More then likely the vibration you are feeling is being caused by slight cavitation. Probably caused by how the water is moving past the hull and prop. If it was something stuck to your prop or damage to it, the vibration would increase as your speed increases. You could also be experiencing a resonant frequency vibration that is just happening to occur between 8-14mph. I had this happen on my boat after the prop (Acme 1235) was repaired when my drive shaft snapped in half. Changed props at the vibration went away. Changing props could make the vibration go away, but it might not as well. Just depends on if the prop is the cause or not. I feel slight vibrations all the time in my boat that no one else ever notices, partly because I do vibration analysis for a living. If the vibration you are feeling starts to increase or changes characteristics then I would try to figure out what is causing it. If it hasnt changed and is minor I wouldn't worry about it too much.