A recent survey of a modern production motor cruiser revealed some significant deterioration to the propeller shaft caused by galvanic action between the shaft and a stainless steel rope cutter.
One often assumes ones shaft to be of stainless steel, but other grades of steel are commonly used which although offer better torque resistance can be prone to galvanic deterioration if not properly protected. See the following report extract below.
“A stainless steel line cutter is fitted forward of the propeller, seen slightly bent on one side although I do not consider this damage to be of a degree that would significantly affect performance.
The shaft was found magnetic suggesting that it is made from EN 1.4462 steel (or similar), a mixture of ferrite and austenitic steel, sometimes referred to as “duplex”. Although this product has high torque resistance it has less resistance to galvanic deterioration.
Close examination of the shaft in way of the line cutter revealed some significant galvanic deterioration of the shaft caused by the different potentials of the stainless cutter and austenitic steel shaft. (See figure 2).
The diameter of the shaft is 40mm but has now wasted to 35.0 mm in the measurable area immediately next to the cutter itself. It may well be that further deterioration of the shaft would be found beneath the cutter should the cutter be removed for examination.
The reason for this deterioration is a lack of galvanic protection to the shaft within sufficient proximity of the cutter. Had a shaft anode been installed forward of the P bracket this damage would have been prevented. As it stands this shaft is of little further use and now requires renewal”.
A recommendation was made to replace the shaft and ensure the shaft was properly protected from future galvanic deterioration especially in the presence of this stainless cutter.
It’s easy to check the material of your shaft with a magnet. As we know stainless steel is non magnetic, duplex (as described above) is magnetic and will be “anodic” to the stainless steel rope cutter. If you have a similar set up, ensure you have good cathodic protection, inspect the shaft annually for any evidence of deterioration and replace your anodes frequently.