Some Basic Information about Sacrificial Anodes and Boats Cont'd: Types of Engines and Drive Systems used in Recreational Boats Most boats, even sailboats have an engine. The engine drives the boat by turning a propeller. Propellers and the drive shafts and housings are almost always made of metal. All this submerged metal needs to be protected; hence the need for sacrificial anodes. To help explain the use of anodes, you first need to understand the various combinations of engines and drive systems. They fall into the following 3 main categories:       1. Inboard       The simplest system, which consists of an engine inside the boat attached to a propeller shaft that drives the propeller outside the boat. This is usually found on larger powerboats and sailboats.           2. Outboard           Unit that fits on the back of a boat with an engine, drive and propeller. This type of engine is used on smaller power and sailboats and even inflatables. Use of “zincs” is limited, since the unit is usually raised out of the water after use.       3. Sterndrive           Like an outboard motor, except that the engine is inside the boat. The sterndrive unit is just a universal joint connection to a gearbox and drive to a propeller. The units are usually made from Aluminum, which causes a lot of corrosion problems. Also known as I/O (inboard/outboard). This type of drive is used for mid size powerboats. Major user of anodes with as many as 8 anodes on one unit, which may require replacement 2 or 3 times per season.