Sacrificial anodes can help control galvanic corrosion, but a generic guide cannot approve a vessel cathodic-protection system. Real protection depends on hull material, underwater metal mix, coating condition, salinity, shore-power exposure, DC stray-current faults, bonding continuity, anode product chemistry, installation details, hull-potential measurements, and the current standards or survey requirements that apply to the vessel.
Use this guide as source-boundary planning context for zinc, aluminum, and magnesium anode conversations. It explains the local prompts used by the sacrificial-anode screen and the questions to carry to a corrosion engineer, marine surveyor, boatyard, ABYC technician, manufacturer, or insurer before buying hardware or changing placement.
Galvanic and Stray-Current Context
Galvanic corrosion involves electrically connected metals in an electrolyte. A sacrificial anode is intentionally more active than the protected metal, so it can supply protective current when the bonding path, water chemistry, coating condition, and installation details allow it.
Stray-current corrosion is a separate source gap. A DC fault, shore-power problem, damaged bonding conductor, wet connector, or marina condition can overwhelm normal anode behavior. A calculator cannot distinguish those conditions from normal consumption; field inspection and measurements are required.
The source-aware anode screen therefore keeps bonding, salinity, shore power, reference-electrode readings, and product data visible as review items instead of presenting a single weight result as proof of protection.
Anode Alloy Selection Boundaries
Zinc, aluminum, and magnesium anodes are not interchangeable product approvals. Alloy behavior depends on specification, salinity, water chemistry, coating, protected metals, mounting, and the actual vessel environment.
The app uses local water-type prompts to suggest a review row, but final alloy choice must come from current ABYC and project requirements, exact product data such as ASTM or military/QPL references where applicable, manufacturer instructions, and a qualified corrosion or marine survey review. Fresh-water magnesium and aluminum-hull decisions deserve extra caution because overprotection and coating damage can be real risks.
When a boat moves between salt, brackish, and fresh water, the review should use actual operating history and anode consumption, not a one-time salinity label.
Sacrificial Anode Weight Calculator
Size zinc, aluminum, or magnesium anodes for hull, shaft, and propeller corrosion protection by water type and surface area.
Sizing Prompts and Placement Limits
The local worksheet uses the familiar relationship between current demand, service hours, anode amp-hour capacity, and utilization. Example shape: weight = current demand × service hours / capacity, with the app keeping its local safety factor and capacity rows visible.
That arithmetic is only a screening prompt. Current density, utilization, anode shape, mounting hardware, electrical contact, flow, coating, shielding, aluminum-hull effects, and placement are product- and vessel-specific. Protected standards and manufacturer data should be used instead of copying local placeholder rows into a work order.
The app also separates entered area from current-basis area. Fiberglass and wood hull area may be useful context, but it is not counted as conductive hull metal in the local current demand. Steel and aluminum hull prompts carry additional warning language.
Inspection and Replacement Evidence
Anode condition should be reviewed with the vessel out of the water, product data in hand, and the bonding path accessible. Consumption percentage, surface condition, looseness, paint, broken conductors, coating damage, salinity changes, and nearby shore-power conditions can all change the interpretation.
A low-consumption anode can be isolated, passivated, painted, oversized, or simply operating in a different environment than assumed. A fast-consuming anode can point to undersizing, wrong alloy, high salinity, stray current, coating failure, or a bonding fault. The app does not decide which is true.
Reference-electrode readings, bonding continuity, galvanic-isolator or isolation-transformer status, DC fault checks, and product/manufacturer limits belong in the qualified review before anode count, material, or placement is changed.