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.