Seawater is the sole lubricant for your boat’s propeller shaft as it spins its way through life, turning thousands of rpm for hundreds upon hundreds of hours while you’re under way. The shaft is supported, typically, at its inboard end by the coupling that’s attached to the transmission; its other end is held in place by a fluted rubber tube referred to as a shaft or Cutless bearing. This bearing can be found, depending on the boat’s design, either within a shaft log that passes through the hull or in a strut some distance from the keel.