Well, I've had a lot of experience with this problem on my boat and I'll pass on what I know.
I've attached a series of pictures documenting my boat's condition. The first picture shows the rust streaking on my boat. It is actually worse than shown. I removed the aft port and starboard sections of my rub rail to see what's going on. The next picture shows the pasty 'muck' captured behind the rub rail at the toe rail drain slot locations. As you probably know, the rub rail is convex shaped, and at the toe rail slot drains the upper portion of the rub rail overlaps the deck with a gap that exposes the back side. Because of the overlap, the drain water and 'muck' can pass behind the rub rail and collect. When I removed the rub rail, the 'muck' was still moist even though the boat had been on the hard for a few months. The next picture shows the distinct rust line left on the hull after the 'muck' was wiped off. The next picture shows the backside of the rub rail immediately after I removed it from the boat. To my surprise, there was only very minor rusting on the back side. However, I did note that there was some pitting on the lower edge of the rub rail, which indicated to me (my guess) that this corrosion (pitting) was caused by the lack of O2 at those sections because they were blanketed by the 'muck.'
From all this I concluded that the major source of my rust streaks was not the backside of the rub rails. It was also not caused by the screws which I replaced a few years back with 316SS. Some rust streaks did come from a few screw holes on the rail, but the screws themselves showed no evidence of rusting.
My theory is that the 'rusting' is being caused by the 'muck' trapped behind the rail. I believe this because the 'muck' could easily cause the O2 pitting. Also, the streaking became much more problematic after I installed my seawater deck wash pump, which I use primarily to wash the 'muck' off my anchor/chain. I seems very probable to me that the 'muck' I found trapped behind the rub rail is the same 'muck' that I washed from my anchor. Now this next part can be a little crazy, but here it goes. I sail in the Cape Cod area. I did a little research, and the soil in this area is high in iron oxide, and contains a lot of rust colored organic matter, which is very leachable. Perhaps the rust streaks are caused, in part, by the rust colored material leaching from the 'muck.' Anyway, regardless I'm going to try to prevent the 'muck' from getting behind the rub rail. The last picture shows the back side of a rub rail at the location of the toe rail drain slot. I filled the back side with thickened epoxy, which I sanded flat. My intent this Spring is to do the same at the mating location at the hull. I'll apply some sealant to these surfaces when I mount the rail, hoping that the seal will hold and this will prevent the 'muck' from the toe rail drain from accumulating behind the rub rail. I'll let you know this Spring how things work out.
Good luck. Hope this is useful.
Attachments: |
File comment: Rust streaks on hull from rubrail
2015-12-10 12.33.01.jpg [ 712.48 KB | Viewed 0 times ]
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File comment: Pasty 'Muck' left on hull at toe rail drain slot after rub rail was removed
2015-12-11 12.27.23.jpg [ 715.16 KB | Viewed 0 times ]
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File comment: Rust line left on hull after 'muck' was removed
2015-12-11 12.28.09.jpg [ 731.92 KB | Viewed 0 times ]
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File comment: Back side of rub rail directly after it was removed from hull; no significant back side rusting evident
2015-12-11 12.28.52.jpg [ 1.7 MB | Viewed 0 times ]
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File comment: Thickened epoxy sanded flat on back side of rub rail at toe rail drain location
2016-02-03 08.24.24.jpg [ 1.55 MB | Viewed 0 times ]
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_________________ Jim Voelxen Odyssey #191 Home Port: Osterville, MA
Last edited by jvoelxen on 15 Feb 2016 20:58, edited 1 time in total
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