Galvanic Aluminum Tunnel Perferated

[Otter Works Alaska]

 

[Ben Okopnik]

The weld-on zinc - which is about as good as can be done in that direction, barring active galvanic protection like Electro-Guard and their ilk - definitely says there's galvanic corrosion (and it should have been changed quite a bit ago; that looks like well over 50% gone. Also, hitting them with a wire brush any time you get the chance is a good idea; they can't protect your boat through an oxidized surface.) But I wouldn't hurry to blame the salt water, or change the type of zinc. I had weld-on zincs on my steel-hulled sailboat that was kept in warm, salt Florida water the year around - and these got consumed at maybe 5% per year. The difference was that I didn't keep it in a marina; I anchored out.

I've done on the order of a couple of dozen galvanic surveys on boats over the years, and here's a plain fact: I have only seen one marina, in all that time, that didn't have ridiculous amounts of electrical leakage into the water. Some of them, you couldn't put your hand in the water without getting a shock. And guess what that grounds through, on your vessel - especially if you're plugged in? In fact, one of the first ones I ever saw at a little marina in the Chesapeake Bay, was a shaft that had been brand new the month before eaten half-through - and the wood around it literally bubbling with white foam. Galvanic isolators can help, but they're mostly going to protect you from anything that's mis-wired on your boat eating your metal; the currents in the water are what's really going to zap you.

If you're in a marina, it would be a really good idea to test the water there first, and to handle the issue with the marina owner (tip: the cost of a survey is usually higher than the cost of a reference electrode, which run ~$100 - and that, plus a regular multimeter, is all you need to do surveys of your own.) Meanwhile, replace that zinc and don't be shy about welding on three or four in its place. I had 4 10-pounders on my hull, which was about 25% more than I calculated I needed for my 36-foot steel ketch, but this was a place where I figured it's better to have more rather than less. Also, docks further away from the marina office tend to have much lower potentials, in my experience. Just sayin'.

 

[Otter Works Alaska]

Greetings Ben, nice, thank you. I do galvanic surveys for a living here in Alaska. I daily see aluminum vessels with jet units with extensive galvanic damage at the water intake tunnels. These seine skiffs replacement values are $400,000 each, and damaged are running $50k to $150k a shot. Every one of these vessels is using old school zinc instead of aluminum anodes. The zinc anodes show no activity, and are completely encrusted as one might expect for a zinc anode being launched in and out of the water. And the number of anodes is down to one or two per vessel. I am seeking information about other aluminum water intake tunnels and how persons are reducing hull potential during maximum jet rpm. I figure if I am seeing this issue on 100 percent of the vessels here, others are seeing it too and have formed a successful mitigation response. I too live on a steel boat year round. I keep my style hull at -16mvdc and up to +30mvdc in reference to a millspec zinc test lead. The tunnel in the photo is about 20” in diameter. Check out more of the projects on Facebook: Otter Works Alaska

 

[buckbuck]

20" diameter?! @swatski needs a couple of those to try on his boat.

 

[Ben Okopnik]

Greetings Ben, nice, thank you. I do galvanic surveys for a living here in Alaska.

Heh, then you know exactly what I mean with this stuff. Marinas just eat boats that don't have good zincs...

I've done boat electrics and electronics on boats for 20 years on the east coast and in the Bahamas/Caribbean, so for me, galvanic surveys were just a part of the job rather than a specialty - and, like I said, correcting the problems is usually a matter of finding the outside damaging factor. You're dealing with something quite different up there.

I daily see aluminum vessels with jet units with extensive galvanic damage at the water intake tunnels. These seine skiffs replacement values are $400,000 each, and damaged are running $50k to $150k a shot. Every one of these vessels is using old school zinc instead of aluminum anodes. The zinc anodes show no activity, and are completely encrusted as one might expect for a zinc anode being launched in and out of the water. And the number of anodes is down to one or two per vessel. I am seeking information about other aluminum water intake tunnels and how persons are reducing hull potential during maximum jet rpm. I figure if I am seeing this issue on 100 percent of the vessels here, others are seeing it too and have formed a successful mitigation response. I too live on a steel boat year round. I keep my style hull at -16mvdc and up to +30mvdc in reference to a millspec zinc test lead. The tunnel in the photo is about 20” in diameter. Check out more of the projects on Facebook: Otter Works Alaska

Sounds like you've got your hull nicely passivated, but the folks around you don't do nearly as well... good business opportunity! Given the kind of costs you're talking about, it might be worth looking into active cathodic protection; I'm all about doing things economically, but at that point, a few grand looks like pocket fluff. I have no experience with it myself, but I've heard of the Navy using them successfully, and met a guy who was doing high-latitudes sailing trips, and just loved his system.

Checked out FB - looks like you're having a lot of fun up there in the high latitudes!

 

[zipper]

@Otter Works AlaskaWelcome to the forum.

 

[swatski]

Greetings Ben, nice, thank you. I do galvanic surveys for a living here in Alaska. I daily see aluminum vessels with jet units with extensive galvanic damage at the water intake tunnels. These seine skiffs replacement values are $400,000 each, and damaged are running $50k to $150k a shot. Every one of these vessels is using old school zinc instead of aluminum anodes. The zinc anodes show no activity, and are completely encrusted as one might expect for a zinc anode being launched in and out of the water. And the number of anodes is down to one or two per vessel. I am seeking information about other aluminum water intake tunnels and how persons are reducing hull potential during maximum jet rpm. I figure if I am seeing this issue on 100 percent of the vessels here, others are seeing it too and have formed a successful mitigation response. I too live on a steel boat year round. I keep my style hull at -16mvdc and up to +30mvdc in reference to a millspec zinc test lead. The tunnel in the photo is about 20” in diameter. Check out more of the projects on Facebook: Otter Works Alaska

@Otter Works Alaska I hope you will contribute here and post on the forum! many of us here (including myself) are not on FB.

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