Any Outdrive Boaters moored on the Tidal Thames?

Zinc, as I spend more time down towards the sea than I do up towards the non tidal bit. That said, I'm moored in central London so I don't know whether zinc are right or not! They do seem to disappear at a reasonable rate though.
 
Thanks! I've been considering the Red Spot Aluminium anodes that are meant to be good for fresh, brackish and salt water. I'm currently on the non tidal Thames so have magnesium but intending to moor around London too.
 
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