A close relation of mine works North Sea SBV's. He has told me that they only train the crews for rescues to 3.5m swell. Over that, it's down to the experience and willingness of the Capt and crew to launch or not. He also said he wouldn't fancy being the person being recovered by Dacon scoop in heavy seas, unless it was just for recovery of bodies!!
This from someone who has been working at sea for 40+years!!
