Isn't ZF the former Hurth?.. the main caution in the manual said (Hurth) not to let the transmission freewheel in forward gear without the engine running.
We've had the Hurth/ZF in our last two boats and always have left it in reverse although now with a Max prop, once the prop's feathered I suppose it wouldn't matter.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)