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For steel and aluminum boat hulls (or underwater structures), the following formula can be used to calculate the total weight (in pounds) 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
Current Density (mA per sq-ft):
1.5 – 5.0 for painted Steel hull / structure
0.2 – 3.0 for painted Aluminum hull / structure
Immersion (hours):
Number of hours in water per replacement
interval (8766 hours per year)
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) in seawater 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:

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