Copper bottom paint on outboard powered boat?

[Sunbeam]

Do you ever re-read an operator's manual that you've read before, and yet this time something jumps out at you for the first time? Well, that happened last night with the manual for my main engine, a Yamaha. It concerns bottom paint, and says not to use anything containing copper, because it will accelerate engine corrosion. I knew that one shouldn't paint the outboard leg itself with copper-containing paint, but what I had somehow glossed over was that they are saying not to paint the boat bottom with anything containing copper. That's a new one on me.

The bottom on my boat is just gelcoat, not painted at all, but this really piqued my curiosity. Is there really much of an effect on the engine if the boat-bottom paint contains copper? Is it exterior, just while the boat is sitting in the water making "copper soup"? Or is it internal while underway and coppery water is getting into the intake?

Or.... is it like the flushing instructions, which tell you to remove the prop each time (but which I'd guess is more of a CYA thing -- not that it wouldn't be safer but I'd guess most people don't do it and the engine is none the worse for it).

Sunbeam

[thataway]

The bottom paints which contain copper thiocyanate are OK (such as Vivid).
I believe that the bottom paints Yamaha is concerned about are ones which contain elemental copper. For example Ultima SR 60 has 60% elemental copper.

Yamaha has some corrosion problems in the engines, and I suspect it is a CYA issue. I would think that the major issue would be copper bottom paint right at the bracket or on the bracket, when the boat was stored in the water. I cannot conceive that the amount of copper coming off the bottom paint and ingested into the cooling system could be a problem.

If there is a corrosion issue you would see it around the bracket area.--and maybe some paint damage there.

I am using Vivid with has two biocides--but not elemental copper.

[Sunbeam]

Thanks for the additional info.

Back story:
C-Brat "Flagold" had waxed the bottom of his boat and incorporated copper (he used the powder type you put on flowers, etc.). He simply mixed a heaping tablespoon into a cup of wax before applying it. He didn't buff it off, like you do on topside wax, but just applied it and left it. He said that he could go up to 3 months (on Florida's west coast, so fairly high-fouling waters) as opposed to only a couple of weeks with just plain wax. So I was intrigued and thought about doing same.

The copper in question is copper sulfate powder, 7%.

Naturally I wouldn't want to harm the engine, so if I am not able to determine whether or not there is a risk to incorporating the copper sulfate, I'll probably just wax with un-adultrated, good ol' Collinite 885 (this is for something like 2-3 weeks jaunts in saltwater, with trailering/freshwater trips interspersed; so I don't plan to bottom paint for the foreseeable future).

Not sure whether to "buff out" the wax (whether coppered or plain) or just apply it and leave it as Flagold did. The latter would probably look a bit weird, but then I don't mind saving the cosmetics for the parts of the boat above the waterline.

Sunbeam

PS: The C-Dory is kind of a new, middle ground for me. I've either had fully bottom-painted boats that were in salt water year 'round; or smaller boats on inland fresh water lakes where bottom paint would be unheard of. I did have a dinghy (in salt water) that I didn't paint -- but that was easy to flip over and scrub every week or so (accidentally went a few weeks once in a dirty harbor.... never forgot THAT chore again).

[thataway]

I can only give the information from what I observed at Knights Key during the winter months. Water in the high 70s. There area a number of center console fishing boats 18 to 30 feet which are kept in the slips with waxed bottoms. They are hauled out every week and presssure washed and rewaxed. If you go two weeks there will be significant slime and I have seen a few small barnacles. These boats are used almost every day. They do not buff the wax on the bottom--but I doubt that buffing would make much difference. If they go 3 weeks, then it is a real mess. Some say they will bottom paint next year. Some do--some don't.

[Sunbeam]

[thataway]

Copper thiocyanate is not soluble in water and is in several bottom paints such as Vivid. Most bottom paints are copper oxide, which is also insoluble in water but apparently more likely to harm aluminum because of more rapid action with sea water to form other copper compounds. Many of the older bottom paints with very high copper content turned green after a few months. My guess is that copper oxide forms copper sulfate more rapidly than does copper thiocyanate (which I believe is more firmly bound. (Some of our chemists can weigh in on this). The copper sulfate attacks aluminum directly.
Washington state apparently has banned copper containing bottom paints:
"to provide that , after January 1, 2013, new recreational water vessels with antifouling paint containing copper may not be sold in the state. After January 1, 2018, any recreational water vessel being sold must have been stripped of copper paint or sealed. After January 1, 2020, no antifouling paint containing more than 0.5 percent copper may be sold in Washington."

Back to the copper powder--it may be protective, depending on how long it lasts. Wax seems to come off the bottom of boats in salt water fairly rapidly. The wax helps to seal the pores in the gel coat--so that slime does not attach (algae grows on the bottom). But I wonder if you are running at planing speed how long the copper will continue to stay on the bottom?