E35-3 Prop Shaft Strut - a look inside - and a zinc question

[peaman]

Why?

The zinc is used to create a favorable voltage difference, and that voltage is set based on the the metals involved. It would seem that a greater area provided for the voltage to act upon would result in greater current which would lead to corrosion or consumption of the zinc.

If the strut were encapsulated in epoxy, the bronze would not corrode because it would not be in contact with the water. It follows logically that some protection of the bronze, presumably provided by bottom paint, will provide some level of protection.

My reasoning could be seriously flawed, as I am no expert, but I know that this is a complex subject.

 

[fstbttms]

It follows logically that some protection of the bronze, presumably provided by bottom paint, will provide some level of protection.

Anti fouling paint is not a waterproof coating. In fact, it works by allowing water to penetrate its matrix. If there is any protection (such as you mentioned) being provided, it is negligible. I certainly have never seen any evidence of it in my 27+ years/30,000+ hull cleanings experience.

 

[Prairie Schooner]

We went to Defender's spring sale opening event on Thursday. There was a zinc company rep there. He was a pretty low-key guy. His most enthusiastic recommendation was for the use of their special aluminum alloy anodes over zinc. When I asked if we should add one to the strut he said something to the effect, "Sure, it wouldn't hurt.” He didn't have a strong opinion either way on painting the strut but did stress the importance of making sure there is a good clean conductive connection between anode and part. He did stress that metal rudders should have anodes.

The company’s info page is pretty helpful and at least gives the impression that they've done their science. I haven’t had a chance to read and digest in depth.

Sacrificial Anodes FAQs

Q. What do sacrificial anodes do? A. All metals immersed in an electrolyte (sea water for example) produce an electrical voltage. When two dissimilar metals are in contact (electrically connected) they produce a galvanic cell (like a battery), with the less noble metal (a bronze propeller for...

performancemetals.com

There were several paint reps as well. I asked one about paint on struts and his answer was non-committal.

So, I shrugged and got two aluminum anodes for the shaft. I'll clean the paint off the strut and leave it bare except for some Propspeed. I’m kicking the strut anode decision down the street and will see what it all looks like in the Fall when we haul.

 

[fstbttms]

I'll clean the paint off the strut and leave it bare except for some Propspeed.

Now Propspeed is a waterproof coating and will absolutely help protect your strut from corrosion.


BTW- the blurb you quoted from the Performance Metals website is really basic stuff that every boat owner should know by heart.

 

[Prairie Schooner]

Now Propspeed is a waterproof coating and will absolutely help protect your strut from corrosion.

BTW- the blurb you quoted from the Performance Metals website is really basic stuff that every boat owner should know by heart.

Propspeed: It was your positive comments elsewhere on this site that moved me to that decision. Thanks.
Blurb: Yeah, when I post a link the algorithm picks the quote/preview. I don't even know what it will be until I hit 'enter'.

 

[Kevin A Wright]

I don't have a strut zinc on my E35 and see no issues. I didn't have on on my E27 for the first 15 years I owned it. Then one time I was diving to clean the bottom and noticed I had pink pits up to 1/4" deep in spots on the strut. I believe someone new in the marina had a grounding problem and It was trying to eat away my strut. So I added a strut zinc and problem solved. You need to drill a 3/16" or 1/4" hold depending on the size of bolt in the zink and I also ground some flats on either side of the strut so the back of the zinc had good contact with the strut. You don't want any paint in the way of the electrical connection.

Never had problems after that. Had to change the shaft zincs every year but could often go 2 years or more on the strut. Think eventually the "hot" boat moved on and didn't have to change the strut zinc after that.

Kevin Wright
E35 Hydro Therapy

 

[fstbttms]

Then one time I was diving to clean the bottom and noticed I had pink pits up to 1/4" deep in spots on the strut. I believe someone new in the marina had a grounding problem and It was trying to eat away my strut.

The day a hear boat owner not blaming another boat for his corrosion issues is the day I die a happy man.

 

[Rhett_m]

I’m rebedding the strut on our ’86 E35. It doesn’t have a zinc on it. It seems to be in good shape with no corrosion. There was a zinc on the shaft. Should I add a zinc to the strut (in addition to replacing the one on the shaft)? What are the factors to consider?
Jeff
View attachment 42098

I personally have one on mine. I think it’s a standard practice to have a zinc on the strut. In theory I suppose the shaft zinc would protect it.

 

[fstbttms]

I think it’s a standard practice to have a zinc on the strut. In theory I suppose the shaft zinc would protect it.

Not standard practice at all. In fact, many more boats have no strut anode than do.

