Speed Log Requirement for Ships Above 50,000 GT
Speed Log Requirement for Ships Above 50,000 GT: What the Regulation Actually Demands
There is a threshold in the SOLAS regulations that changes everything for speed log requirements. Below 50,000 gross tonnage, the rules are straightforward. Above it, the requirements double. Not just in scope. In hardware.
We design and manufacture our own navigation and communication equipment, including speed logs of various technologies. And we have watched enough vessel operators discover the 50,000 GT rule during a port state control inspection to know that this is one of the most misunderstood requirements in the SOLAS framework.
The Two Regulations That Create the Requirement
The speed log requirements for ships above 50,000 GT come from two separate provisions in SOLAS Chapter V, Regulation 19.
First, SOLAS regulation V/19.2.3.4 requires that all cargo ships of 300 gross tonnage and above, and all passenger ships regardless of size, must be fitted with a speed and distance measuring device capable of measuring speed and distance through the water. This is the baseline requirement that applies to virtually every commercial vessel on international voyages.
Second, SOLAS regulation V/19.2.9.2 adds an additional requirement specifically for ships of 50,000 gross tonnage and upwards. These vessels must be fitted with a speed and distance measuring device capable of measuring speed over the ground in the forward and athwartships direction.
The distinction matters. The first regulation covers speed through water (STW). The second covers speed over ground (SOG) in both longitudinal and transverse axes. A ship above 50,000 GT must provide both types of speed data.
Here is where many operators get caught out. In 2012, the IMO Maritime Safety Committee adopted resolution MSC.334(90), which amended the performance standards for speed and distance measuring devices. The key amendment was the addition of section 5.3 to the existing performance standards: "If ships are required to carry speed logs measuring speed through the water and speed over the ground, these speed logs should be provided by two separate devices".
The IMO then issued MSC.1/Circ.1429 on 28 May 2012, which clarified that on ships requiring both devices-that is, ships of 50,000 GT and above-the requirement should be fulfilled by two separate devices. One speed and distance measuring and indicating device capable of measuring speed through water. And one separate speed and distance measuring and indicating device capable of measuring speed over the ground in the forward and athwartships direction.
This clarification applies to devices installed on ships constructed on or after 1 July 2014. For vessels built before that date, the requirement is not retroactive. But for any vessel above 50,000 GT constructed after 1 July 2014, two separate devices are mandatory.
What "Two Separate Devices" Actually Means
The phrase "two separate devices" sounds straightforward. In practice, it has generated significant debate and some creative engineering solutions.
The IMO's intent is clear: the STW measuring system and the SOG measuring system must be functionally independent. They cannot share critical components in a way that a single failure could disable both measurements. The systems must be separate in terms of hardware, processing, and indication.
Some manufacturers have developed integrated speed logs that achieve compliance with a single physical unit by using completely separated internal processing channels.
But the default interpretation-and the safest approach for compliance-is two physically separate units. One STW log. One SOG log with athwartship capability. Two transducers. Two processors. Two displays. Complete independence.
The Performance Standards Behind the Requirement
Carriage is one thing. Performance is another. SOLAS Regulation 18 requires that all installed navigation equipment must be type-approved in accordance with relevant IMO performance standards.
For speed logs, the key performance standard is IMO Resolution A.824(19), adopted on 23 November 1995. This resolution was amended by MSC.96(72) and further amended by MSC.334(90). The technical standards are implemented through IEC 61023, which specifies the minimum performance requirements, methods of testing, and required test results for marine speed and distance measuring equipment (SDME).
The accuracy requirements are specific. Errors in the measured and indicated speed, when the ship is operating free from shallow water effect and from the effects of wind, sea bottom type, current and tide, should not exceed 2 percent of the speed of the ship or 0.2 knots, whichever is greater. Distance measurements must remain within 2 percent of the distance run in one hour or 0.2 nautical miles, whichever is greater.
The display requirements are equally detailed. Digital speed displays must read in increments no greater than 0.1 knots. Analogue displays should be graduated at least every 0.5 knots and marked with figures at least every 5 knots. Distance readings must cover at least 0 to 9999.9 nautical miles with 0.1-mile increments. All displays must remain readable in both day and night conditions.
