Sleuthing deck stains

After crossing the Gulf Stream recently, we noticed that the deck of our boat was stained with patches of yellow. At first we were baffled as to how this had happened. It had been a fairly rough crossing from Lake Worth, Fla., to Lucaya, Grand Bahama Island. Our C&C 40 was busting into good-sized waves and there was plenty of water across the deck most of the night. Our pulpit-mounted running lights were on for the bulk of the passage. During the process of securing our vessel, the dock staff pointed out the yellow patches on our deck. There were two main spots, one near the bow pulpit and one on the starboard shrouds at the chain plate.

There had been no evidence of yellowing in these areas before our crossing. I also noted the evidence of new corrosion on the bow pulpit and proceeded to check with my voltmeter for a path of stray electricity. With the running lights switched on, 12 volts DC was measured between the bow pulpit and battery ground. The culprit was an original equipment red or green bow light unit made by Aquasignal mounted on the bow pulpit. The company used rivets to hold the light bulb bracket to the base molding. When the light is mounted on a stainless bracket, there is a very small gap between the rivet and the bracket. That gap had been bridged with corrosion over time. With the addition of salt water and power to the lights, the circuit was completed and corrosion to the stanchion and bow pulpit continued at a rapid pace while we sailed overnight.

The yellow stain was a result of the electrolytic corrosion of the differing metals. While they looked difficult to remove, the stains actually washed off fairly easily with simple soap, water and elbow grease. Placing a dab of silicon between the rivet and the mounting bracket repaired the light, isolating it electrically from the bow pulpit. It had the added benefit of keeping water from entering the unit and continuing to corrode it. Two days later, another vessel arrived in Lucaya from the Virgin Islands. Yellow stains covered their deck and streaks ran down the side of the boat. It turned out their vessel had the same light attached to its bow pulpit. So, check your bow lights to prevent corrosion of the bow pulpit, yellow deck stains and potentially more serious problems.

By Ocean Navigator