Anode Weight Calculation Formula

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

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 (8760 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) 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