The equipment must perform reliably under typical ship motions-up to ±10 degrees of roll and ±5 degrees of pitch-and must function in waters at least 3 metres deep below the keel.
The Athwartships Requirement
The requirement for measuring speed over the ground in the athwartships direction is what truly distinguishes the 50,000 GT rule from the baseline STW requirement.
A standard single-axis speed log measures speed only in the fore-aft direction. It has no capacity to detect lateral transverse movement. For a vessel above 50,000 GT, that is not sufficient. The SOG device must measure speed in both the forward direction and the sideways (athwartships) direction.
This capability is critical for maneuvering large vessels in confined waters. When a 50,000 GT ship is subject to beam winds or cross-currents, it does not travel exactly along its heading. It drifts sideways. The athwartships measurement captures that drift. When the vessel is docking, the bow and stern move in different directions. The transverse speed data makes that visible.
The Practical Reality for Vessel Operators
For vessel operators, the 50,000 GT requirement translates into specific obligations.
If your vessel is above 50,000 GT and was constructed after 1 July 2014, you need two separate speed measuring devices. One for STW. One for SOG with athwartship capability. Both must be type-approved. Both must meet the performance standards of A.824(19) and MSC.334(90). Both must interface with the bridge systems.
If your vessel is above 50,000 GT but was constructed before 1 July 2014, the two-device requirement does not apply retroactively. However, any replacement or upgrade of the speed log system on such a vessel would typically need to meet the current standards.
If your speed log malfunctions, recognized organizations may issue temporary exemptions, but these are typically limited and require repair at the earliest opportunity.
Why the Requirement Exists
The 50,000 GT requirement exists because vessels of this size present unique navigational challenges. They have large turning circles. They respond slowly to helm commands. They have significant momentum that makes stopping and maneuvering difficult. They operate in ports and waterways where precise control is essential.
Speed through water tells the bridge crew how the vessel is moving relative to the water mass-critical for understanding propulsion performance and current effects. Speed over ground tells them how the vessel is moving relative to the earth-critical for position keeping, docking, and collision avoidance. The athwartships component of SOG tells them how much the vessel is being pushed sideways by wind or current.
Without both measurements, the bridge crew is navigating with incomplete information. With both measurements, they have the full picture.
What to Look for When You Buy
If you are outfitting a vessel above 50,000 GT, or upgrading an existing speed log system, here is what you need to verify.
First, confirm that the STW device carries type approval under IEC 61023 and IMO Resolution A.824(19). Second, confirm that the SOG device with athwartship capability carries type approval under the same standards, with specific reference to MSC.334(90). Third, verify that the two devices are functionally separate-not sharing critical components that could create a single point of failure. Fourth, confirm that both devices output data in the correct NMEA format (IEC 61162 series) for integration with radar, ECDIS, AIS, and VDR. Fifth, check that the SOG device actually provides athwartship speed data, not just fore-aft SOG.
Some integrated solutions can meet the requirement with a single physical unit using separated internal systems. If you choose this path, verify that the classification society has explicitly recognized the design as compliant with the "two separate devices" requirement.
Why We Build Our Own
We design speed logs because we have seen too many vessels fail inspection over the 50,000 GT requirement. We have watched operators discover during a port state control inspection that their "compliant" speed log was never actually compliant at all. We have seen integrated systems that claimed to meet the requirement but failed the independent verification because the STW and SOG systems shared too much hardware.
When we build a speed log, we design for the specific requirements of each vessel class. For vessels above 50,000 GT, that means two independent measurement paths-whether in two physical units or in a single integrated unit with fully separated systems. We test against the IEC standards. We verify the NMEA output. We confirm the athwartship measurement. We do this because our customers cannot afford to discover compliance problems after the vessel is already at sea.
The 50,000 GT requirement is not a suggestion. It is not a marketing checkbox. It is the legal minimum for safe navigation on the largest vessels at sea. And the equipment that meets it is the equipment that keeps those vessels moving.
