Zinc Anode Calculation for Steel Hull

[schmidp]

Hello everyone,

I need some help calculating the required zinc anode weight for my steel boat, because the coating from the last owner came off when washing it with a high pressure washer, I am going to sandblast the hull on the weekend and repaint it from bare metal.

Since I am already stripping it down, I want to ensure I have the right amount zinc anodes as now would be the best time to weld in new bolts.

It's 9.2 meters long, 2.7 meters wide and 1.25 meters draught.
The waterline is about 8 meters long.

I found the following link to calculate the amount of anodes:

Anode Weight Calculation Formula

8m = 26.2467feet
2.7m = 8,85827 feet
1.25m = 4,10105 feet

Anode Weight (lbs) = [(Wetted Surface Area) x (Current Density) x (Immersion)]
[(Energy Content) x (1000 mA/Amp)]

26.25 * (8.86 + 4.1) * 2ma * 8766 hours / (368 * 1000) = 16.21 lbs

16.21 lbs = 7.35kg zinc anodes.

Does that make sense? Or does this seem off?

Currently the boat has 8 anodes at the hull and 4 round ones at the rudder.
Most of the anodes are more than half gone, so I think it is not over-zinced.

 

[bluerm166]

The MG Duff catalogue here https://www.mgduff.co.uk/downloads/Leisure Catalogue.pdf appears to agree with the 8 + 2 for 2 yrs. where your wetted surface exceeds 28 M .S. and you are probably just around that area . They have not changed this advice for many years so I assume experience supports it.
Not sure why you couldn't have 6 + 2 though.But perhaps that would place them too far apart - the patterns they show having significance.If you are in salt water then the total weight of ZD78 zincs will be considerably more than your calc indicates.

 

[schmidp]

Thanks @blurm166, that is an interesting link!

They seem to suggest much more zinc then the link I posted though.

The anode they mention weights 4kg. So 6-8*4kg = 24 - 32KG of anodes, while the link I found I would only need 7-8kg.

Hmm...

 

[schmidp]

Ahh I just learned:

The exposed surface area of the sacrificial anodes should be in proportion to the exposed surface area of the metal being protected. The exposed surface area is what’s important, rather than the volume, or the weight of the piece.

So the weight might be more of a "how long the anode lasts" thing and not really relevant to the correct "sizing" of anodes.

 

[Bilgediver]

Ahh I just learned:

So the weight might be more of a "how long the anode lasts" thing and not really relevant to the correct "sizing" of anodes.

Yes that is quite correct and MG Duff do have a technical dept to answer questions like this . Your information may be pertinent to them when discussing this as yes you may have the incorrect amount of protection.

MG Duff Tel. +441243533336