By: Daniel van Drunen – XEAMOS
10 July 2021

IMO Tier III for smaller superyachts

The IMO Tier III requirements came into force for all superyachts on 1 January 2021. While this eco-friendly measure is globally welcomed by the boating community, it has resulted in a host of new challenges for the builders and users of yacht engines.

The IMO Tier III regulations regarding nitrogen oxide (NOx) emissions took effect for all large vessels with an engine output of 130 kW or more which had their keel-laying on or after 1 January 2016. They specify that NOx emissions must be at most around 70% lower than the maximum under the Tier II regulations. (<2.0g/kWh)

The vessels described above must comply with IMO Tier III when sailing in an Emission Control Area (ECA).

Challenges for yachts below 500 GT

Originally applicable to vessels over 500 GT, the regulations have also applied since 2021 to all yachts above 24 metres LOA no matter their volume. Smaller boats are subject to particular constraints in terms of space and size: simply tuning the engine will not be sufficient in most cases, and a combination of techniques and technologies will have to be deployed instead.

Yachts below 500 GT will often have insufficient space for standard solutions and Xeamos provides serious added value by delivering custom solutions that fit each engine room. It also has the capability to combine a silencer and after-treatment system in a single unit, options that engine manufacturers cannot provide.

Engines and after-treatment units are usually delivered as large standardised boxes, slightly adapted versions from solutions originally designed for onshore or automotive applications. While this may work for tankers or large commercial vessels, it’s a far cry from what many yacht owners need.

There are very few superyachts on the water with the latest after-treatment systems. They are mostly still being built and the first problems will arise – and be managed – in the years ahead. Discussions are already arising about refits related to repowering existing vessels.


Back stories


The International Maritime Organization ((IMO) is the UN agency responsible for the safety and security of shipping and the prevention of marine and atmospheric pollution by ships. As part of the UN family, IMO is actively working towards the 2030 Agenda for Sustainable Development and the associated SDGs.


The International Convention for the Prevention of Pollution from Ships (MARPOL) is the main international convention covering the prevention of marine environment pollution by ships due to operational causes or accidents.

The MARPOL Convention was adopted on 2 November 1973 at IMO. The Protocol of 1978 was adopted in response to a spate of tanker accidents in 1976-1977. With the 1973 MARPOL Convention not yet in force, the 1978 MARPOL Protocol absorbed the parent convention in a combined instrument that came into force on 2 October 1983. In 1997, a Protocol was adopted to amend the Convention and a new Annex VI was added which came into force on 19 May 2005. MARPOL has continued to be updated by various amendments over the years.

The convention includes regulations aimed at preventing and minimising pollution from ships – both accidental pollution and that from routine operations – and currently includes six technical annexes. Special areas with strict controls on operational discharges are included in most annexes.

Annex VI Prevention of Air Pollution from Ships (19 May 2005)

Sets limits on sulphur oxide and nitrogen oxide emissions from ship exhausts and prohibits deliberate emissions of ozone-depleting substances. Designated emission control areas set more stringent standards for SOx, NOx and particulate matter. A chapter adopted in 2011 covers mandatory technical and operational energy efficiency measures aimed at reducing greenhouse gas emissions from ships.

Latest developments

· In light of the urgency for all sectors to accelerate efforts to reduce GHG emissions as emphasised in the recent IPCC reports and the Glasgow Climate Pact, IMO has agreed to initiate a the revision of its GHG strategy.

· IMO’s Marine Environment Protection Committee (MEPC), meeting virtually for its 77th session on 22-26 November 2021, adopted a resolution on the voluntary use of cleaner fuels in the Arctic in order to reduce black carbon emissions.

· A strategy to address marine plastic litter from ships was also adopted as were revised guidelines for exhaust gas cleaning systems (EGCS).

· The MEPC also agreed the scope of work on discharge water from EGCS, and considered matters related to the Ballast Water Management Convention.