zinc anode

I was able to successfully get hauled out on one of our club trailers yesterday. My main goals are bottom inspection, bottom paint, and replacing the infamous pre-valve PVC elbows on three of my through-hulls. While looking around at things on the bottom, I have a few questions I hope to get...

I have a Volvo Penta 2002 in my Olson 34 and I've only just recent realised there is a Zinc Pencil Anode that's supposed to be replaced on a regular (once a year) basis. I've had the boat for 13 years and got on with it as quickly as possible. Bought the anodes from the UK and had them shipped...