For steel and aluminum boat hulls (or underwater structures) the following formula can be used to calculate the total weight of sacrificial anodes required.
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 Metal |
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Current Density (mA per sq-ft): |
1.5 – 5.0 for painted Steel hull / structure
0.2 – 3.0 for painted Aluminum hull / structure |
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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 of Boat Hulls:
= 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
Estimating Current Density:
The amount of electric current required from anodes protecting a metal hull (or structure) is primarily a function of the water's flow rate and the quality of the metal's protective paint coating. BoatZincs.com issues the following design estimates for sea water:
Steel Hull or Structure
Stationary (0 - 0.5 mph):
Well coated = 1.5 mA/sq-ft
Poor or old coating = 2 mA/sq-ft
Uncoated = 3 mA/sq-ft
Low Velocity Water Flow (0.5 - 2 mph):
Well coated = 2 mA/sq-ft
Poor or old coating = 4 mA/sq-ft
Uncoated = 5 to 15 mA/sq-ft
Medium Velocity Water Flow (2 - 5 mph):
Well coated = 3 mA/sq-ft
Poor or old coating = 5 mA/sq-ft
Uncoated = 15 to 30 mA/sq-ft
High Velocity Water Flow (>5 mph):
Well coated = 5 mA/sq-ft
Poor or old coating = 10 mA/sq-ft
Uncoated = 25 to 100 mA/sq-ft
Aluminum Hull or Structure
Stationary (0 - 0.5 mph):
Well coated = 0.5 mA/sq-ft
Poor or old coating = 1 mA/sq-ft
Uncoated = 2 mA/sq-ft
Low Velocity Water Flow (0.5 - 2 mph):
Well coated = 1 mA/sq-ft
Poor or old coating = 2 mA/sq-ft
Uncoated = 4 to 8 mA/sq-ft
Medium Velocity Water Flow (2 - 5 mph):
Well coated = 2 mA/sq-ft
Poor or old coating = 3 mA/sq-ft
Uncoated = 5 to 12 mA/sq-ft
High Velocity Water Flow (>5 mph):
Well coated = 3 mA/sq-ft
Poor or old coating = 5 mA/sq-ft
Uncoated = 10 to 25 mA/sq-ft
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