Foreword

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This document was prepared by Technical Committee ISO/TC 8, Ships and marine technology, Subcommittee SC 12, Ships and marine technology - Large yachts.

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Introduction

This document has been developed in response to growing industry demand for a realistic and transparent methodology to assess and compare large yachts regarding their environmental performance while in operation. Increasingly, yachts are being promoted as “green” or “sustainable” without quantifiable criteria, while simultaneously more clients are looking for references to understand the environmental footprint of yachts.

The International Maritime Organization (IMO), with the International Convention for the Prevention of Pollution from Ships (MARPOL) Annex VI Energy Efficiency Design Index (EEDI),^[18^] has developed an instrument for ships, however yachts are generally not subject to EEDI. Based on voluntary attempts, the EEDI was determined to be inadequate for the specific application to large yachts due to its narrow scope and different operational assumptions.

Elaborating on results of a five-years joint industry project, the document draws on collaboration between major shipyards, naval architects, classification, environmental experts and research institutes. It provides a framework with requirements not covered by legislation for yachts over 30 m and below 5 000 Gross Tonnage (GT), and complementary for yachts over 5 000 GT. It can be seen as pro-active self-regulation to set new industry standards, providing a sector specific, goal-driven approach.

Specifically, the EEDI methodology presumes continuous vessel movement, whereas yachts typically spend approximately 90 % of operational time anchored or docked in harbour.^[1^] The substantial energy consumption occurring during these stationary periods cannot be neglected, as propulsion typically accounts for about only half of a yacht's annual energy use and hotel power the other half. Therefore, it was necessary to develop a robust method, starting with a realistic, standardized operational profile for large yachts. The approach is system-based rather than behaviour-based, using installed equipment data and verified performance across a fixed operational profile, ensuring a repeatable and verifiable calculation method.

While EEDI focuses primarily on CO₂ emissions during ship operation, other MARPOL regulations address additional pollutants such as NOx and SOx. A comprehensive method based on life cycle assessment (LCA) (see ISO 14040)^[19^] has been used to accurately evaluate total environmental impacts, measuring both downstream emissions (CO₂, NOx, SOx) and upstream processes including fuel production, shore-based energy, and urea production. Although the current scope of this document emphasizes operational energy, it is structured to allow future integration of yacht manufacturing, maintenance, and end-of-life stages. This document aligns with MARPOL Annex VI and the IMO Life-Cycle GHG guidelines by accounting for both Well-to-Tank (WtT) and Tank-to-Wake (TtW) contributions to a yacht’s total Well-to-Wake (WtW) impact. In general terms, the calculation of the well-to-wake impact follows the formula below.

Well-to-Wake impact = power demand × operational profile × (specific Well-to-Tank impact + specific Tank-to-Wake impact)

The power demand is based on vessel specific propulsion and auxiliary power input, thus rewarding yachts with either improved naval architecture or improved onboard systems with less environmental impact, or both.

The subsequent environmental impact is expressed in terms of “ecopoints” a common environmental indicator when applying Life Cycle Assessments (LCA) incorporating various environmental parameters including CO₂, NOx, particulate matter (PM) and more. Alternatively, the impact can be translated into CO₂eq, a broader recognized metric.

Since 2019, a joint industry project and other index initiatives^[20^] have emerged to address yacht specific environmental assessments. This document includes elements of these methods and aims to provide the yachting sector with a single methodology and reference for clarity, unity and most importantly to accelerate progress with assessing and subsequently improving the environmental performance of both the existing fleet and new build projects.

Use of this document enables early, consensus-based use of a maturing method and allows timely updates as data availability and alternative fuels evolve.

1 Scope

This document specifies a methodology to calculate the environmental impact of operational energy use for large yachts and enables the comparative assessment of a yacht's environmental performance against a defined baseline fleet. The method established herein is robust and based on the best available data, with transparency and comparability across various yacht types and technical characteristics. This document explicitly addresses operational energy consumption during the yacht's use phase, emphasizing the efficiency of onboard systems benchmarked against the found average operational profile and the environmental emissions coming from this energy, both upstream and downstream. It excludes behavioural variables arising from individual yacht operation patterns. Additionally, the production and maintenance materials and processes (upstream impacts), as well as yacht end-of-life considerations (part of downstream impacts), are outside the scope of this document.

The operational profile specified in this document assumes that the yacht is professionally crewed year-round and capable of independent transoceanic voyages.

NOTE For motor yachts, this operational assumption generally aligns with IMO MARPOL requirements, which apply to ships exceeding 400 GT that must be surveyed and certified for MARPOL compliance. Sailing yachts typically exhibit lower GT for equivalent length; however, their cruising behaviours align closely with motor yachts of comparable length.

2 Normative references

3 Terms and definitions

4 Operational profile

4.1 General

4.2 Speed profile

4.3 Shore power availability

4.4 Sailing correction

4.5 Total operational profile

5 Propulsion power

6 Hotel power

6.1 Determination of hotel power

6.2 Hotel power in various conditions

6.3 Heat recovery systems

6.4 Onboard sustainable power generation

7 Matching power demand and supply

7.1 General

7.2 Shore power use

7.3 Diesel direct, hybrid and diesel electric systems

7.4 Matching procedure

7.5 Batteries

7.6 Lack of factory specific emission data

8 Environmental impact

8.1 General

8.2 Upstream (WtT) impact

8.3 Downstream (TtW) impact

8.4 Use of alternative fuels

9 Fleet reference lines

Bibliography