Got caught in a thunder storm down draft last summer. The storm formed right over us, so there was no ominous dark sky warning to the west like we usually get. Hard to say exactly, but we guessed wind speed at about 35-40kts at the time. Water was nearly dead-calm before the blast. We traveled out, sheeted hard and tried to stay in a semi-parked position at an angle to the wind. Rig shook like I've never seen - the boat oscillated vertically a couple times too. Lasted about 2-3 minutes, shifted about 180°(that part sucked) and then stopped as quickly as it started. I'd give Rick's story some credence if it was something similar.