Sacrificial Anode Sizing
Cathodic protection design following DNV-RP-B401. The current demand comes from the wetted area, the design current density for that water, and how much of the coating has broken down; the anode has to supply it for the whole design life. Three inequalities have to hold at once, and the interesting part is which one binds:
(5) enough metal — N·Ca ≥ Imean·t·8760, the alloy has to last. (6) enough output on day one — N·Iai ≥ Iinitial, to polarise a bare surface. (7) enough output at the end — N·Iaf ≥ Ifinal, when the anode has wasted to its utilisation factor and its resistance has climbed. Mass is about the alloy; output is about the shape. A few large anodes usually pass (5) and fail (7); many small ones do the reverse.
What you are protecting
The anode you propose
How many
Design data and procedure from DNV-RP-B401 (October 2010, amended April 2011), whose current densities and −0.80 V protective potential are for carbon and low-alloy steel. An aluminium hull is a different problem: it needs aluminium anodes, never magnesium in seawater, and a potential window with a negative limit as well as a positive one, because over-protection makes alkali at the surface and attacks the metal you were protecting. Anode distribution, interaction, fastening, and current drain to shafts, propellers and bonded equipment are all outside what this computes. A helper, not a substitute for engineering judgment.