Story and photography by Capt. Vince Daniello
To ensure your boat has adequate zinc protection attach a silver-silver chloride reference electrode (the black device shown at the end of the red lead, above) to a digital multimeter and lower it into the water near the propellers. Attach the other lead from the meter to the common ground point of your engine or engines (again, due to the limitations of studio photography, the lead in the photo is shown connected to the flange of the seacock), as described in the ohm test above. My ProMariner analog corrosion meter, by the way, has both a long reference-electrode lead and a test lead that is also long.
ABYC suggests attaching the engine-connected lead to the positive terminal on the meter. This method will produce negative voltage readings, or below-zero volts as established by the negative post of your battery. (Remember, the highest voltage metal, closest to zero volts, is the one which will erode.) My ProMariner meter is color-coded in reverse so, with the black common (negative) lead connected to the engine, it registers positive voltage.