Forward end of shaft is connected to main engine and at aft end we have propeller fitted on it. Basically it is a cantilever beam with one end fixed and other end free on which propeller is fitted and weight of propeller will try to pull the shaft down. A sealing is provided at a point where the propeller shaft passes through the hull into sea to avoid sea water leaking into the engine room. This seal contains nitrile rubber or viton lip seal which seals against the bronze liner shrunk fit around the cast iron propeller shaft. Lubrication is provided between liner and sealing to avoid heat build up and damage the sealing .
Usually after few years ,grooves are created on the liner surface of shaft because of the seals and sealing is lost resulting in sea water leaking in .This reduces lubrication effect and creates wear in bronze liner on propeller shaft.
Now due to the weight of propeller and the clearance developed due to wear , the shaft will come down by a certain amount. This drop in propeller shaft is termed as propeller wear down or propeller drop.
Propeller drop is measured by POKER GUAGE during dry dock.