A typical canal boat will be made up of a number of different metals that should be protected using anodes. A mild steel hulled boat may have a bronze propellor, a stainless steel propellor shaft and a brass stern glad for example. All these different metals are in contact with one another and interact in the same way the anode and cathode in a battery do. Very simply, electrons are lost from one of the metals atoms (anode) by travelling through an electrolyte (canal water) and are disposed onto another (cathode). Depending where on the Galvanic Series a metal is determines how noble (reactive) it is – gold being the most noble (least reactive) all the way through to magnesium being the least noble (most reactive). The corrosion between metals in the Galvanic Series is called galvanic corrosion also known as electrolysis. This is typically seen on canal boats as bright patches of steel underneath rusty encrustations which can clearly be seen after pressure washing or grit blasting.
By introducing a less noble metal onto the hull of a boat to act as a sacrificial material in this process ensures that the important metals mentioned above are not affected by electrolysis, i.e. welding magnesium anodes onto the hull allows the magnesium to corrode before the mild steel of the hull itself.
Pictures showing typical galvanic corrosion – shallow pitting across a large, unprotected area. The sliver areas are where metal has been removed due to electrolysis, making the hull thinner.
Examples of aggressive pitting from electrolysis due to inadequate galvanic protection. The picture on the right shows a surface after grit blasting. The pitting was so severe that the thickness had become so thin that the blasting process had penetrated the side of the boat.
The myth that you do not need to protect the baseplate in the same way as the sides of a hull more than likely stems from the fact that it is normally much harder to gain access to the baseplate. Pitting can be just as bad on the baseplate as the sides of a hull. The two principal reasons anodes are not fitted to the baseplate are ease of access and the fact that you cant normally see it!
We recommend sacrificial anodes to be welded onto the baseplate every 8-10 foot apart along with the typical configuration of two at the bow and stern. By attaching them to the underside of the boat they are able to provide protection to both the sides and bottom in a 4-5 foot radius enabling the maximum area to be covered by one anode. P&S Marine has been carrying out this practice for over fifteen years and has seen the benefits countless times – even on our own boats. It is not true that anodes on the bottom of boats get “caught up” on the bottom of the canal or in locks – we have never experienced this. A typical rudder skeg hangs down lower than the amount a 2.8 kg anode protrudes.
At P&S Marine access to the baseplate is not a problem and after the baseplate is painted we grind away small areas for the anode tags to be welded to and then reapply bitumen to these areas. This ensures the area under the anode has been painted and has the same level of protection as the rest of the hull. We also recommend installing a galvanic isolator which separates your 240V shore line earth from the rest of your electrics which blocks the majority of the low voltage currents that cause electrolysis. This increases the lifespan of any anodes fitted to the hull and in turn reduces the overall hull protection costs. It is generally recommended that anodes that are > 30% worn be replaced with new ones.
2.8kg Magnesium Anode
- Cost of each anode
- Cost of welding each anode on
- Total cost per anode
- Removal of each depleted anode
Please contact us if you have any questions concerning hull protection or would like advice or an estimate for the fitting of anodes.
The following email was sent to us by M.G. Duff, specialists in marine corrosion control, concerning the placement of anodes: