Just a friendly warning for anyone who has or is considering putting a VHF-AM/FM antenna combiner on their boat....BE CAREFUL! Shakespeare sells a unit designed to allow you to use your VHF antenna to be used as your AM/FM antenna source, and if your rigging/keel ground is separate from your boat's DC ground, this unit will allow a current path between the two. After discovering my brand new Flex-O-Fold prop was suffering from galvanic corrosion, with a zinc installed on the shaft, I found that this antenna combiner was providing a current path for stray DC from the marina water to enter from the keel and exit from the prop, causing galvanic action and eating my beloved Flex-O-Fold! See Don Casey's book "Sailboat Electrics Simplified", page 166-167, for the full explanation on this process. If you do not have a flexible, insulated coupling on your propshaft, any connection between your keel and DC system will eat your prop if any stray DC exists in the water!
Patrick is correct also, your boat doesn't even have to be plugged in to have a problem. In fact, my problem had nothing to do with the boat being plugged into shore power anyway. The problem is simple, if your boat's keel & rigging ground (lightning protection) is connected to the DC ground (batt negative, engine, etc.), there is a path for stray DC in the water to go between the keel and the shaft, and the prop corrodes regardless of whether you have a zinc or not! I did not install the antenna combiner in question, I boat the boat and it already had it. I never gave it a second thought until I noticed my prop corroding and had to find the problem ASAP. My marina has 2-3 volts of stray DC in the water all around the area I'm in, so I don't have a clue where it's coming from, but it doesn't matter now anyway. Hope that clears it up. Tim