parked sailboat accident

During high winds, our 17\' sailboat,that was covered with a tarp, strapped to the trailer and tied to a lightpole in a parking lot, flipped on it\'s side and caused damage to a vehicle parked next to it. Are we responsible for the damage to the vehicle, even though the owner was warned the boat had flipped before?

Normally you would be liable if you had not secured it properly and to withstand winds; but having warned him of the situation you might argue he assumed the risk. Not clear to us if he had to park there etc.?? Those kinds of facts could affect a judge.