Was out a week or so ago and seen a brand new unit not even flinch a reef shark about 2m long. When we tested it on myself it gave me the usual zap in the teeth but nothing to this shark.It was trying to get a nice red brim off a mate of mine so it did several fly bys with no effect. I've seen these units work very well time and time again but nothing to this particular shark. Same unit same day no probs on dif shark.