The combination of cast iron, silicone bronze, naval brass and stainless steel used on a retractable keel boat is a virtual battery just waiting to be damaged
by galvanic corrosion
. Zinc, being the least noble of any of the metals used under water on your retractable keel boat, corrodes first thereby protecting all the other metals.
To install, just drill a hole through the "skinny" part of the keel near the trailing edge. Through bolt being sure you have made electrical contact with the keel and you're done for the season. Replace once a year. Mandatory on boat moored in salt water.