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It should be possible to turn the ERRV without headway or sternway and to hold it in a desired position and on a set heading while carrying out rescue operations, with sufficient accuracy to avoid hazard to the ERRV, other units or persons in the water, in all but exceptional weather.
I do not quite understand the bolded fragment because it is the duty of Emergency Response and Rescue Vessels to carry out rescue operaations in all kinds of weather.
Irrespective of my ability to understand the fact, in all but exceptional weather means in all weather conditions except for exceptional/bad weather conditions.
Do you think I am on the right track?
You are right. It does indeed mean "in all weather conditions except exceptional weather conditions". What their definition of "exceptional" is is not known but I assume that it would have to be really, really bad weather which actually makes it impossible or far too dangerous for them to carry out their job.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.