed and distance measuring equipment
• Clear indication on the display of which mode, bottom-track or water-track, is currently active for Doppler units
• Interface support for the ship's actual ECDIS and VDR models without a protocol converter
• Availability of a diver-serviceable or retractable sensor housing suited to the ship's dry-docking interval

Before trusting a Doppler log's speed over ground figure in coastal waters, check the display is actually in bottom-track mode; a silent fallback to water-track has put more than one ship's dead reckoning out by the set and drift of the current.