I hit an underwater tree with my boat yesterday doing about 15 mph or more on the way to wide open under full throttle. Wow, it sounded really bad and my wife and I almost fell out of our seats from the collision. I thought I might have ruined the transom or the engine/lower unit on my 115 hp Mercury 4 stroke. Luckily the prop and lower unit appear to be ok. A person needs to be very careful there and you never know when you might hit something underwater that you cannot see. I was very surprised I didn't damage anything.