Regarding the shaft anode protecting the strut- that doesn’t happen. The strut is electrically isolated from the shaft by the cutless bearing.

 

[dhill]

A galvanic isolator can help in preventing corrosion due to poor grounding, particularly if you are usually connected to shore power.

Galvanic Isolators Explained

Often overlooked, this piece of equipment is critical in the fight against corrosion.

www.boatus.com

 

[Pete the Cat]

Not standard practice at all. In fact, many more boats have no strut anode than do.

Regarding the shaft anode protecting the strut- that doesn’t happen. The strut is electrically isolated from the shaft by the cutless bearing.

That would be true except in many Tartans where they bonded the strut to the engine block and some of the through hull fittings. it was also common among some Cals and I am not sure how many other production boats in the 80s. On my Tartan, the disc on the strut is eaten before any of the zincs on the prop or shaft. for reasons I do not know. I do agree that ambient electricity in a marina can cause this but I am currently on a mooring.

 

[fstbttms]

On my Tartan, the disc on the strut is eaten before any of the zincs on the prop or shaft.

Well, it isn't because the strut is being protected by shaft anodes, that's for sure. And even in the few cases where the strut may be bonded to the engine block (and God knows why anybody would think that was good idea), the electrical path between the anodes and the strut is extremely long at best and extremely resistant at worst. It's hard to imagine that the strut would actually receive any protection from such an arrangement.

 

[peaman]

Now Propspeed is a waterproof coating and will absolutely help protect your strut from corrosion.

I wonder how good Petit "Prop Coat" is for props and struts? It claims to be 93% pure zinc when dry, is applied in one step (from a spray can), and costs a fraction of that for Propspeed. My marina store (south New England) says they sell a bunch of it each spring.

 

[fstbttms]

I wonder how good Petit "Prop Coat" is for props and struts? It claims to be 93% pure zinc when dry, is applied in one step (from a spray can), and costs a fraction of that for Propspeed. My marina store (south New England) says they sell a bunch of it each spring.

Apples and oranges. Propspeed is a foul release coating and Prop Coat is simply spray paint with a lot of zinc in it. The price difference reflect the difference in effectiveness.

 

[Loren Beach]

Perhaps a related question....
Given that little differences in weight in prop blades can cause vibration, I wonder if a sprayed-on coating might induce some differences in thickness that would put the prop a little out of balance? Or, is the coating typically so thin, and hopefully even, that this is not a concern?

 

[fstbttms]

Perhaps a related question....
Given that little differences in weight in prop blades can cause vibration, I wonder if a sprayed-on coating might induce some differences in thickness that would put the prop a little out of balance? Or, is the coating typically so thin, and hopefully even, that this is not a concern?

It takes a lot more than some spray paint to unbalance a prop. A LOT more.

 

[peaman]

Perhaps a related question....
Given that little differences in weight in prop blades can cause vibration, I wonder if a sprayed-on coating might induce some differences in thickness that would put the prop a little out of balance? Or, is the coating typically so thin, and hopefully even, that this is not a concern?

Propspeed calls for prepping the surface with 80 grit, followed by two coats of primer, applied by roller, and finally a top clear coat. So it seems they aren't too concerned with imbalance.

 

[Pete the Cat]

I have used the Petit product. I didn't find it particularly effective--seemed like it stopped working after a very short time-- and it seemed to react with the bronze surface of the strut. You can buy the same stuff for cheaper as a generic cold galvanizing paint at a hardware store. If you compare the ingredients you can see that Petit just relabeled some off the shelf product. I use it on my anchor and it makes it look better and keeps it cleaner. My thought is that the inherent drag of the strut and propeller (even though I have a feathering propeller) is enough that a little growth in a season probably would do as much to make things more slippery as to help inhibit growth.

 

[Prairie Schooner]

Forensics
I chipped the many layers of ablative paint off the strut today, then wire brushed it. The only sign of corrosion was in the area where the raw bronze was inside the fiberglass. We're rebedding it because it was wiggling a little and this area had some salt water seepage. The area which was covered by bottom paint had no corrosion. (You may notice little dings. That was my heavy handed chipping.) I take this to mean that ablative bottom paint is acceptable protection against corrosion, at least in the brackish tidal river where the boat used to live. That said, our plan is still to leave this raw bronze and coat it with PropSpeed.

I'm thinking of getting bronze bolts to replace the SS. Available at McMaster Carr. Do you think this is overkill?

 

[Pete the Cat]

This looks a little pink at the top. I might have someone with metal expertise take a look at it before reinstalling it. This might argue for a disc zinc on the strut. Is this bonded to something? Bonding is controversial, I am just asking for my education, not advocating.