CHAPTER ONE
Anodes must be used properly to be effective. Simply fastening pieces of zinc to your boat in random locations won't do the job. You must determine how much anode surface area is required to adequately protect the equipment's surface area. A good starting point is provided by Military Specification MIL-A81800 1, which states that one square inch of new anode will protect either: a. 800 square inches of freshly painted steel. b. 250 square inches of bare steel or bare aluminum alloy. c. 100 square inches of copper or copper alloy. This rule is for a boat at rest. When underway, more anode area is required to protect the same equipment surface area. The anode must be fastened so that it has good electrical contact with the metal to be protected. If possible, the anode can be attached directly to the other metal. If that is not possible, the entire network of metal parts in the boat should be electrically bonded together so that all pieces are protected. Good quality anodes have inserts of some other metal around the fastener holes. Otherwise, the anode could erode away around the fastener. The anode can then become loose or even fall off, removing all protection. Another Military Specification (MIL-A18001) defines the type of alloy preferred that will corrode at a uniform rate without forming a crust that could reduce its efficiency after a time.
Impressed Current Systems An impressed current system can be installed on any boat that has a battery. The system consists of an anode, a control box and a sensor. The anode in this system is coated with a very noble metal, such as platinum, so that it is almost corrosion-free and will last indefinitely. The sensor, under the boat's waterline, monitors the potential for corrosion. When it senses that
corrosion could be occurring, it transmits this information to the control box. The control box connects the boat's battery to the anode. When the sensor signals the need, the control box applies positive battery voltage to the anode. Current from the battery flows from the anode to all other metal parts of the boat, no matter how noble or non-noble these parts may be. This battery current takes the place of any galvanic current flow. Only a very small amount of battery current is needed to counteract galvanic corrosion. Manufacturers estimate that it would take two or three months of constant use to drain a typical marine battery, assuming the battery is never recharged. An impressed current system is more expensive to install than simple anodes but, considering its low maintenance requirements and the excellent protection it provides, the long-term cost may actually be lower.
PROPELLERS The propeller is the final link between the boat's drive system and the water. A perfectly
