3rd Edition

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Understanding how an anode works
(abridged version)

At home, the problem of grounding is usually simple. It consists of the green ground wire in the AC cabling system and has as its goal to prevent shock or electrocution. The ground connection is usually done by clamping a metal water pipe or by pressing a copper post deep into the ground.

On a boat, this concept is much more complicated. In addition to AC supply (if equipped), we need a DC ground and negative return, a grounding due to lightning and RF ground (radio frequency) for radio systems and VHF (if on board). Our first instinct might be to simply make the connection to the ground in a submerged metal such: a through-hull, a propeller shaft or other metal submerged. Unfortunately, this type of connection will probably give rise to serious electrolytic corrosion problems in these parts. The use of sacrificial anodes will protect the metal components which are subject to deterioration and corrosion.


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Anode: negative polarity metal piece (-) that is installed on the vessel or, more often, over the immersed part of the engine. It helps protect the engine and / or boat against galvanic corrosion and damage caused by power losses.  

Anode types: zinc, aluminium and magnesium.

  1. Zinc Anode : preferably used for salt water,   but can also be used in freshwater.
  2. Aluminium Anode : Offers a good compromise for those who can or should move from salt water to fresh water.
  3. Magnesium Anode: only used for fresh water.


Several factors affect the lifespan  of an anode. Among these include electrical leakage which, carried by a conductor, attack the integrity of the vessel and enhances corrosion of immersed metal or of two metals of different kinds in contact with each other.

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It is recommended in most cases, to replace the anodes annually.
Here is an example of a virgin anode and a corroded anode:

AngAnodeSchema
  1. Regularly check your anodes.
  2. Identify the type of anode required (aluminum, zinc or magnesium)
  3. Other useful information to identify good models of anodes:
  • Motor manufacturer (base)
  • Engine Model
  • Year of the engine (or base if it has been replaced)
  • Engine type (i.e. outboard, or semi outboard inside)
  • In the case if propulsion shaft anode (inside the engine), you need the diameter of the shaft
If you are unsure of your needs, bring your anodes to the branch.

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