Found this text at www.boatdesign.net site, talking about zinc and how much should a boat have it:
Here’s something to go on from a guide I wrote a while back. This is the ’ideal’ protection regime. If the paint job is a good thick epoxy job and the boat is hauled a least annually then trying fewer anodes won’t hurt. If you are in warmer water then go and look at the anodes and you’ll see what they are doing. Bolt on anodes are easily replaced in-water with breathing gear.
Guppy anodes larger anodes hung externally when tied up or moored long term are good practice since the anode is well away from the hull.
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To calculate hull anode requirements
The level of protection required is quoted as a current density (Amps per square metre) and can vary from 10 to 40 mA per square metre (a milliAmp is 1/1000 of an amp)
coastal vessels need a slightly higher protection than ocean going ships and should be provided in clean coastal waters with around 20 mA/m^2.
To calculate the number and weight of anodes on a new vessel:
1: Work out the area to be protected, ie the area of your hull/structure under the water in square metres
2: Multiply this area by the current density of 0.02 to get the total continuous current required (use other values only if previous experience dictates or on professional advice).
3: Multiply the continuous current by the number of hours in a year (8760 hours) to get an amp-hour requirement for the period you want protection (usually one year, otherwise whatever period is required in hours).
4: The total anode weight required in kilograms is now easily found by dividing the amp-hour figure from step 3 by the amp-hours per kilogram potential of the anode material ( Zinc 730Ahrs/kg, Magnesium 1250Ahrs/kg, Aluminium 2500Ahrs/kg ).
5: The number of anodes required is found from the total weight calculated from step 4 divided by the individual net weight of the anodes to be used. This will be co-dictated by the spacing required (see anode location below).
Anode location
Evenly around the boat, each anode gives a protective radius of around 3m. Space the anodes up to 6m apart either welded or bolted to the hull and keel. Best sited on the lower curve of the hull in protected positions (from collision) and on the keel sides. The anodes must be continually immersed to work. Tend towards greater protection with the dissimilar metals at the stern. If you are unsure of how to arrange your spacing, start at the stern and work forwards.
Note that streamlined anodes should have the blunter end be closest to the bow for least resistance.
When calculating the mass for 2 or 3 year protection; rather than using lots of small anodes it is better to use fewer anodes of greater mass to reduce the surface area and avoid problems from over-protection.
www.boatdesign.net/threads/corrosion-on-steel-hull-around-zincs.47123 MikeJohns, Senior Member