We had to tabernacle our Ericson 35 under an 18 foot bridge. We could see the cabin top flex visibly when lowering the telephone pole that passed for a mast. Our Santana had "oil can" stiffeners put in by a PO specifically to deal with the San Francisco Bay chop.I sail a C&C Mega 30 OD. A 4,500 pound late 70's ULDB with 2,400 pounds of ballast the hull twists and flexes so much you don't need a string you can watch it flex around the keel lifting mechanism. On rough days you can watch the hull tin can in and out over a 1/4". Pretty freaky when you are trying to sleep looking up and watching the fiberglass flex everywhere.