Spark Erosion

Technically, when two current carrying dissimilar metals are in contact, a sparks travels at the point of contact which erodes the small metal by making a cavity.

In a Vessel, different metals are used to building propeller, hull, bedplate, crankshaft, bearing etc. The current from the cathodic protection system is generally present in these parts, which eventually creates the perfect situation for spark erosion

In short -Spark erosion occurs if electrical potential in the crankshaft discharges through main or thrust bearing

EFFECTS OF SPARK EROSION

When the propeller is at rest, the stern tube, propeller shaft and bearings are in contact with each other. Similarly main engine bearing and journal are in contact with each other, maintaining continuity of the circuit. When the ship is running, due to the rotation of the propeller and lubricating oil film the shaft becomes partially electrical insulated. It may also happen on the tail shaft using non metallic bearing which acts as an insulation.

The propeller at the aft is a large area of exposed metal which attracts protective cathodic current which produces an arc while discharging from the lubricating film. This results in spark erosion of bearings, which can lead to worse situation if lube oil is contaminated with sea water.

If this effects continue for a considerable amount of time, it may lead to overheating of Main engine bearings caused by improper lubrication resulted by cavities from spark erosion. It may also lead to formation of oil mist, emergency shutdown of the engine or in extreme cases crank case explosion.