On our Max Prop we have one of those shaped conical zincs; but we also have one or two clampon zincs on our 1-3/4" shaft. They sit about 2' forward to the prop.
Our transmission and vibration isolator has a good connection through drive train. The Yanmar engine doesn't even have an engine zinc. (The throughbolts do the job.) Sometimes you don't get a good ground through the shaft on some older transmissions. (That's why some people used to use "brushes' on the shafts aft before the exit through the hull.)