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.


Part wasted magnesium anode with deep irregular pits Fully wasted zinc anode – ‘well sucked lollipop’


So how can we make best use of this knowledge to keep our boats in good order? The deductions we make here are very dependent on what type of vessel we are considering, what is has been built from and where it is being used. 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


94 | The Report • September 2020 • Issue 93


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.


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