Anode needed on bronze shaft & prop?

[preservedkillick]

The setup on my boat makes it difficult to put a zinc on the shaft or prop. There's not much shaft showing in front of the prop, and behind the prop there's little room between the nut and the rudder. A 2.5 cm thick shaft collar will fit, but underway the thrust the engine/flexible coupling shifts forward a little and the zinc rubs on the cutlass bearing.

I've ground the zinc thinner, and it helps but need to take another mm or two off.

My question is this..do I really need an anode? Everything down there is bronze. My boat has been on a cold freshwater lake since it was repowered 10 years ago and I'm guessing previous owners did without. Now that I'm in salt water I'd like a zinc to protest against electrolytic corrosion. The reason I'm doubting myself is that I had a talk last year with the owner of a 50 year old wooden boat with bronze shaft/prop and he didn't use a zinc. His claim was that the boat sitting in cool (salt) water on a mooring, far from any marina with shore power and similar metals made the anode unnecessary. His boat was out of the water, and his prop and shaft did look to be in very good condition. I have my doubts..am I missing something?

[JimsCAL]

Many experienced boaters in my club don't put zincs on because they seem to attract the growth of barnacles on the prop and shaft. Boats are on moorings. I went for 20 years without zincs on my boat with bronze shaft and prop and saw no problems with deterioration of the bronze. When I changed my shaft (SS) and prop (bronze) this spring, I put a zinc on. Barnacles were covering the prop and shaft after a month or so. So I guess I will skip the zinc next season.

[Fstbttms]

Regardless of what flavor water your boat lives in and regardless of what meterial your running gear is made of; go without anodes at your peril. Zincs are cheap, props are not.

BTW- magnesium anodes for freshwater, not zinc. FYI.

[RichH]

A zinc anode doesnt necessarily have to ON the part/component thats being protected. All that is needed is the zinc anode be 'electrically bonded' to the component - you could mount the zinc 'anywhere' underwater and run proper wiring (including an electrical 'wiper' on the shaft) to make 'contact'.

Magnesium anodes are for use in fresh water.

[celenoglu]

anodes offer protection for different types of metals (different voltages). If your shaft and prop are bronze, they will not protection. If they are connected (electrically) to another under water metal such as your keel or other metal, you might need protection (not for the bronze) but for your iron keel. You ight use an anode on your keel if this isthe situation. Otherwise bronze is the best underground metal which doesnot degrade easily.

[Bellita]

Ok, I have a varient on this question: I too have a bronze shaft and prop. If I want to keep a zinc on the shaft as a just in case mechanism, since all items are bronze, can I strap it to the shaft where it connects to the transmission, inboard, vs near my cutlass bearing strut, which is also bronze, but in the water and prone to sea critter collection? any electron flow would come from good connection to the bronze, regardless of being in the water, yes?
any downside to doing this?

[Fstbttms]

Originally Posted by celenoglu

If your shaft and prop are bronze, they will not protection.

Hate to disagree with you chief, but you could not be more wrong. Bronze running gear is more susceptible to corrosion-related problems than other commonly used metals and absolutely needs protection.

[k1vsk]

Originally Posted by Bellita

Ok, I have a varient on this question: I too have a bronze shaft and prop. If I want to keep a zinc on the shaft as a just in case mechanism, since all items are bronze, can I strap it to the shaft where it connects to the transmission, inboard, vs near my cutlass bearing strut, which is also bronze, but in the water and prone to sea critter collection? any electron flow would come from good connection to the bronze, regardless of being in the water, yes?
any downside to doing this?

The way I read this, you are asking if you can effectively put the anode inside the boat - that would be useless.

[mitiempo]

I differ with celenoglu - there are different types of bronze. The bronze shaft is probably tobin bronze, a different alloy than the propeller which is likely manganese bronze. Tobin bronze is usually 60%copper, 39%zinc, and 1% tin. The prop of manganese bronze will be a different alloy even if only by a slight amount - probably 1% manganese instead of 1% tin. A zinc is sacrificial and will be eaten away instead of the zinc in your prop or shaft. You are dealing with 2 different metals even if they are both called bronze.
Brian

[mitiempo]

To be effective zincs have to be electrically connected to the metals they are trying to protect and must be immersed in the water. Often a zinc is installed around the shaft inside the boat AS WELL but this is only to prevent the shaft from exiting the boat if a coupling lets go.
Brian