A/C Thru Hull Discharge Color

[wizard]

Please disregard this post. I figured it out. I can't seem to find out how to delete the post.
Hello,
Our '91 Grand Banks classic has three A/C units. The seawater pump feeds a common manifold, and there are three thru hulls for the discharge. The seawater discharges from all three, regardless as to whether an individual unit is cooling or not.
One of the thru hulls, that is right at seawater level, develops a black film over it, on the hull. It seems sooty, and is easily wiped off, but reappears in less than a day.
The A/C unit associated with the particular through hull works well. It did receive a professional Barnacle Buster (or related) cleaning a few months ago.
Anybody got any thoughts on this? I'm at a loss as to why this may be occuring.

Thanks,
Mike Wise
GB Isabell

 

[Easternshore]

Wizard, what was the cause of the black? Thx

 

[Comodave]

We can delete the post if you want us to, just let us know.

 

[Frosty]

We can delete the post if you want us to, just let us know.

But only after we find out the answer to the mystery, right?

 

[Comodave]

He said he figured it out but didn’t say what it was. So I guess we will hope he posts the answer and then we can leave the thread for educational putposes.

 

[wizard]

Hello,
Sorry about the deletion thing. I don't post often, and I must be mis-reading the deletion instructions.
When I first encountered the "black" stain, I wiped it off with my hand, and rinsed my hand in the water. When it re-occurred, I used a paper towel, and looked at the what had come off. It was actually gray, and a bit slimy - something organic. So my theory, weak as it may seem, is that somehow the frequent salt water flow (we're in the Keys, so the A/C is frequently pumping), somehow stimulates a growth from the splash pattern of the seawater. And the only reason that the other two A/C thru hulls are not producing the same thing is that they are more elevated from the waterline. This one is right at it.
Thanks for your responses, and sorry about the wasteful post.