Scubakevdm:!!!
They said PG-13
Having been using back plates since 1988 and having manufactured thousands of them over the years I can say without any reservations a 2-12 hour boat ride will vibrate loose a hand-tightend set of wing nuts.
I was gearing up for a tie-in on the Doria a few years back and forgot to tighten my nuts. Just before I stood up I felt that small tiny clicking behind my back. I asked my crew-mate to "stick your hand behind my back and tighten my nuts" she did and away I went.
It's important to note that its the top nut that really needs to be tight. It's the one that holds the whole thing in place. The bottom nut is not that critical, but, you need at least one nut.
Off to the office ................
Regards,