SYSTEMS TO CALCULATE THE PROTECTION OF THE HULL OF A SHIP

1. Density of the current:

The electrochemical and mechanical conditions have a large influence on the design of the cathodic protection system.

Other conditions to bear in mind are: temperature, salinity, resistance to the dissolving of oxygen, etc.

The specifications of a cathodic protection system are normally expressed by the density of the electrical current required to give the surface which has to be protected a sufficient potential.

The normal density of the current for ship hulls varies from 10 m A/m2 to 30 m A/m2, although it can be increased in special cases.

2. Life of the Anode:

With anodes, you can usually calculate a life from one to four years while for Aluminum Anodes you can expect them to last for approximately five years.

3. The number of Anodes:

The total current needed is calculated by the following formula:

Area (m²) x density of the current in mA/m² 


Current in Amperes = 


1.000 



The total weight of the anodes:

Current (A) x life of the Anodes (Years)x 8.760 


Weight (Kg.) = 


Capacity of the material (A Hora/Kg.) 



The number and type of Anodes to compensate for the total of the current and the weight required:

Current required

Number of Anodes = 


Current leaving each anode


Calculated weight 

Number of Anodes = 


Weight of each anode



4. The position of the Anodes:

The anodes must be distributed conveniently around the wet surface of the hull, near, above and below the balance keels, increasing in number in the zone of the stern due to the high density of current originating from the propeller. We also recommend the installation of anodes in the sea valves, tuyeres, propellers, etc.

This normal practice can be varied depending on the geometry of the ship, its system of painting or also on the expected use of the vessel.


5. Recommended practice for the installation:

a) Our anodes are in the form of plates (stripper bars) so they can be welded directly onto the hull. We recommend that the ends of the plates are not galvanized to avoid the toxic gases which are produced when they are mounted. 
b) The distribution of the anodes will have to be according to our design 
c) Anodes must not be placed on the bottom due to problems in entering and leaving the dock 
d) The anodes must never be painted