Rudder vibration is another source of irritation in my book, usually caused by worn rudder ports or packing not tight which in time causes wear, in any event if this is the case, you could experience the cold sweats that come over you as you notice your boat is missing, and then notice the antenna tips just sticking out of the water, this is described as SINKING, not only the boat, but your heart as you try to remember if the policy is current.  Pleasure type boats without a skeg to support the bottom of the rudder ,depend on the through hull port which contains the packing to act as a bearing, a built down (lobster type) hull with full keel has a rudder bearing attached to the skeg. Rudder vibration is noticed more when turning the helm hard down in either direction, this is caused by the prop-wash. A conventional stuffing box should be allowed to drip a few drops a minutes, this applies to rudder ports as well, some boats are equipped with drip less boxes, they speak for themselves, but not to be ignored or forgotten .