Getting a vibration at about 6 kts when under sail - doesn't seem to be coming from the (fixed 2 blade) propellor - boat does have a lifting keel - could this be the source? any ideas? boat is a First 260.
On my lift keel Hunter the lift rope vibrates dependent on the tension and to some degree the point of sail. Try tightening it up to tension, or letting it go really slack and see what happens. usually one tweak stops it until the next outing.
JimJ.
I had a similar issue (occasional thus as the keel moved on a run) on my Jeanneau 32.2, tighting the line seemed to change things, so should at least rule the cause in or out.