About RGM
RGM is a modern framework for analysing vessel performance under time charterparties. It is based on legal principles and maritime expertise, providing an alternative to traditional and often unreliable ‘good-weather’ assessments.
What is RGM?
The Reasonable Grounds Model (RGM) combines legal reasoning with seafaring judgment to determine when underperformance is likely to have occurred, even in the absence of good weather – offering an objective, tribunal-facing approach to performance evaluation.
Who is This For?
Shipowners and charterers involved in performance claims
Weather Routing Companies to supplement voyage audits
Maritime lawyers and arbitrators
Technical managers and consultants seeking robust methods
Why This Matters
Performance disputes are rising, but expert opinions often rely on questionable data. The RGM offers a principled alternative – helping parties understand performance objectively and avoid costly disputes based on flawed reconstructions or biased logbook entries.
Our mission
To promote fairness, transparency, and analytical integrity in assessing vessel performance under time charterparties, the Reasonable Grounds Model (RGM) was created to replace outdated, unreliable methods with a practical and legally robust alternative that accurately reflects how ships perform and how tribunals decide.
Our Vision
To establish ourselves as the leading methodology and digital platform for resolving maritime performance claims, we aim to empower charterers, owners, lawyers, and arbitrators through the RGM and RGM Guru. By providing clear, evidence-based insights, we seek to reduce disputes, discourage manipulation, and enhance outcomes for all parties involved.
The Reasonable Grounds Model (RGM) is a contemporary approach to evaluating voyage performance under time charterparties. It moves beyond narrow weather-based filtering to ask whether the vessel had a fair opportunity to fulfil its warranted capabilities in the real-world conditions it faced.
Developed by a maritime lawyer and master mariner, the RGM rests upon two core pillars:
Technical Indicators — RPM, %MCR, SOG, STW, slip, fuel consumption, and ship-specific performance data
Legal Reasoning — Evidentiary standards drawn from key cases such as The Didymi, Triton Lark, Apollonius, Pearl C and Divinegate
By integrating these elements, the RGM tackles prevalent issues associated with traditional approaches, including:
Over-reliance on filtered good weather segments
Misleading reconstructions of voyage conditions
Ignoring trends that show consistent underperformance across the voyage
What Makes the RGM Different?
Traditional performance assessments often fail because they attempt to isolate ideal conditions that may never have occurred during the voyage. RGM accepts that "real world" conditions are rarely perfect and poses a more practical question:
On the evidence available, were there reasonable grounds to expect the vessel could have performed as warranted?
This holistic, evidence-based approach is more equitable for owners and gains greater credibility with tribunals, particularly in complex or contested cases.
Introduction
Brian Williamson is a Master Mariner, former Solicitor, Chartered Arbitrator, and LMAA Arbitrator with more than six decades of experience in shipping. His career spans sea-going service, legal practice, and arbitration, culminating in the creation of the Reasonable Grounds Model (RGM) — a methodology that brings transparency, consistency, and legal compliance to vessel performance disputes.
Sea-Going Career
Began sea-going service in 1963 as an Officer Cadet.
Served at sea for 13 years, progressing through the ranks to Master Mariner.
Experience across a range of vessels provided practical knowledge of navigation, performance, and the operational realities of logkeeping and compliance.
Academic & Professional Qualifications
Graduated with a BSc (Hons) in Science and Technology in 1975.
Qualified as a Solicitor of the Supreme Court of England and Wales in 1979.
Full Member of the LMAA from 1987 to 1990, but temporarily stepped down as legal practice conflicted with full membership obligations.
Rejoined the LMAA as a Full Member in 2003, upon ceasing legal practice, and in the same year qualified as a Chartered Arbitrator.
Legal Practice
Practised as a shipping lawyer for 25 years (1979–2004), specialising in charterparty disputes, performance claims, and litigation for owners, charterers, insurers, and P&I Clubs.
Developed a reputation for blending technical expertise with thorough legal analysis.
Arbitration & Determination
Arbitrator since 1983 when he published his first award, practising part-time alongside his legal career.
Full-time LMAA Arbitrator since 2003, with over 600 published shipping-related arbitration awards, many concerning vessel performance and charterparty disputes.
Accredited as an Expert Determiner by The Academy of Experts in 2012. His dual qualifications as Master Mariner and Solicitor, and his frequent appointments in complex technical matters, have naturally led to expert determination as well as arbitration.
Regular contributor to international maritime law discussions, including presentations at the International Congress of Maritime Arbitrators (ICMA).
Development of the RGM
Drawing on decades of sea-going, legal, and arbitral experience, Brian created the Reasonable Grounds Model (RGM) to address persistent weaknesses in traditional performance claim analysis.
RGM integrates voyage data, environmental evidence, and English law, replacing fragmented, adversarial approaches with a transparent, probability-based framework.
The digital tool, RGM Guru, now enables charterers, owners, and insurers worldwide to access this methodology.
Key Message
From cadetship in 1963 to my current roles as an arbitrator and expert determiner, I have experienced performance disputes from every angle — on board ships, within legal and expert practice, and in arbitration. The Reasonable Grounds Model is the product of that journey, designed to ensure that performance claims are fair, transparent, and defensible.
Brian Williamson
Author of the Reasonable Grounds Model
(RGM Guru)