right Steve. Yes you are correct that an anode will errode at different rates in different situations. Salinity, water temp, closeness of other vessels to you all have an affect on the errosion rate. However, the rate an anode errodes is actually irelevant. It is the level of protection an anode gives you that is the important part. You can easily have too much anode as well as not enough. Different metals close together in salt water
act like a battery
. An electrical current
flows between them. As the current
flows, carried along with it is the least noble metal. If the other more noble metal was close enough, it would slowly attract the other metal and eventually be plated in the other metal. By fitting another metal, in our case Zinc, the Zinc becomes the least noble metal. So the current flows from it to the most noble. The anode errodes away as it's electrons are carried away with the current. Due to water flow, seldom does the Zinc actually arrive at the other parts of the boat being protected. Fo good protection, the current needs to be within a parameter as Gord has shown above. If the anode is too big, it can actually cause too much current and this is sometimes seen as paint
being blown off hulls. Sometimes the anode can actually corrode and take up minerals from the salt water
. This then stops the anode from working and the result is as if the anode isn't fitted. To small an andode or badly worn will reduce the level of protection and the next metal on the nobility scale starts erroding. This is often the Zinc that is locked up in the Bronze props and the result is a damaged prop. I have seen alloy stern legs and otuboards that Anodes have corroded on. The owner still see's what he thinks is a good anode. But later to his horror, he finds ally that you can quite literaly poke your finger through.
So to concise all the above, yes you select an anode based on the variables around you and you replace the anode when the anode is getting about 50% down. You don't, although many do, leave it till the anode is just about gone, or you will loose your prop or worse.