electrolysis vs. corrosion

[Doug Fallin]

In a few weeks I will be setting up a mooring. I am using a helix type screw rather than a mushroom or pyramid. My concern is where I shackle the chain to the end of the screw shaft (the eye of the screw). The screw hot dipped galvanized steel and the shackle can be stainless or galvanized. If I use a stainless shackle with galvanized screw, will I experience greater degradation in the metals due to electrolysis of dissimilar metals or should I use a galvanized shackle which will experience corrosion where the galvanization is rubbed off? Does anyone have an idea at what rate degradation/corrosion is expected to occur in these two settings? The mooring is near Tampa, Fl. in a mud/silt bottom. I understand that if I can get this connection in the mud, the rate of degradation is slowed. Can any of the experts out there help me before I set the anchor?

[Oswego John]

Doug,

First off, I'm not an expert but I can talk from my experiences.

I can't speak on the corrosive action in southern waters where you are situated. I can relate some info on what I have observed in northeastern salt water.

It is true that you can have the galvanize abrasion of the links and also electrolytic corrosion, some hot areas moreso than others.

I have no idea on the scope allowed in your mooring area. The more, the better.

Some people use all chain from the mushroom or screw up to a mooring pickup float and then use line pennant from the float to the cleat or bitt.

Others use 1/3 chain on the bottom and use line on the upper 2/3 of the mooring line. There are several reasons that they don't use all chain.

In some areas, there is concentrated electrolytic action nearer the surface than down below in the mud. I have noticed the top three or four feet are the hottest Hence, chain below, line on top.

It is believed by many that when chain is laying on, or has worked it's way a little under the mud surface, the electrolytic action is greatly reduced.

When on a mooring with line on the upper third, most often a mild to medium blow will have the chain act as the anchor. When the wind pipes up strongly, the mooring line will tend to straighten out and the weight of the chain will arc and lift some off the bottom. This, in effect, acts as a damper to eliminate shock to the parts of the mooring as well as the cleat on the boat deck. Keep in mind that an all chain mooring will also reduce shock. The down side of an all chain mooring line is it's initial cost as well as periodical replacement. Replacing the upper portion of line is more practical for some owners.

I have reservations about mixing metals under water, especially salt water. If your screw type mooring stake is galvanized steel, myself, I would use galvanized fittings throughtout. It will reduce the possibility of electrolysis. Even if you did use a stainless steel shackle, it would abrade the zinc plating on the screw stake just as much as a galvanized shackle. Your mooring chain will probably be galvanized steel, also. I say stay with the same metal as much as possible.

Whatever route you choose to follow, don't forget to use a swivel on either end of the mooring chain. Don't neglect to check the mooring a minimum of every two years. In a hot area, even more often.

Just my viewpoints. I'm sure that others have more to add or alter.
Good luck,
O J

[Sea Hunt]

In the March/April 2007 issue of Good Old Boat (Issue 53), there is an article on various types of mooring systems beginning at page 29. For a newbie like me, the article was very informative, although it did not directly discuss the issue of galvanic corrosion, electrolysis, etc.