Speed Log for Voyage Data Recorder: Why Your Choice of Sensor Matters More Than You Think

There is a quiet assumption that runs through most VDR discussions. People talk about the protective capsule, the float-free memory, the playback software. They worry about audio channels and radar image compression. But almost nobody talks about the speed log feeding data into the black box in the first place.

That is a mistake. And it is a mistake that usually gets discovered during the first port state control inspection after a VDR performance test fails.

We design and manufacture speed logs-Doppler, electromagnetic, and satellite-based-alongside our other navigation and underwater detection equipment. We have seen the inside of enough VDR installations to know that the speed log interface is where many otherwise solid systems develop weak points. Not because the VDR is faulty. Because the speed log feeding it was never designed with VDR integration in mind.

What the VDR Actually Needs from a Speed Log

Open IMO Resolution MSC.333(90), the revised performance standards for VDRs adopted in May 2012 and applicable to installations on or after 1 July 2014. Section 5.5.3 is short and direct: "Speed through the water and speed over the ground, including an indication of which it is, derived from the ship's speed and distance measuring equipment, as required by SOLAS regulations".

That is it. Two numbers. One indicator telling the playback system which number is which.

But the simplicity of that requirement hides a more complex reality. The VDR does not care whether your speed log uses Doppler, electromagnetic, or satellite principles. It cares about three things: accuracy, format, and reliability under failure conditions.

The accuracy requirements trace back to IEC 61023, the performance standard for speed and distance measuring equipment. Speed must be accurate within 2 percent of the vessel's speed or 0.2 knots, whichever is greater. Distance measurements must stay within 2 percent of the distance run in one hour or 0.2 nautical miles, whichever is greater. Those numbers are not suggestions. They are the floor.

The Interface That Makes or Breaks the Connection

Here is where the technical details start to matter. The VDR receives speed data through serial interfaces defined by the IEC 61162 series-the international standard that governs NMEA 0183 communication for marine navigation equipment.

The specific NMEA sentences that carry speed information are VBW (Dual Ground/Water Speed) and VLW (Distance Traveled through Water). VBW carries both longitudinal and transverse speed components, along with status indicators that tell the receiving device whether the data is valid. VLW carries cumulative and trip distance totals.

A speed log that outputs these sentences in the correct format at the correct baud rate-typically 4800 baud for IEC 61162-1-will talk to the VDR. A speed log that uses proprietary sentences, or that fails to include the status flags the VDR expects, will cause the VDR to record garbage or nothing at all.

We have seen installations where the speed log output worked perfectly for the radar and the AIS but failed the VDR interface test because the manufacturer had never tested the VBW sentence structure against the VDR's specific parser. The data looked right on a scope. The VDR rejected it anyway.

 

Three Technologies, Three Different Considerations

The choice of speed log technology affects VDR integration in ways that are not always obvious.

Doppler speed logs measure speed through water by transmitting ultrasonic pulses at an angle to the keel and measuring the frequency shift of the return signal reflected off particles in the water column. They are accurate down to very low speeds, which makes them valuable for maneuvering and docking. For VDR purposes, Doppler logs typically output VBW and VLW sentences over IEC 61162-1 interfaces. The key consideration is whether the log provides both longitudinal and transverse speed components-some single-axis units only output fore-aft speed, which limits the VDR's ability to reconstruct the vessel's full motion during an incident.

Electromagnetic speed logs measure speed by generating a magnetic field through the water and sensing the voltage induced by the water's movement through that field. They have no moving parts and require minimal maintenance. Units like the Wärtsilä SAM 4642 provide dual-axis measurement and relay speed information to radar, AIS, and VDR via NMEA-format connections that comply with IEC 61162 specifications. The electromagnetic principle gives stable readings but can be affected by fouling or changes in water conductivity. For VDR integration, the critical factor is whether the log's NMEA output includes the status flags that tell the VDR when the data is suspect.

Satellite speed logs measure speed over ground using GNSS signals. They provide speed over ground (SOG) rather than speed through water (STW), which is the key distinction. SOLAS requires speed through water for the primary speed log on most vessels. Satellite logs are useful as secondary sources or for vessels above 50,000 GT that require separate SOG measurement. They output standard NMEA sentences like VTG (Course Over Ground and Ground Speed). The integration challenge with satellite logs is that they depend on GNSS  availability-if the satellite signal drops, the VDR loses speed data entirely.

 

What the VDR Does with the Speed Data

The VDR does not just store speed numbers in isolation. It correlates speed data with position, heading, radar images, ECDIS display signals, audio, and alarm status to reconstruct the complete picture of what happened before, during, and after an incident.

Speed through water tells investigators how the vessel was moving relative to the water mass-critical for understanding propulsion and maneuvering performance. Speed over ground tells them how the vessel was moving relative to the earth-critical for understanding drift, current effects, and position accuracy.

If the speed log feeding the VDR drops out, or if it provides STW when the VDR expects SOG, or if the status flags are missing, the playback loses resolution. The investigation becomes harder. The questions become unanswerable.

The Practical Checklist for VDR-Ready Speed Logs

When you are selecting a speed log for a vessel with a VDR-and that means virtually any passenger ship or cargo ship of 3,000 GT and above-here is what you need to verify:

First, confirm that the speed log carries type approval under IEC 61023 and the relevant IMO resolutions, including A.824(19) and A.694(17). This is the baseline for accuracy and environmental performance.

Second, verify the NMEA output. The log must produce VBW and VLW sentences in IEC 61162-1 or IEC 61162-2 format. Check that the status flags in the VBW sentence are correctly populated. Some logs output the sentences but leave the status fields blank, which causes the VDR to treat the data as invalid.

Third, confirm that the log provides both speed through water and, where required, speed over ground. For vessels above 50,000 GT constructed after 1 July 2014, separate devices are required for each measurement.

Fourth, test the interface under real conditions. We have seen speed logs that passed the bench test but failed when connected to the actual VDR because of grounding issues, baud rate mismatches, or electrical noise on the serial line. The interface standard is only half the story. The implementation is the other half.

 

Why We Build Our Own

We design Doppler logs, electromagnetic logs, and satellite-based speed measuring equipment because we have seen too many vessels where the speed log and the VDR simply do not get along. Not because either device is defective. Because nobody ever tested them together before installation.

When we build a speed log, we test it against VDR interfaces. We verify the VBW and VLW sentences. We check the status flags. We run the unit through the environmental tests required by IEC 61023 and IEC 60945. We do this because our customers do not have time to discover integration problems after the vessel is already at sea.

The VDR is only as good as the data it records. And the speed log is one of the most critical data sources on that list. Choose accordingly. The investigation after an incident will depend on it.