Are the zincs that attach the the transom thru bolts beneficial? I noticed a zinc on my lower unit above the cavitation plate... do I need more than that?? I saw other zincs that look like they just screw onto the remaining bolt that extends past the nut... are they effective? how much zinc is enough based on my boat being used almost always in Nacl H2O .... I keep it on the trailer when not in use. Suggestions?