A 5% reduction in fuel is significant, especially as cargo ships are responsible for a disproportionate share of petro-pollution. They burn the cheapest petro gunk, emitting tremendous volumes of soot and such.
Using this thick soup thats close to coal, the process of burning is always far from optimal for most of them. Thus black sud gets emitted - basically unburned carbon - a huge waste financially but also ecologically as its comparable to cigarette Fumes regarding unhealthy compounds. Plus fine particle dust. Chances its toxic for marine life are huge.
Those are all externalized costs no one of the polluters pays for besides the collective life on this planet.
How can this be fair?