" TIP SPEED"  A condition that creates a vibration in the propeller area of a boat.
The tips of the propeller blades are in essence moving through the water during its rotation faster than the water can displace the blade on its trailing edge.To avoid confusion, this action described has little to do with the forward speed of the boat, and caused by excessive shaft speed & to large diameter propeller. Also some people describe this action as a boiling action, I'm not going to argue that point other than to call it  cavitation as is mentioned on other pages is proper.  To correct tip speed sometimes involves choosing a different style propeller, or pitch and diameter combination, or even transmission ratio.
  In our business we encounter repowering of lobster boats by their owners, sometimes very frustrating, as we recently experienced when a customer was looking for new or used propeller to match the new 400 HP diesel engine he installed in a 10 year old boat. This 400 HP replaced a 165 HP gas engine, needless to say the reduction gear, as well as many other things had to be replaced or modified at a large cost to the customer,even though he did most of the work himself.
  If your in doubt always ask, it could save money in the end.