The final rule granted recreational marine engines an exemption from having to install Tier IV catalyst engines, which would create cleaner emissions. Commercial marine engines over a certain size will need to install the catalyst.

The NMMA had both submitted comment and testified on behalf of exempting recreational diesel engines, citing the difficulty of installing and maintaining urea-based catalyst systems on recreational boats and the difficulty of accessing ultra low sulfur diesel and urea, which are required for catalyst, in foreign ports.