I have attached two photos showing the location of the zinc anode in the heat exchanger end cap. This location covers all the BF2803 engines after model year 1998.
Prior to that the zinc anode was in the end cap at the front of the manifold.
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At first sight, you have to drain the seawater side of cooling to change the anode.
We found it easier to close the input sea****, and pump sal****er from the galley or head pump to create a vacuum in the system. This avoids mess in the boat.
Of course, this trick works only if you piping is appropriate.