[Silmaril]

A bit of flex is sometimes expected, but yours sounds extreme. Does it also happen on port tack to the starboard hull? Also how old is the boat. Did you have a survey completed if you just purchased it? If so, did the surveyor note any previous hull damage and or repair to the hull?

If you have a cored hull section in that area I would suspect a delamination, and in that case, proceed to your nearest surveyor/navel architect for a concidered opinion. You may also want to visit your nearest pub, as you will need to settle your nerves a bit, because proper repair of a severe hull delamination is riotessly expensive, if not down right impossible.