Anode Quantities & Placement
When determining the cathodic protection requirements for your vessel the main factor to consider is the type hull material on the vessel and type of water.
How many anodes do I need?
Steel Hull GRP
When determining the cathodic protection requirements for a steel hulled vessel the first criterion is the surface area of steel hull below the water line - the wetted surface area.

This is calculated by multiplying the water line length by the sum of the beam and draft i.e. LWL x (Beam+Draft). This calculation will apply to most motor cruisers and sailing boats.

Once the wetted surface area is calculated, it can be applied to the selection tables below from which the number of anodes required can be taken.
Salt Water Brackish Water Fresh Water
For one year in Salt Water fit the following Zinc anodes:
Area Hull Anodes per Rudder
Up to 28m2 Stud Fixed 2 x ZD78B 2 x ZD56
(300ft2) Welded 2 x ZD78 2 x ZD76
28.1 - 56m2 Stud Fixed 4 x ZD78B 2 x ZD56
(300 - 600ft2) Welded 4 x ZD78 2 x ZD76
56.1 - 84m2 Stud Fixed 6 x ZD78B 2 x ZD56
(600 - 900ft2) Welded 6 x ZD78 2 x ZD76
84.1 - 102m2 Stud Fixed 4 x ZD72BM 2 x ZD58
(900 - 1100ft2) Welded 4 x ZD80 2 x ZD77
102 - 148m2 Stud Fixed 6 x ZD72BM 2 x ZD58
(1100 - 1600ft2) Welded 6 x ZD80 2 x ZD77
For two years in Salt Water fit the following Zinc anodes:
Area Hull Anodes per Rudder
Up to 28m2 Stud Fixed 4 x ZD78B 2 x ZD58
(300ft2) Welded 4 x ZD78 2 x ZD77
28.1 - 56m2 Stud Fixed 8 x ZD78B 2 x ZD58
(300 - 600ft2) Welded 4 x ZD78 2 x ZD77
56.1 - 84m2 Stud Fixed 4 x ZD72B 2 x ZD58
(600 - 900ft2) Welded 6 x ZD73 2 x ZD77
84.1 - 102m2 Stud Fixed 6 x ZD72B 2 x ZD78B
(900 - 1100ft2) Welded 6 x ZD73 2 x ZD78
102 - 148m2 Stud Fixed 8 x ZD72BM 2 x ZD78B
(1100 - 1600ft2) Welded 8 x ZD73 2 x ZD78
Anode Placement
Anode Placement for a 2 Anode System Anode Placement for a 4 Anode System Anode Placement for a 6 Anode System Anode Placement for an 8 Anode System
When fitting and replacing bolt-on anodes always ensure that the serrated fan disc washers are replaced
Related