NIGEL CALDER REPLIES

Normally speaking, the aluminum saildrive leg is more vulnerable to corrosion than the propeller, so I suspect your propeller corrosion had more to do with stray current (against which a sacrificial anode provides no protection) than with galvanic corrosion (which is held at bay with sacrificial anodes). The isolation transformer may have fixed this problem. With respect to anode choices, you are correct, you should not mix anode materials in the same cathodic protection system. The key thing here is determining what constitutes the same system. If your propeller is electrically connected to the saildrive via metal-to-metal connections, then the two are essentially the same system, and the magnesium anodes will get eaten up by the aluminum anode. If, however, the propeller and saildrive are electrically isolated (by, say, some sort of plastic sleeve or a composite propeller hub) then the two can be considered to be different systems, and there should be little to no interaction between the anodes. My guess is you do not have electrical isolation, in which case you are somewhat between a rock and a hard place. All but Volvo-Penta saildrives are not electrically isolated from the rest of the boat, and as a result corrosion— be used in salt water because it becomes too reactive.) However, this may be resulting in over-protection and hence the blistering of paint on the keel and the coating on the propeller. Aluminum anodes are reasonably effective in freshwater as well as saltwater, and may well do an adequate job for you with a greatly reduced rate of anode consumption and no paint blistering or propeller coating. I think