The vast majority of failures are related to improper fit between the shaft taper and the propeller hub taper, either due to imperfectly mating surfaces or high keys. The contact area should be a minimum of 70%. Over the years, we have seen contact areas as low as 20%. When the contact area is too small, the forces created by the blades can set up a slight wobble in the propeller, which may or may not be felt by the boat user. If the frequency does not hit a harmonic wavelength in the boat’s propulsion system, it is not amplified, and you may never perceive that your hub is wobbling. However, this wobble sets up conditions which will ultimately result in a fatigue failure.