MAGNESIUM

Q:

Boat Doctor, we are planning a trip down the Mississippi to the

Gulf of Mexico with our boat. Should we change the zincs before we get to salt water?

Emmett and Ellen Stuckey Montgomery, Alabama

A:

I get this question often, and the answer is pretty critical. In fresh water, most zincs are actually made from magnesium—though they may be made from either actual zinc or aluminum.

While any of the three will protect from corrosion to varying degrees in fresh water, magnesium should

never be used in salt water. Magnesium will cause corrosion in short order if used in the salt chuck.

Therefore, if you know what type of sacrificial anodes are currently installed, change to the metal your engine-maker recommends for saltwater application, which will be either zinc or aluminum.

• Humphree X-Series X300 interceptors; system includes a pair of 1113/16-inch-wide interceptors with servo actuators, a control unit, helm control panel, GPS antenna, wiring and Humphree wrench ($4,000, humphree.com) • Allen wrenches (4, 5 and 6 mm) • Socket wrench (36 mm) • Power drill with 9 mm bit • Hole saw (60 mm) • Spring scale or torque wrench with optional torque adapter (for torque measurement) • Threadlocker • Straightedge • Sander (to create flat mounting surfaces) • Fine sandpaper • Grease • Marine sealant, such as Sikaflex 291 Rags to wipe up sealant Marking pencil • •