Propeller Shaft Components

Components of a boat's propeller shaft either hold the shaft in position or provide the link to transfer rotation and torque from the engine, through the hull, to the propeller. The shaft must withstand heavy shock loads from rapid shifting of the transmission during maneuvering. Most shafts are machined straight to within very small tolerances, preventing excess vibration while under way.

  1. Shaft

    • The propeller shaft stock is usually made of stainless steel or another high-strength and corrosion-resistant alloy. A machined taper is sized to fit snugly inside the propeller hub. Threads cut into the end of the shaft accept the propeller nut and lock nut, and a keyway cut into the tapered area accepts the shear key.

    Transmission Coupling

    • Power transfers from the transmission to the shaft through the coupling. Couplings can be clamped on the shaft, or may have a shear key for added engine and transmission protection in the event of a propeller lockup. The coupling clamp bolts must be very tight to support, and transfer, the torque loads. The machined faces between the transmission coupling and the shaft coupling should have less than .005 inch of run-out, or transmission bearings might be damaged at high speeds.

    Propeller

    • Propellers are made from stainless steel, bronze or an alloy made of nickel, bronze and aluminum called nibral. The hub, machined with a taper, will accept standard shaft diameters -- usually in 1/8-inch diameter increments -- and a keyway to correspond to the keyway in the shaft that will accept standard key stock. Use nylon bushings to increase the diameter of the taper on the shaft to allow for the use of a propeller hub with an oversized inside diameter.

    Stuffing Box

    • The stuffing box mounts on the front of the shaft-log area of the boat, inside the engine room where the shaft penetrates the hull or the horn timber. Square flax rope, saturated with wax, wraps around the shaft and is packed in the stuffing box. A compression plate is then tightened over the stuffing box to compress the flax and form a seal. The plate should be loose enough to allow a slow, steady drip of water to prevent the flax from overheating or chafing.

    Strut

    • The strut mounts to the bottom of the boat and holds the shaft in position while supporting the heavy radial loads generated by the shaft at high rpm. A rubber component called a cutless bearing is pressed into the strut to separate the shaft from the metal parts of the strut. A slot in the cutless bearing allows water to travel through the bearing when under way to provide additional lubrication and prevent overheating of the rubber.