For those of you who have older SM's with MaxProps, and who use zincs on the prop...
Frequently you will hear complaints about either premature loss of the zinc, or having it come loose from two for the attachment bolts and spinning around--wildly out of balance. This could happen in theory if the anode was simply a chunk of cast zinc, but this should never happen, if you get properly made anodes. What is "proper"? There is an internal non-zinc frame cast into the zinc that the bolts go through. Basically, this secures the zinc until it is nearly all gone.
Here is a photo of a "proper" zinc anode at the very end of its useful life, and maybe a little past it, that clearly shows the internal frame. This was a Martyr brand zinc. Even though rather eroded, it is still tightly secured against the metal face of the prop as it should be and doing its job.
Unfortunately, I know of no way to be sure ahead of time that the frame is actually there... It is not magnetic, and is completely buried inside the casting when it is fresh.
Annapolis, MD, USA