How to Save Your Sailboat from Being Eaten Alive!

Captain John from shares tips on how to prevent damage to your boat in the long run.

Do you know the #1 problem professional boat surveyors run across time after time with used boats? Hull blistering? Nope. Deck core water damage? Uh-uh. Cracks in the hull or gel-coat? Not even close!

Galvanic corrosion wins the prize–hands down–as the biggest problem for both sail and power boats anywhere in the world. Master surveyor John Marples says “This is the most common fault in the boats I’ve surveyed in the past 10 years”.

It’s simple to prevent but easy to forget until it’s already taken a firm grip to destroy your costly sailing rigging and equipment. This creeping-crud disease can topple your boat mast, destroy a boat boom, or cause your winches to rip from their bases under the enormous loads of a headsail or spinnaker.

Look at this simple chart, called a “Galvanic Series” table. It shows those metals that are less noble (most likely to corrode) and those that are noble (less likely to corrode).

Metals on the anode side (less noble) lose mass to metals on the cathode side (more noble). Metal immersed in salt water loses mass much faster than metals in salt air. Salt water creates a current that flows from the least noble metal to the more noble metal.

Check the table first before you mount a piece of sailing gear (block, winch, cleat, propeller, pad-eye, wiring, metal fasteners) to another metal surface (shaft, mast, boom, keel, stanchion, rail, pulpit).

If possible, choose two metals close together in the table (i.e. brass and bronze; bronze and stainless steel) to slow down the effects of corrosion.

Below your boat’s waterline, you install a zinc (anode) on your propeller shaft (stainless or bronze–cathode). Current flows from the zinc to the shaft. Ions leech from the zinc into the water and it crumbles. Note how far apart zinc and stainless (or bronze) are in the table.

Above your boat’s waterline, galvanic corrosion slows down because no longer are your fittings immersed in sea water. But you still must deal with the corrosive effects of salt air and spray.

A boat boom (aluminum) loses mass to a cheek block (stainless steel) mounted on your boom. Those white deposits that you see near the edges of the cheek block are aluminum granules.

Fortunate for us sailors, the solution ranks among the cheapest and easiest of all marine maintenance projects. Follow these three simple rules from master Marples.

Marple’s Magic Rules

1. Insert an isolation disk between all metal fittings that show wide separation in the table. Any non-metal material works, but plastics like nylon or polypropylene are best for high load sailing gear. Make isolation disks at least 1/32″ thick.

2. Extend the disk 1/4″ beyond the edge of the fittings. This creates a protective “drip” edge to channel salt water away from the fitting seams.

3. Coat stainless steel fasteners used on aluminum with an anti-seize compound. Otherwise, oxidation buildup beneath the stainless screw heads can make screw removal or replacement a tough chore.


Follow these easy-to-use secrets to lower the maintenance and replacement costs on your small cruising sailboat. This will save you a small fortune in repair costs and protect your sailing rigging from the deadly plague of galvanic corrosion.

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