Question: I am in mild shock. When I hauled my boat last fall I noticed the propeller shaft had a pretty big accumulation of barnacles on it. I hadn’t used the boat much last summer as I got busy with work etc., so I wasn’t too surprised to see some buildup on both the shaft and the bottom of the boat. I simply figured I’d deal with it all this spring.

prop shaft crevice corrosion

This close-up photo of a propeller shaft shows how barnacles, if they're allowed to make a home, will starve the surface of oxygen, eliminating its resistance to serious crevice corrosion.

Well, last Saturday I went down to the boat to get going on my spring get-ready and I started out by scraping the barnacles off my stainless propeller shaft. You can see in the photo that underneath some of the areas where barnacles were attached, there are now areas where it looks like the barnacles have been feeding on my stainless steel propeller shaft. What is going on here? What am I going to have to do here?

Answer: Oh boy, all I have for you is bad news. What you see here is a profound example of what is known as crevice corrosion. The barnacles aren’t eating the stainless steel, but once attached to the shaft, they are starving the surface of the metal underneath them for oxygen. You see, stainless steel develops its “stainless” properties only in the presence of oxygen. The metal develops an invisible oxide coating that essentially keeps rust from forming. In a case where there is still exposure to salt water, but a minimal oxygen exposure, rust will indeed begin to attack the metal in question.

We do know that the extent to which this will occur varies a bit from one stainless alloy to another. In the case of propeller shafting for example, we know that Aquamet 22 is far less prone to crevice corrosion than Aquamet 17, which is also used as a less expensive alternative to Aquamet 22. Aquamet 17 is a fine choice for propeller shafts on boats that are in constant use, thereby oxygenating the metal on an almost daily basis, as well as making it tough for barnacles to find a comfortable spot to hang out. The Aquamet 22  is a superior choice for boats that spend more time at the dock than underway -- it is less prone to crevice corrosion.

So, the bad news is that the shaft in the photo you sent in is going to have to be replaced, and it’s not going to be cheap. As for maintenance recommendations, I think you have taught yourself this lesson: Either dive on your boat and get the bottom scrubbed and barnacles scraped off mid-season or start using it more. Since you are going to need a new shaft anyhow, why not spend a bit more and spec it out for Aquamet 22? I’ll bet the shaft in the photo is Aquamet 17.