Sez Capt. Jeff
"...
removing two
drains on the manifold elbow things on each side of the engine and one
on the heat exchanger. The latter one has a pencil zinc, the other two do not.
Would it be prudent to install pencil zinc anodes on those two also? The other
one corrodes pretty quickly in salt water and...."

Capt, it depends on if your manifolds are fresh water cooled as well. Fresh
water cooling comes in several levels. (with mine the engine, manifolds and
risers are fresh water cooled, the elbows are not).
Putting zincs into the FWC portion of your system would not be a help.
With the engine running and warmed up, put your hand CAREFULLY on (maybe just
close would be best) the various cooling system components, The ones that are
HOT are part of the FWC system. The ones that are just barely warm or cool are
in the raw water circuit and should probably have some zinc protection.
Best wishes
Mike