Is my rudder OK?
It is a little hard to tell what you have found based on your description. A lot of boats have a delrin or teflon impregnated nylon washer that fits around the rudder post and acts as a bushing between the top of the rudder and the hull of the boat. On the other hand this might have been an admission strap from a recent rock concert.
I like Nigel Calders books for a general overview of yacht systems.