That is to say, the convention is to consider the boat as external to the water ... even though it might seem more logical to think of the water as external to the boat. This may seem arbitrary, but at least it is consistent with the aforementioned electrochemical corollary ( item 12 ), so that oxidation reactions take place at the anode, and reduction reactions take place at the cathode. This leads us to the useful concept of a sacrificial anode , which is just a cheap, easily-replaceable electrode that is placed in the water and arranged to have a large positive voltage with respect to the rest of the boat. That makes everything else on the boat a cathode, greatly reducing corrosion, because most forms of corrosion involve oxidation reactions. To say the same thing in other words, inside the water, highly corrosive anions such as OH – and Cl – are flowing toward the anode and away from everything else, in accordance with the anions-to-anode rule. The anode, of course, corrodes rapidly, and needs to be replaced on occasion.