No Good Weather Clause Triggered? There Are Still Three Ways to Prove Underperformance

When the Weather Doesn't Cooperate, the Claim Doesn't Just Disappear

A vessel performance claim traditionally depends on a “good weather” period — conditions clean enough to isolate hull fouling or engine issues from weather effects. But good weather periods aren't always available. Skuld's guidance sets out three alternative methodologies used to establish underperformance — hull fouling, an engine problem, or both — without waiting for one.

The Three Methods

The RPM Method works from engine log recordings, calculating the theoretical distance a vessel should travel per propeller revolution based on propeller pitch, and comparing that to the actual distance covered. The Reasonable Grounds Method (RGM) runs through seven stages: testing reported weather against hindcast data, examining speed, RPM, engine setting and slip, building speed/power curves from warranted data, then comparing promised versus achieved performance to calculate time lost, overconsumption, and resulting damage. The Digital Twin Model goes further still — a virtual replica of the vessel built from its technical parameters, using live AIS and weather data to calculate what consumption and speed should be at any given moment, and comparing that against warranted performance regardless of whether good weather occurred at all.

What This Means for Masters and Chief Officers

• Accurate, complete engine log and noon report data matters more than ever — every one of these methods depends on the quality of your own vessel's recorded data
• Don't assume a lack of a clean weather window ends a performance dispute either way — for owners defending a claim or charterers pursuing one
• Understand that RPM, slip, and speed data recorded routinely on your vessel may become the central evidence in a commercial dispute months later

Related Reading

• Heavy Weather Ship Handling — why accurate weather logging matters beyond just safety