When ship is in Drydock normally it is required by Class to take a wear down measurement at the aft stern tube seal.
Reasons for taking a wear down measurement is to get an indication of the wear and tear of the stern tube bearing. The measurement taken will be compared with the previous reading. Previous readings are normally registered in a service report as well as on a form in the box containing the poker gauge or the wear down gauge or in the last Drydock Reports.
As class requires a reference reading, it is important that every reading is taken under the same conditions. This means always using the same tool, always having the shaft in the same position and of course always measuring from the same location within the seal. The plug hole that must be used for the wear down measurement is normally indicated on the drawing in the manual. Sometimes, the subject plug is also marked.
The reference position should be stated in the poker gauge toolbox.
Normally measurements are taken both at the top and at the bottom of the seal.
If the new reading indicates a considerable difference compared to the previous reading, this indicates stern tube bearing wear. The limits of this wear depend on the type of bearing and the limits set by the manufacturer. In case of suspected excessive wear, further investigation by a specialized engineer is required.