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Zinc anode protection

The zinc anode is probably the only protection for your boats drive shafts propellers and metal through hulls needed. Anode manufacturers produce anodes that can last all summer and still protect your in water metal. The yacht designer's choice of contraction materials and whether the vessel be used in freshwater or saltwater will determine the type of anode required. Other relevant factors in deciding the chemical makeup of the anode are pH water temperature and flow. The use of sacrificial anodes is a juggling act if the anode produces too little negative voltage the vessels underwater metal will not be protected. Further if the anode produces two great a negative voltage, the anode will exhibit a shortened life and probably result in some form of damage to the boat. The most reactive and electrical negative element used in anodes is magnesium the next least reactive element is aluminum then zinc. Metals used in vessel construction also possess these properties electrical properties when in solution. Of the metals commonly used in boat construction the most electrically negative is aluminum followed by steel then bronze. These voltages may be measured with volt meter using a silver-silver chloride half cell Electrode as reference. In seawater voltages produced by the different metals would be as follows magnesium will produce -1.63 V, aluminum will produce -0.90V, zinc will produce -1.03V, steel will produce -0.7 V and bronze will produce -0.4 V. If environmental factors i.e. temperature, salinity, pH, and flow remains constant each metal will produce an individual voltage specific to that metal. Changes to temperature, salinity, pH or flow will produce variations of the voltages produced.


Over protection will occur if the differential electromotive force between the boats metal and anode installed is too large. Magnesium anodes have a very strong electronegativity charge if used in saltwater they will not last long the reaction so vigorous damage to the boat is likely. Magnesium anodes are used in freshwater, but should be removed and replaced with zinc anodes before the boat goes into salt water. Wooden boats if overprotected can experience galvanic wood rot. Once wood rot has started the deterioration of wood produces acids that increase the electromotive force. This accelerates the damage to metal components and adjacent wood. When choosing anodes for boats with aluminum in water components care should be taken not to over protect the boat. So Mercury, Bombard, Johnson, and Evinrude recommend and sale aluminum anodes to protect outboards and outdrives components. Here's why is the voltage of aluminum is -1.1V compared to -1.03V of zinc making the aluminum outdrive or hull the anode relative to zinc. So the hull or outdrive would be eaten away would be eaten away. Also aluminum boats in saltwater using magnesium anodes may develop hydrogen or oxygen bubbles on metal surfaces causing paint to blistering off. Magnesium and aluminum anodes are used in freshwater in place of zinc anodes which form an oxide skin insulating the zinc and reducing electron flow interfering or halting protection of the boat.


When attaching anodes do not obstruct water flow to the Cutlass bearing allow a distance of 1 1/2 shaft diameters between Cutlass bearing and anode collars. The area where the zinc will be attached must be cleaned. This is can be done with sandpaper paper a wire brush or wire wheel when clean the metal should be bright and untarnished. The anode should be tightened to make significant electrical contact with the metal component and not come loose or fall off during operation. When attaching the anode to shafts I will strike each half of the sink with a small hammer to ensure they are seated before checking the final torque of the fasteners.

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copyright2008 Terry Capp

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