How many Anodes?

quick tip that a lot of people don't know...
use a magnesium anode for fresh water and zinc for the sea.

As for size, try one and see how long it lasts. You can don a swimsuit and add another. I hear of anodes lasting a month or a year depending on the local chemistry.

If you are not on passage, use a hanging anode at the transom.  If it starts wasting quickly there is an electrical problem nearby and you should move the boat and check your hull anode.  I have got six seasons (salt water estuary and UK coastal) so far out of a large pear shaped zinc hull anode, cleaning it up each time the boat is hauled.    When it gets down to about 50% I'll replace it.

Christopher, In answer to your question, the one hull anode I have is through bolted and a big wire runs from the bolt to a big copper grounding plate that the engine ground is also connected to (looks like to me that it is connected to the entire electrical ground system of the boat, battery included).  When I was trying to remove it today, I broke it but I think that this is just as well as it was pretty severely eroded and the bolts also seemed to be bedded rather well in compound and their electrical connection to the anode itself looked pretty iffy to me. 

I take it that this is so the anode will be electrically connected with the prop through the engine, transmission and prop shaft and therefore wear away instead of the prop?

And how do you seal the bolts of the anode without losing the electrical connection? 

Ian, Where do you electrically connect your hanging anode when not underway?  to the battery negative?

It looks like my anode is the small pear shaped one in the WM catalog.   And my engine (the Univ 5411) doesn't have an anode.  It was before engine design got that clever.