Hull Zinc

[Capt. G]

A question for those that have a slip or mooring. I know that every harbor is different but how often do you find that you need to replace the hull zinc on your boat? I pulled mine after being in the water for 30 days and I can see that the zinc is working but it does not appear to be excessive. BTW...the bottom (unpainted) had only a little slime on it after 30 days and the pressure washer took it right off...looks like new again. I plan on pulling her every 30 days for inspection, washing, and engine maintenance.

[21ftcc]

You should replace your anodes when they appear to be half their original size. A season is considered 6 months. There are many factors that determine how long you anodes will last. If you use the boat longer or if you stay in the water year-round, you will have to change your anodes mid-season. If you dock near a steel bulkhead or near a vessel that has an electrical issue the anodes will also dissolve quicker. Other issues include water salinity, temperature, stray currents, and other external influences, you only know the right weight by actual trial. To lengthen the replacement interval, increase the weight of zincs used. Another issue is shore power. You could theoretically disconnect the ground connection in the shore power and avoid electrolysis, and in some cases this is a solution, however ABYC regulations require the AC ground be connected to the DC ground so an electrical fault on the boat won't electrocute swimmers in the vicinity.


The formula below 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)]

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

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
Source: ABYC Corrosion Certification Study Guide