Mixing Zinc and Magnesium anodes

[Copious]

What are the issues with mixing anodes and having half zinc and half magnesium to cover time spent in salt and fresh water zones during a season?
Copious

 

[AntarcticPilot]

Basically, the magnesium anodes will protect the zinc, so the magnesium anodes will erode while the zinc anodes remain unchanged until the magnesium is all gone.

 

[Quandary]

I sail in the sea and berth in fresh water, I now use aluminium, they are now available for most props and for saildrives. The anodes do not surface passivate like zinc but erode a good bit quicker. My hope is that I can convince myself that the alloy Flexofold prop anode will last two years so that a bi-annual lift out would be enough, but there is a lot of difference between 6 months and 18 months exposure though more than 2/3 of the latter would be to fresh water. However, when using zinc the prop. anodes eroded normally probably because the thrashing blasted the passivation off the surface.
Before when I was using zinc in fresh water I had a hanging anode connected to the leg that I could grind down to clean metal with coarse sandpaper before going to sea.

 

[Copious]

Hi Quandary,
Where do you attach the cable end of your hanging anode? The VP 120S has the two anodes which I can only surmise is to have one (ring anode) for the leg and one (collar anode , in 3 sections) for the hub and prop.
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[Boatface]

I have a zinc anode on the prop (autoprop) and an aluminium anode on the hull (wired to the engine block).
The prop anode is in electrical continuity with the engine block through the shaft and gearbox etc.
I change the aluminium anode each year but the zinc on the prop has lasted at least 5 years as it is protected by the aluminium.
The main function of the prop anode now is to keep crustaceans out of the nut hole in the back of the prop.
Aluminium is a satisfactory anode in sea water.
(also ... Autoprop anodes are not cheep)