If I may add to this:
For saltwater use a zinc anode
For Sweetwater use an alu anode
My father swears by his magnesium anodes (these are mounted on wire ends
welded to his steel hull, he changes them easily but these is an annual
thing) but I also know of people swearing at them... paint falling off the
hull etc.
On the Drivfjäder we have a little alu ring anode mounted (very thin) just
behind the prop, leaving enough room for waterflow to the rubber
end-bearing. This one lasts for about two years, so I think it does has a
job...
Have a great weekend all!
Hielke