>> So they are in the same time zone <<
Well, to be exact, Zulu Time is always the same as UTC, which is the same as GMT.
So for example, 1730Z is just the way military, aviation and diplomatic traffic refers to 1730 GMT, or 1730 UTC, or 5:30 p.m. London time. It’s that simple.
Oh, so the boats weren’t in the same time zone as GMT?