4. Shaft Support
When you haul the boat, make sure the propeller shaft angle is 10 degrees or less (below the horizontal plane) for maximum efficiency. Also, the strut should be foil-shaped in cross sections to reduce drag, with the top of the strut recessed into the hull to minimize turbulence. The best material for the strut is nibral, and it should have a rubber cutlass bearing in the strut barrel. This holds the weight of the shaft and propeller — putting a lot of pressure on the bottom of the bearing — so make sure it is inspected carefully for wear. Tiara designs its boats with sufficiently large-diameter prop shafts — machined from Aquamet 22 high-strength stainless steel — to eliminate the need for more than one strut, Eggerding says. “This eliminates the alignment issues experienced with multiple struts.” This is also the time to remove the prop and inspect the entire shaft for signs of wear or damage.
Carefully look at the zincs on the propeller shaft for proper installation and wear. If the zincs have slid down against the strut, they were not installed properly. Also, if they’re eaten away at all, put on new ones while you have the boat out of the water, and make sure the fasteners are tightened properly.
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.