What is propeller drop?

The propeller shaft in the after peak tank is provided with inboard and outboard seals.these seals contain nitrile rubber or viton lip seal which seals against the bronze liner shrunk fit around the cast iron propeller shaft.after a few years it creates grooves on them and naturally looses sealing and sea water can easily find its way inside.this reduces the lubrication effect and creates wear if the bronze liner.now as there is enough clearance the shaft will come down by certain amount because of the propeller weight.this drop in propeller shaft is termed as propeller drop and is measured by POKERS gauge.

Clearance Measurement 

The clearance between the propeller shaft and the bearing should be measured during the bottom inspection. There are two methods of 
measuring clearance, depending on the water—sealing method for the shaft. For the rudder, the clearances in the longitudinal (fore-aft) and 
transverse (p-s) directions of the rudder shaft are measured, but for the propeller shaft, because the lower surface of the shaft is in contact with the bearing, only the clearance of the upper surface of the shaft has to be measured in case of a water--lubricated system. 


Fig.11-5 Clearance　of stern tube bearing
For an oil lubrication　system, also measure the clearance at the lower surface.

However, for an oil—lubricated system, clearances of the upper and lower surfaces have to be measured, because the shaft sink-age is small. Clearances in the transverse direction need not be measured because the shaft is rotating all the time. 

click the link if you are not satisfied: propeller drop and how to measure it?