One of the most critical elements in the propulsion system of a marine vessel is the shaft. Propulsion systems with shafts that become corroded over time due to electrochemical potential between the metal shaft and seawater lose their efficiency at transferring work or energy from the vessel’s engine to its exterior propulsion system, which provides thrust to move the vessel forward. Overly corroded shafts can lead to system failure. Therefore, cathodic protection is provided for the shaft in order to protect the working metal (transferring engine energy) from corrosion. An outer “sacrificial” layer of metal is provided around the shaft such that the electrochemical potential from seawater will be attracted to the outer layer as opposed to the shaft itself.
In passive systems (such as with reinforcing bars in concrete), the additional outer layer of metal alone is acceptable. However, with rotating propulsion shafts, the shaft workload and constant contact with seawater make passive cathodic protection insufficient as the outer metal will corrode very quickly itself. Thus, a small direct current (DC) electrical supply is delivered to the cathodic protection layer in order to continually maintain the electrochemical reaction between the outer metal layer and seawater, and not the shaft metal and seawater. An open-ended electric circuit (as created by the DC supply) needs to be grounded. Since the rotating shaft is typically not connected directly to the vessel’s hull (providing a ground for other vessel electrical systems), this added electric current will seek its own discharge, which could damage other critical propulsion system components such as bearings, seals, etc.
As its name indicates, Active Shaft Grounding Systems deal with this nuisance discharge of electricity from cathodic protection systems on rotating shafts. Given the logistical issues associated with connecting a rotating shaft to a relatively stationary object, Active Shaft Grounding Systems are specialized marine products that provide a means of getting the discharged current from cathodic protection to flow to the vessel hull ground via a very low-resistance electrical connection. Since electricity seeks the path of least resistance, discharge current will seek appropriate grounding to the vessel hull, protecting other vessel equipment. In conjunction with cathodic protection, Active Shaft Grounding Systems extend the life cycle of marine propulsion equipment, reduce maintenance and repair costs, increase the vessel’s return on investment, and promote a higher level of vessel safety.
For more information on how these systems work and improve marine vessel performance, please visit cathodicme.com.
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