M/V or mv.

To finish up something from this past spring, I've gone to the old usage of M/V for towboats, as in motor vessel.

I've seen mv. used in some places over the past three decades, such as the Waterways Journal. I prefer it, as it's shorter and less obtrusive than M/V. But I've found that people who prefer mv. don't mind M/V, but some people who prefer M/V don't like mv. and have a bad reaction to it.

It's not all that important to me, so I went with what the readers seemed to prefer. So it's M/V.