Beyond Didymi: navigating the challenges towards a more continuous performance assessment of ships

In the vast realm of commercial shipping, assessing a vessel's performance is a multifaceted challenge that extends beyond the confines of calm seas and clear skies. Weather effects play a pivotal role in determining how a ship performs under various conditions, significantly impacting consumption, efficiency, and overall operational success.

In this blog, we will delve into the complexities associated with weather effects on commercial ship performance and explore the merits and drawbacks of using the traditional Didymi and Gas Enterprise method* ( "Didymi") for assessment, and the benefits of using a new approach.

The importance of weather in ship performance assessments

Commercial ships operate in diverse weather conditions, ranging from serene days to tumultuous storms. The interaction between a vessel and its environment is a delicate dance influenced by wind, waves, currents, and temperature. Understanding how these elements affect ship performance is crucial for maintaining operational excellence.

The Didymi method is a historical benchmark used for assessing a vessel's performance based on a 'good weather' approach. While this method provides a standardized framework for evaluation, it has its limitations. The key issues with The Didymi Method are:

1. Limited Realism:

  The Didymi method relies on data obtained in relatively calm weather conditions. This limitation hampers the method's applicability as (depending on seasonality and agreed weather limits) up to 80% of the voyages may be classified as insufficient good weather to make any assessment.

2. Subject to (intentional) misreporting:

  A vessel performance is only assessed during good weather days. As this is known to the vessel, they may intentionally report in line with the charter party on these days. Any overconsumption may be reported on bad weather days when no warranty is applicable.

3. Not taking into account the direction of weather:

The Didymi method relies on a strict threshold for weather, such as Beaufort 4 or sea state 3. Besides the fact that these limits are often ill-defined, the direction of weather relative to the heading of a vessel is not taken into account at all while thay have a significant impact on performance.

Recommendations for improved ship performance assessment

The last years has seen an increased access to reliable satellite data, improved AIS coverage and reliable weather data. Combined with the use of dynamic simulation models, performance assessments that incorporate real-time data to assess a ship's performance are becoming a reality.

This approach offers a far more realistic representation of how vessels perform in dynamic environments. Furthermore, the risk of (intentional) misreporting on speed, weather or consumption is taken out, as all sailing days are considered, and human input is reduced.

Charterers should strongly consider using these new methods when evaluating ship performance. It also allows them to benchmark reported weather and vessel speed with satellite observations. This approach reduces errors and provides a more comprehensive understanding of a vessel's capabilities and limitations.

Besides tracking true ship's performance in real-time, these insights allow for adaptive strategies, enabling operators and crews to make informed decisions based on the evolving weather conditions.


As the maritime industry navigates an era of increasing climate unpredictability, the assessment of commercial ship performance must evolve to meet the future requirements.

While the Didymi method has historical significance, its limitations highlight the need for a more dynamic and comprehensive approach. By embracing modern technologies and considering a broader range of factors, the shipping industry can enhance safety, efficiency, and sustainability in the face of an ever-changing weather landscape.

*  The Didymi [1987] 2 Lloyd’s Rep. 166 and The Gas Enterprise [1993] 2 Lloyd’s Rep. 352