Sacrificial Anode Needed on Bronze shaft?

[preserved_killick]

On my Alberg, there's not much space between the cutlass bearing and the prop, so I've had to grind down a shaft collar zinc to fit. I've got a bronze shaft & prop. Last fall, I was speaking to someone while poking around his old wooden boat and asked "where's your zinc?". He told me he's never used one. His shaft and prop looked ok to me. Since then, I've made it a little personal mission to query people I meet with classic boats (likely to have a bronze shaft) about zinc usage. Surprisingly, most people answer no, they have never used one..which surprises me.

The bronze in the shaft and the bronze in the prop are likely different, and thus one's got to give..correct? One must be less noble than the other. Yet, these people are going years (20 + in one case) with no zinc, and no problems (that they know of). Could the oxidization process process be so slow with the similar metals that in the time frame of these owners, it is not an issue? Could these guys have brittle significantly weaker shafts and simply have not stressed the metal enough to break anything?

Note that my survey, about 12 people, was conducted in Maine.

Opinions?

[DaleH]

A great synopsis ...

"Despite the complexity of the issue, boats with galvanically compatible underwater metals usually will not be affected (unless the problem is extreme), while the ones with lesser compatible metals, the ones that become anodic, suffer the consequences."

... from this site here, source = http://www.yachting-life.net/new%20inde ... _frame.htm

It's a great read!

[Tim]

I have a stainless shaft, bronze prop, and no room for a zinc--so I don't use one. This does not seem to cause any issues.

Most shaft zincs in Maine that I have seen come out nearly new-looking at the end of the season. The galvanic difference between various bronzes, and between stainless and bronze, is really quite minimal, and these metals will react together very slowly/negligibly unless you add in some stray current (such as that found in and around most marinas with shore power), which exacerbates the process. Still, barring atypical circumstances, the two components (shaft and prop) won't seriously react with each other without external influence, or extreme amounts of time.

Some boats have serious stray current issues contained wholly within the boat, usually escaping via a poorly-maintained or ill-conceived bonding system. These boats will also tend to waste their zincs extremely quickly, but this is not the norm.

No scientific data here--just years of anecdotal evidence and observation. I can speak only for Maine as well; results elsewhere may vary.

In my opinion, this is another one of "those" issues that tends to get blown way, way out of proportion on the Chicken Little scale, and which is also strongly divided between two opinion camps. I say look at what really happens, not the theory of what could happen.

[Rachel]

Sounds like you probably don't need one, but just as a data point if you decide to add one at some point: On my A-30 I (perhaps unnecessarily as it sounds now) put on a new zinc that was part of the prop nut system. I don't have a photo with it installed, but here's one showing the general fit of things when I first got the boat, so you can see how it compares to yours. I could dig up the zinc info if you did want it (I know I tried a couple of different ones for the best fit).

Rachel

[Tim]

I should add that it certainly won't hurt anything to install a shaft zinc (or prop nut zinc) if you can fit one. But if you can't, don't let worries of instantaneous metal disintegration cloud your enjoyment of the boat--it'll be fine. (Unless you have external influences that exacerbate the process, as indicated above)

[Quetzalsailor]

As Rachel noted, you can buy the nut-mounted zinc. It requires a special prop nut which is sort of castellated and extends out past the end of the shaft, and has a threaded hole to receive the zinc. The buullet-shaped zincs are readily available in at least two sizes. My observation on Quetzal's during my recent spring horrorshow of a fitout (documented elsewhere on this site), was that this kind of zinc is electrically more remote (more joints) from the boat and my stainless shaft end was badly corroded. The bronze nut and propeller were fine. I put it all back together with an annular zinc bolted to the prop shaft between the stern fitting and propeller, a nice and direct connection.

When we bought the boat in 2004, there were two zincs in addition to the propshaft zinc. These were hull-mounted and teardrop shaped. They had been screwed to the hull with wood screws. The reason that they were uncorroded was that they were not attached to any of the metal in the boat. Such zincs would be functional if they were through-bolted to a connection and wire leading to the engine. Clearly, some thought should be given to how easy or difficult changing them would be; mine would have been a bear, way down in the skeg.

[preserved_killick]

Thanks everyone.

I was able to fit a shaft zinc, after I hack-sawed it down to 17mm in width. It fits fine, so I'll leave it there. Over the winter I may look into the nut-zinc setup if it looks easier. I'm just surprised to learn that's it's less of an issue than I thought.

-Jeff

[Rachel]

Jeff,

I haven't seen your installation, so this is more of a general question, but when I see a collar zinc that's a tight fit between the cutless bearing and the prop, I always wonder if it might impede the water flow to the cutless bearing and thus cause a problem.

Can that happen? Or is there always enough water-flow space, even it if seems very minimal?

Rachel

[preserved_killick]

Rachel,

From what I understand it can impede water flow. In my situation, the zinc barely fit on the shaft before I cut it down, but under engine thrust it would rub on the cutlass bearing as the engine moved forward on it's mounts. I checked and still had drips at the stuffing box though, plenty of them in fact but that's a separate issue. The new zinc is just under 25 mm in width, I cut it down to under 17 mm and it fits fine about 10mm space to the bearing exit. I still have a good drip at the stuffing box.

One thing I "think" I notice is the engine runs a little rougher with the cut down zinc than without. I'm sure that my zinc is not perfectly cut. With this warm weather I enjoy the swim so I may experiment taking the zinc off and motoring around with & without.

-jeff