Shaft anode small bubbling

[thinwater]

And if there aren't two streams of bubbles? Because in my experience, there is invariably only one.

Only oxygen if it were freshwater, so you are absolutely correct.

In salt water the 2 "streams" are not H and O, but rather sodium (from the salt) and chlorine (also from salt), because they have lower potentials. The sodium reacts instantly in the presence of water into caustic and hydrogen gas (the bubbles you see). The chlorine rapidly reacts to form hypochlorous acid (bleach) which disolves. Thus, only one stream on the zinc and the slight smell of bleach on the cathode, though certainly too dilute to notice. (This is how Electroscan heads work. This is also how bleach and caustic are made.)

 

[Allex]

Well of course it's hydrogen!

 

[Bill Marrero]

As a start I will take a closer look at the isolator and test it.
Thanks .

 

[fstbttms]

Only oxygen if it were freshwater, so you are absolutely correct.

In salt water the 2 "streams" are not H and O, but rather sodium (from the salt) and chlorine (also from salt), because they have lower potentials. The sodium reacts instantly in the presence of water into caustic and hydrogen gas (the bubbles you see). The chlorine rapidly reacts to form hypochlorous acid (bleach) which disolves. Thus, only one stream on the zinc and the slight smell of bleach on the cathode, though certainly too dilute to notice. (This is how Electroscan heads work. This is also how bleach and caustic are made.)

I'm still not buying it. I have seen anodes and props produce a single stream of bubbles in saltwater many, many times and it never occurs until the object in question is touched with a tool (wire brush, screwdriver etc.) or is otherwise disturbed. The electrochemical reactions described in this thread should be producing visible signs regardless and all the time, given the conditions that cause it are present, yes?

 

[JamesG161]

given the conditions that cause it are present, yes?

No, not in normal protection.
There has been a mix of info, because they are focusing different possibilities to help the OP.

Two situations of no external electrical interference and in salt water.
1) Replacing existing Zinc in kind.
2) Cleaning submerged metals and not changing Zincs.

1) As long has your Zinc anode has been correctly sized, the bubbles should be short lived and Oxygen
2) Depends the metal, but if Bronze, it would be Hydrogen as the prop takes the Oxygen to protect itself again leaving the H from H2O to bubble. Stainless is very slow to oxidize after cleaning so would not expect bubbles.

Now all bets are off if , as Charles noted, there is electricity being added to the boat or water, thus an electrical survey is needed.
Jim...