Attaching sacrificial anodes to boats with aluminium hulls

Key instructions

  • To provide the best protection for the stern gear, the propeller shaft should be fitted with an electro-eliminator brush and earthed to the hull.
  • The gearbox should be bonded to the hull using 4mm² multi-stranded cable and any flexible couplings in the shaft should be bridged with a copper strap or cable.
    • When fitting and replacing bolt-on anodes, always ensure that the serrated fan disc washers are replaced.
    • Do not paint the surface of the sacrificial anode.

Positioning the anodes

  • The anodes should be positioned on the outside of the hull, below the waterline.
  • Prepare and paint the hull in accordance with a reputable paint manufacturer’s specifications before fitting the anodes.
  • Fit the anodes using galvanized mild steel set bolts with fan disc washers and ensure that the anodes are fixed tightly against the hull.


To ensure an optimum level of protection, anodes should be replaced when 80% consumed or if it is anticipated that they will go beyond this level of corrosion before the next scheduled docking.

Diagram of two, four and six anode systems.

ZD42 strip anodes

  • ZD42 strip anodes are low-profile zinc anodes with non-reactive aluminium inserts and are ideally suited for fitting to high-speed aluminium-hulled boats. The anodes are not recessed, but are prominent on the surface of the hull and can be attached after construction.
  • Each type of ZD42 strip anode requires three or more fixing positions. The ZD42/26 and ZD42/54 anodes need three fixing positions, while the ZD42/72 requires five fixing positions on standard boats, rising to seven on fast craft.
  • ZD42 anodes are supplied in 1,830mm lengths and can be cut to length. They can be supplied un-drilled or drilled and counter-bored to standard centres. Fixing holes are drilled to suit 10mm studs and it is imperative that the fixing hole is counter bored down to the aluminium core.

Hull-mounted ZD55 recessed disc anodes

  • 225mm diameter hull-mounted ZD55 recessed disc anodes are generally used for flush mounting when minimum drag is required. Hence, the anode is normally incorporated during the construction process.

Diagram of hull mounted ZD55 recessed  disc anodes Diagram of hull mounted ZD55 recessed  disc anodes

ZD72B(M) zinc hull anodes

  • Low-profile cast ZD72B(M) zinc hull anodes are suitable for lower-speed craft on which the surface mounting will not result
    in excessive drag. These anodes can be attached after construction.

Diagram of ZD72B(M) zinc hull anode

Diagram of ZD72B(M) zinc hull anode