Is it necessary to wire the anode to the keel? The reason I ask is
that I have a zinc anode connected to the engine & the prop but not
the keel. When I put a volt meter from the keel bolt to the engine I
got a smal reading, about .5volt!

It's difficult to generalize on electrical grounding / bonding /
corrosion protection issues; the vessel's entire metal infrastructure
needs to be factored in, especially if a sailboat with a mast that
requires lightning protection.

See Chapter 4 of Nigel Calder's excellent 'Boatowner's Mechanical and
Electrical Manual' for a thorough discussion of the topic.

MW