For steel and aluminum hull boats, the following formula can be used to calculate the
total weight (in pounds) of sacrificial anodes required on the hull.
Anode Weight (lbs) = [(Wetted Surface Area) x (Current Density) x (Immersion)]
[(Energy Content) x (1000 mA/Amp)]
Wetted Surface Area (sq-ft): |
Actual or Estimated Wetted Surface of Hull |
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Current Density (mA per sq-ft): |
1.5 – 5.0 for painted Steel Hulls
0.2 – 3.0 for painted Aluminum Hulls |
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Immersion (hours): |
Number of hours in water per replacement
interval (8766 hours per year) |
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Energy Content (amp-hours per pound): |
368 for Mil-Spec zinc anodes
1108 for Mil-Spec aluminum anodes |
Estimating Wetted Surface Area:
= 1.0 LWL x (Beam + Draft) for full displacement vessels (motor yachts and sail boats)
= 0.75 LWL x (Beam + Draft) for medium displacement vessels
= 0.50 LWL x (Beam + Draft) for light-displacement vessels
Source: ABYC Corrosion Certification Study Guide
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