Superfast is smokin', but not in a good way

I've noticed recently that the new Stena Superfast vessel operating from Dublin consistently is smoking as it leaves port. It looks like something you would have seen 20 years ago before emissions controls came in everywhere.

I was looking at other vessels leaving port in the last few days and they don't seem to be as bad. The Ulysses had nothing visible (from shore) as it left this morning.

I don't claim to be an expert and I know that no visible smoke isn't necessarily a good indicator for having no polluting emissions, but it's probably fair to say that visible emissions are generally a bad sign. Does anyone know what the story is with Superfast X - why it smokes so noticeably and others don't?

z