Keeping Tabs On Things

Today I took a short stroll through the boat yard and snapped the photo of a trim tab you see below. It made me think back to an article I posted here back in December entitled Rust Never Sleeps. That article turns out to be one of the most popular I've written since I started blogging last year. Looking at the trim tab below made me think of another area of maintenance you need to think about as spring approaches, and it ties in directly to that original post.  The concern here is the matter of crevice corrosion common to stainless steel. ...

17th February 2010.
By Ed Sherman

Today I took a short stroll through the boat yard and snapped the photo of a trim tab you see below. It made me think back to an article I posted here back in December entitled Rust Never Sleeps. That article turns out to be one of the most popular I’ve written since I started blogging last year. Looking at the trim tab below made me think of another area of maintenance you need to think about as spring approaches, and it ties in directly to that original post.

 The concern here is the matter of crevice corrosion common to stainless steel. I explained that phenomena in the December article so check out my archives to learn about that specifically.

So to begin, let me start out with a true story that I hope will drive my point home.

Several years ago I received a call from a boat builder that I had recently done some training for with the ABYC. It seems they had a rather interesting problem that nobody could figure out. Several of their boats, equipped similarly to the one shown above, had been motoring along when suddenly the trim tabs came flying off the back of the boats. So the question was why?

Since I had been to the factory a short time before the call I remembered watching the folks on the production line actually installing the trim tabs on a boat and remembered quite specifically what I had seen. This all made answering the builder’s question that much easier. The fasteners, which are nothing more than # 8 or 10 self-tapping screws had not been properly sealed when they were installed. This allows water to seep in around the screw head and is the beginning of crevice corrosion to the screw. Eventually the heads of the screws corrode off and the trim tab pulls away from the back of the boat’s transom.

In the photo above you can see lots of sealer all around the hinge and oozing out around the screw heads holding the hinge to the transom, so originally things were done correctly. But a closer look shows that some of the sealer is breaking away, and the screw to the far right on the hinge is actually beginning to seep some rust, a sure sign that crevice corrosion is setting in. On the boat above, part of the spring maintenance procedure should be to remove the tabs, clean up the old sealer and rebed the screws, replacing them as needed. This is something that nobody does until something breaks, just like with my boat builder. The installers were putting a tiny dab of sealer on the screws and driving them into the transom. No sealer was oozing out from around the heads of the screws. Crevice corrosion got em….

I should also point out here that the long term problems can go way beyond a few broken screws. The screws I’m talking about thread directly into the boat’s transom laminate, which is most often cored with either wood or in some cases foam. Not totally sealing them will allow water to eventually work its way into the laminate. Now add a few years of freezing and thawing cycles to this moisture and you will eventually have rot on the inside of the laminate…you get the idea.

So, this is an area where more sealer (3M 5200 works great here) is a good thing. Make sure that all of the fasteners have fresh sealer oozing out around the screw heads and leave it to cure. It may look a little messy, but don’t forget, all of this gear is underwater anyhow once the boat gets launched so nobody is going to see it.


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About the author:

Ed Sherman

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Ed Sherman is a regular contributor to boats.com, as well as to Professional Boatbuilder and Cruising World, where he previously was electronics editor. He also is the curriculum director for the American Boat and Yacht Council. Previously, Ed was chairman of the Marine Technology Department at the New England Institute of Technology. Ed’s blog posts appear courtesy of his website, EdsBoatTips.
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