These are sacrificial anodes to help reduce the electrolysis between two different metals in salt water. Check your engine owner’s manual for pencil zinc inspection frequency. And, scroll all the way down to check out these photos of a zinc replacement procedure aboard a boat.
Want to learn more? Read this article for a little more detail about where to find zincs…and to become more comfortable in your engine room come to a two day seminar Introduction To Boat Systems where you can Ask Captain Chris all about boating. Need MORE???? 772-205-1859