On a channel island trip last year, we were doing 20+ on the way back, flat calm, not a wave in sight. Somewhere in mid channel the skipper had to do a hard to starboard, he saw what looked like a wheel hub (?) float at the surface. He did a u turn and had a closer look, it was some sort of a tower had to be at least 15' under water, with only a round hub with 4 bolt sticking above the water. Had we hit it, I think that would be it.....