Do It Yourself: Replacing Sacrificial Anodes
by Steve Noury
If you're like many people who keep their boats in the water, you may be neglecting to periodically
replace your sacrificial anodes. Galvanic corrosion can damage or even destroy the underwater
metallic boat parts. When two different metals are touching each other, or are electrically
connected by an electrically conductive fluid like fresh or salt water, the "least noble" of these
metals (called the anode) will corrode faster than it normally would. In this example, Martyr Fresh
Water Magnesium Anodes supplied by West Marine (www.westmarine.com) are installed.
Step 1
A knowledgeable
understanding of the problem is the first place to start. Galvanic corrosion can often happen in a
single piece of hardware — particularly alloys, which contain more than one metal. Interactions
between the differing metals in the alloy will dissolve the least noble of the two. That's why it's
so important to use marine-grade fasteners (bronze, monel and 316 stainless steel) in underwater
applications so they're not destroyed, which can cause a hose clamp to fail or your propeller to
fall off. Keep in mind that the higher the salinity and temperature of the water, the greater the
likelihood of corrosion. Breaking the electrical circuit between exposed metals by connecting them
to a sacrificial anode will prevent galvanic corrosion and damage to your boat's metal parts.
Replacing the anodes is an inexpensive and easy project.
Step 2
Next, determine which
anodes are most appropriate for your boat. Some stores, like West Marine, offer technical advice on
how to choose and install them. If you're a saltwater boater, you can choose to install either
aluminum or zinc anodes — however, aluminum anodes last longer while also providing the same level
of protection as zinc. If you're a freshwater boater, then magnesium is the correct choice. The
project boat pictured above is a 2001 Tahoe 202 deck boat with a MerCruiser Alpha One
Inboard/Outboard. For this particular motor,
BW purchased a freshwater kit that had all of the different anodes. Before you begin,
you'll need to pull your boat out of the water and clean the lower unit. The tools needed are an
American socket set, a prop wrench (or appropriate-sized socket for your prop nut), and an Allen
wrench set.
Step 3
First, remove the old
anodes. You'll need to remove the propeller to get to the bearing carrier anode. Place the engine
in gear to keep the propeller from turning. There's a locking washer on the propeller nut. You'll
need to pry the tabs up with a screwdriver before loosening the propeller nut. Once you've done
this, remove the propeller. Loosen the two bolts, and the anode should come off. You might have to
gently pry it loose with a screwdriver. Next, take off the anodic plate, which is located under the
lower unit. Remove both trim cylinder anodes. To remove the trim tab anode, you'll need to pry the
rubber boot on top of the trim tab with a screwdriver, and then use an Allen wrench to loosen the
bolt. Last is the gimbal housing anode, under the lower unit close to the transom. A small gasket
may be stuck to the gimbal housing, so be sure to remove it.
Step 4
Once the old anodes
have been removed, it's time to begin installing the new ones. Clean all the exposed engine parts.
Be sure to use new bolts and lock washers. Replace the bearing carrier anode first. When replacing
the propeller, be sure to tighten the propeller nut flush and snug, and don't forget to bend down
the locking tabs. Next, replace the anodic plate, and then replace the two trim cylinder anodes.
Return the trim tab anode to its original position, and tap the rubber boot back in place with the
screwdriver's handle. Finally, replace the gimbal housing anode, and be sure to install the new
gasket between the anode and the gimbal housing. You're now good for two or three years, depending
on the frequency with which you use your boat during the season.