Sacrificial anodes are something every boat owner should be aware of, yet not every boat owner will need them. The principle is quite simple; the material that your boat is made of and the environment in which you keep it has the propensity to cause serious metal loss through corrosion if things get out of balance. Sacrificial anodes are there to help mitigate minor imbalances in this situation but cannot in themselves fix a problem that is out of control. That all sounds rather nebulous doesn’t it?
The process we are concerned with here is galvanic corrosion; the corrosion of one metal in the presence of another where both are electrically connected by a conducting medium. The variables here are the types of metal involved and the medium in which they are used (fresh, brackish or salt water). We can deduce which metals are more susceptible to galvanic corrosion by considering their relative potential in a series known as the Galvanic Series. Here is a simplified galvanic series for a selection of typical marine metals in salt water (the relative order of the metals may change slightly in different marine environments).
- Gold – Most ‘noble’ metal
- Stainless steel 316 (passive)
- Stainless Steel 304 (passive)
- Silicon bronze
- Stainless Steel 316 (active)
- Monel 400
- Phosphor bronze
- Admiralty brass
- Brass plating
- Yellow brass
- Naval brass 464
- Stainless Steel 304 (active)
- Chromium plating
- Cast iron
- Mild Steel
- Zinc anode
- Aluminium anode
- Magnesium anode – Least ‘noble’ metal
So whilst it would be really useful to build our boats out of gold and titanium, they are not the most practical of hull building materials for several reasons. You may be surprised to see plain old mild, steel so far down the table and that is probably the most significant observation to make. Mild steel is readily corroded by almost all of the other marine metals, however that is mitigated by what is called the surface area effect, whereby a less noble metal (e.g. a steel hull) can lead to the corrosion of a more noble metal (e.g. a small brass skin fitting) due to the imbalance in their relative surface areas.
This brings us to the final consideration, how effective is any given anode? The choice of anode material is dependant on the medium in which the vessel is predominantly used. Magnesium anodes are best suited to fresh water, aluminium for brackish waters and zinc for salt water. An easy way to remember this is to remember how water flows with this cunning euro-mnemonic “Mountains Am Zee”: M (fresh water – magnesium) A (brackish – aluminium) Z (sea – zinc). Next to consider is the size of the anode and how much metal it can ‘protect’. There is no precise answer as to how effective any individual anode will be as its position, electrical contact and the size and composition of nearby metals all influence its performance. As a rule of thumb however, we should expect a typical anode to be effective to a distance of 6-7 times its own length. So for a typical 3.5kg magnesium narrowboat anode that is approximately 300mm long, it should be effective to a distance of about 2 meters in any direction.
Narrowboat anodes can be fitted as either welded or bolt on.
A sacrificial anode’s effectiveness is directly proportional to the purity of the metal from which it is cast. Zinc anodes must be more than 99% pure zinc if they are to function effectively and their performance is greatly reduced by the presence of impurities such as lead or iron. So whilst high quality zinc anodes from a reputable supplier are expensive, you are paying for this guarantee of quality that the anode will do its job. There are plenty of lesser quality anodes available online which come at a greatly reduced price. These are typically cast by scrap merchants melting down old zinc fittings and part worn anodes without the equipment or expertise to refine the resultant metal. As is nearly always the case; you get what you pay for.
So how can we make best use of this knowledge to keep our boats in good order? The deductions we make here are very dependant on what type of vessel you have, what is has been built from and where you use it. Narrowboats are most commonly found with a pair of anodes fitted at the bow and another pair fitted at the stern, typically on the swim, with occasionally a smaller anode on the rudder. Noting our observation regarding the effective range of an anode, this leaves much of the mid-section of a narrowboat unprotected. Given that this is where we often find the heads and the galley with their associate through-hull fittings, this can lead to galvanic corrosion in this area of either the hull or the skin fittings if they are made of cheap non-DZR (dezincification resistant) brass. So for a narrowboat, the optimum anode fitting plan is to fit them in pairs on either side at 1m from the bow, 1m from the stern and then evenly spaced at 4m intervals along the hull sides, fitted below the waterline and just above the chine. A final few key ‘rules’, don’t fit them on the bottom plate as they will get snagged or rubbed off or the hull will rest on them when the hull is slipped and never paint anodes as that stops them working completely.
The use of anodes on FRP sailing and motor yachts is slightly more nuanced given the absence of a large steel hull and instead a non-conducting hull material. Here we are concerned with the various metal through-hull fittings, the propshaft/s, propeller/s, p brackets, outdrives and any below the waterline hull fastenings used in the construction. Most modern boat propellers are cast from either manganese bronze or stainless steel mixed with several other alloying components to deliver a strong, durable propeller which is corrosion and erosion resistant and all within a cost envelope. Skin fittings found on FRP vessels on the other hand range from domestic grade yellow passivated brass bathroom fittings to top quality cast fittings from reputable companies such as Blakes. Here the surface area of the propeller can outmatch a nearby low quality fitting or the presence of a couple of top quality fittings can outmatch the propshaft or rudder bearings. Due to the lack of electrical connectivity offered by the hull itself, we now introduce the principle of bonding to enable the sacrificial anodes to be suitably ‘connected’ to the full suite of skin fittings and other machinery to be protected. This is done internally with suitable marine grade tinned copper strand wire and marine grade fittings.
A final consideration in how to protect your boat from galvanic corrosion is the risk posed to it by other vessels via shore power connections. The earth wire is connected as a common ground for all vessels and the pontoons themselves. There have been many instances of vessels suffering dramatic and in some cases terminal corrosion due to the faulty connections of shore power terminals or nearby vessels, which can turn your own hull into one massive anode and wreak havoc with propshafts, props and if a skin fitting is compromised the vessel is at risk of sinking.
A galvanic isolator (sometimes also referred to as a zinc saver) is fitted into the vessel’s earth wire immediately after the shore power connection socket and before the earth wire connects to the boat electrics in any way. It is a solid state device which blocks any stray currents and prevents your boat’s anodes being used to protect nearby vessels and the pontoon! They are not particularly expensive but are worth the outlay for the protection that they offer, especially if your vessel spends a lot of time connected to shore power.
A more thorough (and expensive) solution is to fit an isolation transformer, which introduces a physical break in all three incoming shore power wires and transfers power onto the boat magnetically via the transformers windings. This can provide a very safe electrical environment for working on the vessel’s electrical systems and for any swimmers in the water as the boat is no longer physically connected to the shore side power circuit. They are also excellent at preventing local stray current corrosion, but that is a subject for a blog in itself.
- Fit the right anodes for your environment “Mountains-Am-Zee”
- Buy quality anodes from a reputable source
- Fit them according to your vessel type at the recommended scaling
- Make sure they are below the waterline but not on the bottom plate
- Consider whether your vessel needs bonding
- Examine your shore power connection and upgrade it if possible
- Do not paint your anodes or grounding plates
© David Pestridge 2020