Not So Straightforward
Any scheme of anode protection is quite difficult to work out. It will depend upon the quality of steel, what metals are close or in contact with one another, the quality of paint protection and finally the reactive nature of the water itself.
The majority of technical information available relates to ocean going vessels and is biased towards the stern as this is the most vulnerable area.
As such, the information relating to anode requirements for steel hulled craft on fresh water inland waterways is at best scant, and at worst, non existent.
A leading global manufacturer of anodes
for marine protection has worked with us to try and clarify how we should be using anodes
to protect our hulls in the most efficient manner.
They have advised The FitOutPontoon that anodes
are only really effective over a certain area. A rule of thumb is they are effective in all directions for about 7 times their length. A more acceptably easy way to gauge this is to place anodes
along the sides of the boat centered every 4 to 5 m.
That's the rule of thumb, but as anodes
are different sizes, what’s the theory behind what size we need?
The manufacturer has produced a table which shows the weight of magnesium required to cover a certain surface area of steel.
up to 28m² = 4.5kg-9kg
28.1m² to 42m² = 11kg-13kg
42.1m² to 56m² = 14kg-20kg
56.1m² to 70m² = 21kg-26kg
Firstly we need to work out the weight of anode material we need relative to the square m of the steel we need to protect.
This easy on a canal boat and is done by the underwater portion of both of the hull sides plus area of the base plate in m2.
i.e. a working example of a 42’ steel narrowboat with an average draft of 24”
(12.8 x .61) x 2 plus 12.8 x 2.1 equals 42.5m2
Therefore in this example the manufacturers recommended weight of magnesium anodes
would be 14kg to 20kg.
are sold by weight. Common sizes are 1.5kg, 2.5kg and 3.5kg ingots for standard weld-on canal boat anodes
. Smaller sizes are available, especially in a bolt on format for protecting propellers and stern gear on smaller river cruisers.
In our example above, the hull would require 6 x 3.5kg anodes
or 8 x 2.5kg versions.
The above figures are for a 2 year scheme, but in reality as most owners tend to look after their investments a 3 year plan of inspection and replacement should be fine for most.
Where to Put Them
Now we have calculated the weight, it is time to think about placement. Conventionally anodes
are placed 2 at the stern and 2 at the bow. Perfectly acceptable practice but now we understand how they work we can perhaps consider a slightly different approach.