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Boat Anodes | Protecting your Boat

Welcome to our information page on choosing, fitting and understanding anodes for your boat. Below is some helpful advice on what protection the boat anodes offer and common problems and misconceptions experienced.

We stock one of the most comprehensive range of boat anodes you will find in the UK, from MG DUFF to genuine Honda, Yanmar and others. If you want to look through our selection of anodes from the catalogue, please select the relevant link below.

Please Click Here to view all our specialist anodes.
Boat Anodes | We Need Them

Boat Anodes | We Need Them

Essentially, when you submerse metallic items in any kind of water or conductive medium for any period of time, galvanic corrosion will occur due to differences in the potential (charge) on the metals. The amount of corrosion will depend on many factors, including the type of metal, conductivity of the water, surrounding electrical activity, and the bonding and type of metals which are in contact with the water.

To reduce the effect of this corrosion, boat owners fit sacrificial anodes to their craft, which effectively concentrates the galvanic corrosion onto these anodes by offering the destructive galvanic currents a more favoured target. When bonded with a metal with less favourable galvanic properties (or a lower cathodic index), the boat anodes (which have a higher cathodic index) will naturally create a more favourable electrical potential and therefore protect the more valuable metal parts (propellers, stern drives and the like). This property is even true when considering the application of paint & antifouling, as copper-based antifoul, if applied directly to a metal substrate, may cause similar problems.
Protecting your boat

Protecting your boat

It is obvious that the rate of decay of boat anodes will vary considerably, even from boat to boat in the same area. It is therefore essential that you check the decay ("wear") on the anodes periodically, and ensure that there is no possibility of them "wearing out" and offering no protection.

A yearly inspection is advised, though if problems have been experienced in the past, more frequent checks may be necessary. When the boat is lifted out for annual maintenance, if any specific anode has more than 60% wear, it is normal to have it replaced.

It is important to realise that the effectiveness of the sacrificial boat anodes will depend on how well it is bonded to the material it is protecting. This is true in the case of fitting shaft anodes, where the surface area of contact between the anode and the shaft may be small in comparison to the surface area of metal being protected. Be aware also that loose or badly fitted anodes will have a reduced capacity for protection, and that you should never paint over an anode or in between the contact surfaces where it is fitted.
Anodes | Zinc, Aluminium and Magnesium

Anodes | Zinc, Aluminium and Magnesium

Boat anodes are manufactured from 3 different types of metal - Zinc, Aluminium & Magnesium. These metals are chosen for their cathodic index and for their other properties that make them suitable. The metal chosen for your protection will depend on where the boat is situated and its area of use.

Zinc anodes - These anodes are mainly used in salt water however they can also be used in brackish water.

Aluminium anodes - Most commonly used in brackish water.

Magnesium anodes - Used in fresh water.

It is vital that you choose the correct type of boat anodes as if used in an environment outside that stated, the ability of the anode to work will almost certainly be compromised. If you are likely to use your boat in waters other than those for which the current fitted anodic protection is designed for more than a few days (2 weeks maximum), you should consider fitting the correct type of anode for the waters you are visiting.

Hanging anodes are also available which can be easily dropped into the water so you can be sure that the hull and stern gear on your boat is protected if you are likely to travel between water types.
Zinc and aluminium boat anodes, whilst in fresh water, will develop a white oxidisation, which will effectively prevent the anode from operating. Similarly, magnesium boat anodes are more reactive than zinc or aluminium and when used in salt or brackish waters will decay far more rapidly and often cause deposits on other protected surfaces.
Common Boat Anodes

Common Boat Anodes

Most craft are protected by a standard set of boat anodes - namely the ones you find on the shelf in most chandlers. The availability of such boat anodes is normally good and the range offers solutions to most manufacturers/owner's needs.

Many engine manufacturers have specifically designed anodes for their engines and drive systems, but in most cases these are common parts that can be ordered or are in stock. Certainly, the range of hull anodes has been well defined over the years and a variety of manufacturers produce very similar weight or design boat anodes, albeit with some offering more hydrodynamic shapes and better structure. They are all designed to fit or place their counterparts and they are generally compatible with each other - if you ensure you keep the same type of metal for the anode - using a mixture of, say, zinc and Aluminium anodes on a single boat would offer only a fraction of the required protection.

Boat anodes fall into one of the following categories:

Props or rudders- Propellers are normally protected using a collar. The collar is made in two pieces which fasten together around the shaft. Before fitting a prop anode, it is important that the shaft is clean. Outdrives and outboard props are usually protected using ring or prop nut anodes. Metal rudder are easily protected using disk anodes that bolt directly onto the metal. Rudder anodes have a shallow dome shape which makes them streamlined and minimises any drag.

Hull plates- These are used to protect boats where all underwater fittings are bonded together electrically. If these anodes deplete or the electrical current to them deteriorates then the metals they protect will begin to corrode.

Outdrives -Typically outboards will have trim tab anodes and zinc plate anodes. Sometimes anodes are found in the exhaust cavity and in the cooling-water jacket. The engine manual from the manufacturer will tell you where on your engine the anodes are located.

Pencil anodes- These are designed to protect heat exchangers. Heat exchangers are often made from a copper alloy which puts them at risk of corrosion. Pencil anodes are often found under a brass plug in the exchanger and can be unscrewed when they need replacing. Pencil anodes may sometimes also be found inside the cooling-water jacket.
Boat Anodes | Fitting

Boat Anodes | Fitting

As said above a boat anode will only be effective if it is bonded to the part it is protecting and its surface is free from contaminants. Make sure that your anode is "clean". It should not be covered by paint or any other substance that will prevent the bare metal being exposed. If your boat is out of the water and the anodes have sufficient wear left in them, then at the very least give them a clean with a wire brush to ensure that fresh metal is exposed.

If you are replacing your boat anodes, ensure that the anodes you fit are suitable and check for any issues that would indicate problems. If any issues are apparent (pitting of propellers etc.) then decide whether your current protection is working adequately - it may be that anodes have become displaced or worn, or possibly not correctly fitted. If in any doubt, check with a professional to advice on what you need to do or change to protect your craft.

When re-fitting boat anodes, always take time to prepare the bonding surface that the anode attaches to which will ensure clean bonding - these areas often suffer from build-up of debris etc.
Leisure anodes are often attached by use of bolts to the hull or drive leg etc. Some boat anodes have integral plates which ensure that even when worn, the anode is still securely attached to the craft. Boat anodes that are internally bonded using wires to their respective part must be checked to ensure that the bonding cables and fixings are not corroded or loose.

The use of a thread locking compound is commonplace when attaching boat anodes with bolts as this helps prevent loosening and subsequent loss of the anode, make sure that the compound does not interfere with the attachment of the anodes to the surface it is protecting.
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