A knockdown happend to me once on Lake Superior on my C&C 36. It was a beautiful sunny day and was blowing about 15 all day. I could see a puff of wind coming across the water but it looked the same as all the other puffs we had that day. No big deal, right?
The wind hit us while we were on a beam reach, port side. The gust had to be in the 30+ range. We were sailing with a 105 and main and the wind just laid us flat over on the starboard rail and held us there. The spreaders were about 4 feet from the water. My friend Fred, who is 6"8", was holding on to the port primary winch and his feet were in the water. I was standing on the starboard cockpit coaming and hanging onto the wheel and backstay.
We stay in this position for about 10 seconds, but it felt like an enternity.
We eventually came up and there was very little damage. I saw another gust coming so turned to starboard and let the wind push us. The second gust was not quite as strong, but I figured if I am going to lose the rig, let it fall forward, away from the people.
The entire ordeal last less then 15 minutes. After it was over there was barely enough wind to sail, so we motored 5 miles back to the marina. When we came in we were the only boat without major damage. Other boats in the marina that were out had shredded sails, furler drum failures, and one broken boom. Fred's wife had been sleeping below on the port settee and was thrown into the saloon table. She got a big bruise and the table suffered a bent hinge. We were very lucky.
That's Lake Superior for you.