Introduction
As we noted in our previous issue of the Skuld Charterer[1], an increasing number of dry cargo charterparties feature ever more restrictive good weather definitions in owners’ favour[2]. The actual conditions experienced during a voyage often fail to meet the prescribed thresholds[3] or create difficulties for WRCs to identify contractual ‘good weather'[4]. The practical result is that, in many cases, not a single day of the voyage qualifies under the ‘good weather method’ for performance assessment.
This article provides an overview of the three most prominent non-good-weather and non-sensor-based[5] methodologies, which are often referred to as ‘Alternative Methods’. They pursue the same goal: to establish either hull fouling, an engine problem or both, translating into either a breach of the vessel’s delivery condition, a breach of the maintenance obligation, a breach of cl. 8 NYPE, or a defect under cl. 15 NYPE. The individual routes to that goal are examined below.
The RPM Method
The RPM method (Revolutions-per-Minute method)[6] was examined in detail and fully court-tested in The Divinegate[7].
The method attempts to establish underperformance from the recordings in the engine log, calculating the theoretical distance a vessel can travel per propeller revolution over a given period based on propeller pitch. In the first step of this method, the nautical mile engine distance in one hour is calculated as follows:
Engine distance = Propeller pitch x Revolutions per minute x 60 (minutes in one hour) ÷ 1,852 (metres in one nautical mile)[8].
This figure is then adjusted for ‘slip’ in a second step and can be used to determine the RPM required to achieve the vessel’s warranted speed. The engine log will show whether the vessel was operated at that RPM and, if not, how many hours were lost during the voyage as a result.
The Divinegate not only clarified the position as to ‘no adverse currents'[9] but also looked at – and ultimately rejected – calculating the loss of speed from the RPM. It would be easy to leave it there and rely on the judgment as a total rejection of the RPM method. And yet, the speed a vessel moves through the water is a direct result of how fast the propeller turns. There are ways to use the RPM in combination with other aspects such as slip, engine power and a forensic analysis of the engine logs to show if the vessel could, or could not, perform at the warranted speed and consumption. We have seen that much can be deduced and proven by looking at the performance holistically, which includes giving some attention to the engine speed. Indeed, the RPM method was recently successfully relied upon by charterers in LMAA 01/26[10].
The RGM Model
The Reasonable Grounds Model (RGM) was developed by Brian Williamson and introduced in 2023[11].
Following The Divinegate, Williamson proposes a sequential evidential methodology directed to the question of whether there are reasonable grounds for alleging underperformance where the conventional good-weather method is unavailable or inconclusive.
The method proceeds in seven stages[12].
It first tests the vessel’s reported weather against hindcast data, then examines key performance indicators including speed, RPM, engine setting and slip. It then builds /power curves from the vessel’s warranted data and compares the promised and achieved performance.
From that comparison, a performance forecast is produced to identify the speed and consumption that should have been achievable in the relevant conditions. The final stages calculate time lost, overconsumption and resulting damage.
The purpose of the method is not to bypass the charterparty warranty, but to provide a reasoned evidential basis for an equitable set-off from hire that satisfies the requirements of The Kostas Melas[13] .
The Twin Model
The Digital Twin Model is a vessel performance monitoring system benchmarking the vessel against a digital twin sailing in good weather conditions[14].
The model creates a virtual replica of the vessel based on technical parameters. This replica behaves as if the vessel had a clean hull[15].
Using AIS data and live weather inputs, the model calculates total hull resistance every 15 minutes for the actual sailing conditions, then works backwards through the propulsion-train (resistance to thrust, thrust to power, power to fuel) to derive what consumption and speed should look like under those precise conditions.
The key output is the speed the vessel would achieve in good weather. This figure can then be compared directly against the warranted speed, regardless of whether the charter period provided good weather as per the benchmark definition[16].
The model has been criticised for using new-building parameters to create the digital twin. However, this criticism misunderstands the model’s purpose as the digital twin is used solely to quantify and isolate the effect of the weather on speed. A new-building baseline is not the performance standard the vessel is expected to meet under the model[17].
Conclusion
Alternative methods had come under scrutiny in the wake of The Divinegate. However, the further development of new approaches – such as Brian Williamson’s RGM Model and the Twin Model – gives charterers an expanded toolkit. Furthermore, it appears that LMAA 01/26 supports the RPM Method going forward.
Finally, the four cases presented in the most recent Skuld Charterer[18], in particular LMAA 15/23, give reason to consider that alternative methods may carry sufficient evidential weight in themselves, without diver’s reports on hull fouling or detailed evidence of machinery issues.
Both the RGM Model and the Twin Model are yet to be court-tested.
How can Skuld help?
Speed and performance claims are a common source of dispute and are covered under Skuld’s standard FD&D cover, and the Skuld claims handlers can also assist with clause reviews when needed.
Should you have any comments or questions, please do not hesitate to contact us.
On behalf of your Skuld team of underwriters and claims handlers who serve our charterers and traders /365.
Source: Skuld (–part-ii-alternative-methods)
