Corrosion of metals is greatly promoted by the influence of an electrical current. Anodes are commonly used to help protect the hull in the case of galvanic corrosion caused by electrical current flowing between connections and between dissimilar metals.

Stray current corrosion is different in that it is caused by electricity from an outside source flowing into the boats metal components and out through the water for a ground.

Commonly caused by problems in the DC circuit, the solution is to have the DC system installed correctly in the first place!

Anodes will offer protection based on the Noble Series of metals. The theory is that by attaching a piece of metal to the hull that is more reactive than the hull itself then this piece of metal will corrode first. For our inland waters we use sacrificial magnesium anodes for protection. As a note of general interest, aluminum is used in brackish water and zinc in seawater.

In order to understand what an anode is and why we need them we firstly have to consider how a battery works.

Chemical Reactions

Two plates or rods of different metals are placed in a conducting liquid. The rods are known as electrodes and the liquid is known as the electrolyte. If the two rods of dissimilar metal are in direct electrical contact a simple battery is formed and a small DC voltage drives a current from one electrode to the other via the electrolyte.

This process of generating voltage which drives the current (amps), results in electrons being lost from the metal atoms in one of the electrodes. This loss of electrons produces electrically charged particles called ions which dissolve into the electrolyte. Very simply, they travel from one rod to the other through the liquid. The top & bottom of it is that one of the rods or plates looses material in the form of ions, known as the anode, the other rod gains material, known as the cathode, and which metal looses the material depends upon how noble it is.

Noble Metals

The most noble metals are the ones that are the most resistant to corrosion with, for simplicity, the highest one being gold. Starting with gold, metals can be arranged into a Galvanic Series with gold at the top being the least reactive and magnesium at the bottom being the most reactive. The corrosion between metals in the galvanic series is known as galvanic corrosion.

Mixed Metal

So, back to our boats and their metal hulls.  A typical mild steel hulled canal boat will have a bronze (alloy) propellor, a stainless steel propellor shaft and a brass stern gland. All the metals in the system are in some form of direct contact with each other. In the galvanic series, mild steel is more active than the other metals so when they are all immersed in fresh water (a weak electrolyte) it will become the anode and start to loose metal and corrode.

Magnesium Anodes for Canal Boats

Introduce magnesium, or magnesium alloy into the system and this will corrode before the mild steel, hence the use of weld-on magnesium anodes on hulls in fresh water.

Zinc or Aluminium Anodes

Sea water or brackish estuary water are stronger electrolytes so anodes of zinc and aluminium respectively are used as magnesium, being the most reactive, would corrode too quickly to be cost effective.

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.

Expert Advice

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 Fit Out Pontoon 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?

By Weight

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.

Luckily anodes 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.

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.

Central Anode Placement?

Some hull builders include a specially constructed indented box midway along the sides of the hull for exactly this. The idea being that the anode ingot is protected from fouling on underwater obstructions such as lock walls. There are plenty of boats on the system which have centrally mounted anodes with no boxed protection that have cruised the system for years with no fouling issues, of even a scrape for that matter.

The fear of fouling up on centrally mounted anodes comes from anecdotes and guesswork rather than any hard evidence.

If you are in any way unsure, or are concerned about fouling, drop a plumb line from the upper rubbing strake down to the base plate next time your boat is out of the water. This should indicate the amount of tumble home there is on the hull sides & whether any anode would project further than the rubbing strake. There is always the option of having anode boxes welded into the hull sides as a retrospective fit but this would be expensive. There are low profile anodes available so it’s worth shopping around.

Protecting The Base Plate

In reality very few boats have anode protection on the base plate. Pitting can be just a bad on the base plate as it can be on the hull sides. Again, anecdotes and misguided gossip suggest that “the lack of oxygen and light” prevents corrosion. It is a fact that the murky depths of our canals are a less effective electrolyte than highly charged, bright & well oxygenated sea water, but corrosion still occurs. Walk around canal boats that have been pulled out of the water for repairs or blacking and you will see that the older hulls have base plate pitting.

The principle reasons anodes are not fitted to the base plate are two fold.
Firstly cost, secondly, we can’t see it! If anodes were £1.00 each, we’d have them all round the boat. If the base plate was more accessible, we’d black it and fit anodes!