Electrolysis Protection?

[GeoffD]

I'm fitting out a monohull, which has stainless steel prop shaft and prop tube, and a bronze prop and gland. I'm wondering whether a zinc near the prop on the hull exterior is sufficient to protect the top of the prop tube where it joins the gland? Or do I need to get some zinc up inside the prop tube? More broadly, which book/s are worth reading on protection against electrolysis - there seems to be such a diversity of opinion!

[zeehag]

add a dry zinc inside-- works. i have been advised to add one to my stern tube to protect it from the bronze of the packing gland and vice versa--m stern tube is ss, also.....

[GeoffD]

Dry zinc? You mean inside the hull, not in the water?

[David M]

An anode must be in an electrolyte for it to work as a sacrificial anode. It must be in the water or some sort of liquid which is conductive. The electrolyte is part of the electrical circuit.

Bronze is reactive with stainless but not all that much. Place an anode on the shaft near the propeller and on the shaft where it exits the hull and that should provide adequate protection. Leave about an inch gap to make sure you have adequate water flow to the cutlass bearings.

[zeehag]

my stern tube isnt so dry as yet, so on my boat it just may work.....

[GeoffD]

Hi David, thanks for that, it's what was puzzling me. What are your thoughts on my original query? (Hope it's OK to address this to you as a moderator). Geoff.

[David M]

No problem.

If I am understanding correctly, you want to attach an anode near the propeller but not attached to any metal. Is this correct?

A sacrificial anode must be electrically attached to the metals in the circuit for it to work.

The way I suggested earlier is a very common way of protecting your propeller and other dissimilar metals in the submerged portion of your propulsion system. Is there a reason this would not work for you?

[GeoffD]

Hi David, yes, I understand that the zinc needs to be in the water, and connected electrically to the metal items needing protection. And the theory behind the electrochemical series.

My concern is practical. I've read that the zinc needs to be physically close to the items to be protected, with a water path of no more than 4 or 5 feet. If I mount the zinc on the hull near the bottom of the prop tube, the water path is maybe 4 feet, which sounds OK - but it is very narrow, since it goes through the narrow gap between the prop shaft and the bottom bearing. I imagine this reduces the conductivity of the water path dramatically, and so reduces the effectiveness of the anode in its job of protecting the interior of the prop tube.

What I'd like to be clear about is whether this is a real issue, or just a figment of my imagination. If a zinc outside the hull works OK, that's great. But if this arrangement allows electrolytic erosion of the SS of the stern tube, near where the gland is attached, maybe I need to place some zinc nearer the site of this erosion. Yes, I know that inspecting and maintaining a zinc inside the prop tube would be a bit of an issue...

Hope that clarifies my question, sorry it was a bit unclear. What thoughts?

- Geoff

[David M]

I would go ahead and try the easiest method first. As long as your shaft tube is getting some water circulation then I don't see a problem. Somewhat stagnant sea water still conducts electrons. If the sea water is sealed up then you have other problems, such as lack of oxygen in the water causing stainless steel corrosion. Not trying to worry you but if your shaft had no pits up in there the last time it was pulled then you probably don't have a sea water circulation problem.

[GeoffD]

Hi again David, thanks for your thoughts. I have no practical experience around prop shafts, and appreciate your taking time to help me. I just had a natter to an old boat builder I met ages ago, and he says he has never heard of anybody putting an anode inside the prop tube. So I guess it isn't needed. Except maybe in oil tankers! So now I've got two opinions that agree. I'll just use external anodes. Thanks again. Geoff.

[redhead78]

Hey all I installed a doughnut on my shaft, I noticed inside the area where the bronze prop tube was there was a wire attached from Jeanneau, that I physically could not reattach. I can run a wire from the inside of the engine compartment, to where ??? do I need this with the doughnut zinc??? Thanks Red

[s/v Jedi]

Originally Posted by redhead78

Hey all I installed a doughnut on my shaft, I noticed inside the area where the bronze prop tube was there was a wire attached from Jeanneau, that I physically could not reattach. I can run a wire from the inside of the engine compartment, to where ??? do I need this with the doughnut zinc??? Thanks Red

You mean you noticed that there was supposed to be a wire but it isn't there? Or did you remove it first?

I would check this out on a sister ship first. Also, if Jeanneau took the effort and expense to indeed install that wire, you can bet that you need it. It might be the safety ground connection for your AC system or another crucial part of the systems design where a donut zinc on your propshaft would do nothing to replace the wire.

ciao!
Nick.

[goboatingnow]

have a read of this http://www.kastenmarine.com/_pdf/mbqCref.pdf good summary on zincs etc

Dave

[redhead78]

nick, when I pulled out my stern tube the 2 bolts holding it were just turning, as you can,t get to this area. Turns out there was a small ground strap in there, its so tight I could not even see in there, I can attach one at the othe end in the back of the engine compartment, I just do not know where to terminate it. I am putting the zinc on the prop shaft either way. Thanks Red

[osirissail]

There appears to be some nomenclature problems here - ?stern tube? or do you mean cutlass bearing? Normally to me the "stern tube" is the fixed in place, part of the boat normally referred to as the "shaft log" which contains a cutlass bearing in one end and a stuffing box or dripless system on the other end.
- - If the propeller shaft is attached to the transmission flange with an "isolator" such as a "DrivSaver" or similar plastic type device then a wire is necessary to reconnect electrically the prop shaft and propeller to the main engine for the bonding system or electrolysis mitigation system.
- - In any case there are occasionally available "prop nuts" with a covering zinc for boats where there isn't room enough for a prop shaft zinc.