Check the space between the propeller and the strut. Too much space can place excessive stress on the prop shaft. Yet, with too little space, the prop can strike the strut, because the prop blades flex and there is a minor amount of fore-and-aft play in the shaft. A strut that's too close can also create turbulence that impairs prop efficiency. Tiara Yachts specifies minimum spacing of 1 inch and a maximum equal to the shaft diameter. Diameters range from 1½ to 3 inches, depending on the engine power and configuration. Look for excessive pitting, corrosion, wear or dings on the prop that might throw the drive train out of balance.