Prop shaft zinc mystery-HELP PLEASE

[RodeoClown]

Good morning All-

In preparation for a coastal trip departing this Saturday morning, I dove my 1991 Beneteau Moorings 38 yesterday to discover that the 22-25 mm propeller zinc anode was gone---4 mm allen head bolt and all. This is a cone-shaped anode that screws in to the end/tip of the propeller shaft.

I had dove the boat on 1 April 2018 and had noted the existing zinc was at 75% AND the bolt loose---which I tightened hard.

I have only taken the boat out a couple of times (estimated engine/prop run times ~3 hours) since then.

I have changed zincs many times before without issues and am really baffled.

The obvious claim would be that I did not tighten adequately...but I really don't think that's the case.

Is there any equivalent for marine locktite that can be applied while in the water?

Humbly...

David
s/v Rodeo Clown

[GordMay]

See ➥ http://www.cruisersforum.com/forums/...ml#post1318178

[Vasco]

The zincs I get for my Beneteau come with an allen screw that has blue Locktite like stuff already on it.

[2big2small]

When your anode is being eaten away around the bolt, then the bolt is no longer compressed against the former and able to wiggle loose with the vibrations of the shaft and of the loose anode.

There are anodes on the market with embedded aluminum washers to minimize this issue but I don’t know whether they make them for your type.

[mikereed100]

In addition to whatever thread lock you use, you might consider coating the screws in Tefgel or something similar to reduce galvanic action between the stainless bolt and zinc. This may help prevent zinc loss under the bolt head. Some people use nail polish. Another option would be lanolin if you have no Tefgel.

[Shrew]

Originally Posted by mikereed100

In addition to whatever thread lock you use, you might consider coating the screws in Tefgel or something similar to reduce galvanic action between the stainless bolt and zinc. This may help prevent zinc loss under the bolt head. Some people use nail polish. Another option would be lanolin if you have no Tefgel.

I've heard this before and you are correct......t absolutely slows the zinc corrosion. However, it is a SACRIFICIAL anode. This is the same as using teflon tape on the threads. You are isolating the zinc from the metals it is trying to protect. This is rendering the zinc useless. The purpose is to corrode first and prevent the running gear from corroding instead.

In other words, this is the same effect as using no zinc at all. DO NOT DO THIS!!!

[billknny]

Just to be 100% clear, you lost just the zinc, not the bronze nut is is attached to?

[S/V Alchemy]

Originally Posted by Vasco

The zincs I get for my Beneteau come with an allen screw that has blue Locktite like stuff already on it.

Same with my Variprop. I do this once a year before launch, however. In the water could get awkward just because dogging down an Allen key with nothing to brace against is problematic in a way that scraping off barnacles with a paint scraper isn't.

[mvmojo]

Originally Posted by 2big2small

When your anode is being eaten away around the bolt, then the bolt is no longer compressed against the former and able to wiggle loose with the vibrations of the shaft and of the loose anode.

This is likely what is happening. Try putting a stainless steel lock washer between the zinc and the prop and compressing it all the way when you install the zinc. As it is eaten away, the lock washer will expand to keep pressure on the zinc threads. As others stated, don't put anything on the threads as that will insulate the zinc from the shaft and render it useless.

[Emouchet]

I always paint the lock washer and screws with anti-fooling paint. This makes sure the corrosion in that area is minimized, allowing a longer life for the anode.

[2big2small]

If anything you’d probably want to use a plastic washer and nothing else.

[Jim Cate]

Hmm... Tef-gel and lanolin are lubricants as well as anti-corrosion agents. They don't play nicely with Loc-tite or other thread lockers IMO. Also, putting Loc-tite on the threads will not cause lack of conductivity between anode and shaft, for there will still be metal to metal contact in the mating of the threads.

FWIW, in similar installations I have used loc-tite and a lock washer under the head of the screw with good success.

Jim

[mikereed100]

Originally Posted by Shrew

I've heard this before and you are correct......t absolutely slows the zinc corrosion. However, it is a SACRIFICIAL anode. This is the same as using teflon tape on the threads. You are isolating the zinc from the metals it is trying to protect. This is rendering the zinc useless. The purpose is to corrode first and prevent the running gear from corroding instead.

In other words, this is the same effect as using no zinc at all. DO NOT DO THIS!!!

I should have been more clear. The Tefgel is applied to the head of the screw where it contacts the zinc. I am thinking more of a shaft zinc application here, where you would apply it to the nut as well. The electrical contact is made between the zinc and shaft, not the zinc and bolt. Maybe not pertinent to the OP's situation as his is a prop zinc with which I am not familiar.

[Sunsetrider]

I switched to aluminum anodes. Better than zinc in salt water, lasts longer.

[fguptill]

Sunsetrider,
Aluminum anodes may be a good idea in fresh water, but not in salt water. After all, if longevity of the anode were an important criterion, we would all use anodes made of wood. Very cheap, last a long time - and totally ineffective for protection against galvanic corrosion.
Fair winds