I was out Saturday in the same rig (380 + 15) and it's probably the roughest I've taken the Aerotec out in. F5 with 2m waves maybe some approaching 2.5m. We were playing outside the harbour against wind, tide and waves - hard work, a bit scary and very wet as to make any progress you really needed to tack across the waves meaning each one chucks a bucket of water into your face off the starboard bow. Due to the conditions we were really just playing - out as much as we dared then surfing back in on the following sea every now and again nipping into the channel for some respite.
I think the boat is more than capable and once again I was reminded how amazing the Aerotec is compared to other SIB's I've owned but really it was conditions for a 5m+ RIB TBH and I chickened out after a while. You could not make any progress in a SIB in such a sea, even an Aerotec and reading and negotiating each wave gets tiring very quickly.
A F6 with 3 - 4m waves (Richard mentions 2m above but Beaufort gives F6 as 3 - 4m) is another step up and would be silly and not something I would want to be out piloting a 12ft rubber boat in...
The other point you mention: "...a few miles off shore..." - again, this is more than I'd go unless in transit to a definite point of course preferably with others. Where we are Sussex/Kent coast we cruise along rather than out and 1/2 mile out is enough for me.
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