Matthew,

As Rob says, right the boat from the main beam which would be in line with your righting rope anyway. There should be a frame at the front of the centreboard case. Does it flex at that frame point too ?

And apart from that try to stand as close to the keel line to get the maximum leverage.

Fixing the flex would involve adding more stiffeners and you really don't want to do that (more weight). Just try standing on points of maximum strength and avoid obvious weak areas.

Regards,

Peter