Props and anodes

[AKDoug]

Hello all,

I am wondering if the metallurgy in a propeller changes with age, requiring it to be replaced at some point due to loss of "something" in the alloy? I am still spinning the original prop and it has no issues, but I read somewhere that as the prop "pinks" up that the metallurgy was changing. Just wondering...

Also, after reading through the anode threads that come and go, I wondered if there were any other Willard owners who do not have an anode on their prop shaft? Mine does not, has no issues, and the aluminum anodes on each side of the keel show pretty normal wear and tear.

There isn't adequate space on the shaft for an anode to fit between the cutlass bearing and the prop, the space between being less than the diameter of the prop shaft. The two zinc anodes (original) were replaced when I had the bottom barrier coated and re-painted and I replaced them with aluminum anodes.

PS just had an article on zinc vs aluminum and the aluminum was superior in every respect except marine growth on the anodes. That's not an issue up here with seasonal haul outs and the cold water, the bottom just accumulates a little "slime" instead of growth unless you leave it in the harbor tied to the dock. Then you grow hay on the sunny side of the hull.

I used high copper Trinidad SR, and there was no sign of incompatibility with the aluminum anodes even in close proximity to the anodes. Turns out the SR is probably not my best choice with seasonal haul outs... Live and learn.

Just wondering Inquiring minds want to know!

 

[DavidM]

"Pinking" is due to electrolysis that removes the zinc in bronze propellers. It is generally due to not having a zinc or other sacrificial anode close by. I do believe that it reduces the strength of the prop, otherwise why have the zinc as part of the alloy. Nibral props which have little zinc and form an aluminum oxide protective layer probably do not "pink", but they may suffer from gross corrosion.


Sacrificial anodes produce less protection the further they are from the metal they are trying to protect. So prop shaft or prop anodes are good and your hull anodes may be too far away to do any good. There are ways to attach an anode to the aft end of the prop and shaft by replacing the nut with a nut/anode attachment. See Propeller Nuts Complete With Zinc Anode


I had this system on my Cape Dory 28 and it had the same prop shaft clearance limitations as yours.


David

 

[Ted]

In some cases you can actually cast the anode over the
prop nut.
This is a slow rpm prop, <400, 53" dia

Ted

 

[AKDoug]

I have no pinking going on currently, it was something I read about somewhere in all that stuff I ingest about boats and boating... My anodes are about 2' from the prop on both sides. I did think it odd there were none on the prop or shaft, but I can't find any kind of an anode in my raw water cooling system either and the Yanmar manual says nothing about one being either present or needed.

The boat only had 46 hours on it when I bought it, so I assumed it was set up properly by the manufacturer. I have learned not to assume much...

My rudder is fiberglass, no anodes on it either...

 

[Nomad Willy]

Doug,
Here's my setup w the zinc as per David's post #2 for 1 1/4" shaft.
I have the original prop but it was a bit thin and some pinking was evident. Probably strong enough for my 37hp but I bought a new prop. Now I use the specialized nut in David's post. Hate putting them on. It's hard to get the cotter pin right in the slot on the nut. Always manage to do it though.

However on my previous boat I used the typical double nuts and cotter pin through the prop shaft. I had a multi-strand copper wire that had a zinc on one end and a scissor snap on the other. Put the snap (like alligator clip) on the prop shaft inside the boat and put the end w the zinc on in the water .. over the side near the prop. Taking it off and putting it on got to be old though.

 

[Capt.Bill11]

Beware the Brass - Professional BoatBuilder Magazine

 

[LaBomba]

Hello all,


There isn't adequate space on the shaft for an anode to fit between the cutlass bearing and the prop, the space between being less than the diameter of the prop shaft.

Just wondering Inquiring minds want to know!

AKDoug, I also had very limited space between the prop and cutlass bearing but was able to find a limited clearance collar anode which was only 1" wide for a 1-1/2"shaft. They are available for many shaft sizes and most marine stores.

 

[Phil Fill]

Some engines props shaft are protected by being connected to the zinc loop, green wire connected to the big main zincs. So make sure the connection of the green wire is conducting. Our perkens gen set has not zincs, so I ran green wire driectly to the zincs. As an additional ptotection I have a zinc grooper hanging out the engine room window. It's a big zinc that looks like a fish with a 12 foot cable with a clamp on the end. Sold at most marine stores.

 

[boatpoker]

The vast majority of propellers although referred to as "bronze" are actually a brass alloy with a much higher zinc content than true bronze. Galvanic action between a brass alloy prop and other metals can cause de-zincification and this loss of zinc is what turns the brass pink. Stray current will cause a more dramatic and much faster loss of zinc. "Electrolysis" is the change in an electrolyte due to the passage of electrical current. Do not take advice from anyone who uses the term electrolysis is this regard as it is an entirely different process.

 

[DavidM]

The vast majority of propellers although referred to as "bronze" are actually a brass alloy with a much higher zinc content than true bronze. Galvanic action between a brass alloy prop and other metals can cause de-zincification and this loss of zinc is what turns the brass pink. Stray current will cause a more dramatic and much faster loss of zinc. "Electrolysis" is the change in an electrolyte due to the passage of electrical current. Do not take advice from anyone who uses the term electrolysis is this regard as it is an entirely different process.

Hmmmm! "Electrolysis" is probably like "black iron".

David

 

[AKDoug]

Lay terms work fine for me. I understand the leeching of zinc from the alloy causing pinking, and that anodes prevent the leeching. Also understand that the movement of the propeller causes field activity that is the root cause of leeching of the alloy.

If the alloy pinked at rest it would be caused by field activity not related to the movement of the propeller but would probably eat up my aluminum anodes first (and rapidly). The field activity would probably be caused by improper grounding of either my vessel or a nearby vessel or dock.

This post has been very productive for me, I appreciate the input you have all given me. I will be looking into the anode that mounts on the nut on the propeller if I can find the right search terms for it. Sometimes that's the hard part... I will also consider mounting another anode on the rudder shoe, and will look again at the spacing on my shaft between prop and cutlass bearing. Also will look to see if my anodes are grounded to the prop shaft as suggested.

Great comments, thank you all!

 

[psneeld]

If you are worried about your prop and don't have room on the shaft nor can live with a nut zinc....

Then a shaft brush tied to a hull zinc will work.

My shaft brush seems to do all the work as every year my collar shaft zinc looks brand new

 

[boatpoker]

Hmmmm! "Electrolysis" is probably like "black iron".

David

Yup, almost 4,000 surveys and never seen an iron tank, black or otherwise.

 

[Phil Fill]

General information

All metals have a galvanic rating. The lower rated protects the high rated metals. On a scale of 1 to 100 in general zinc is 5, most aluminum is 5 to 20, mild steel is 25 to 35, ss is 35 to 50, bronze coppet is 50 to 75, silver, 75 to 80, gold 80. So wider the spread the more the lower protects. So when working with metals their rating has to be considered.

During the winter I hang 2 zinc grooper over the side one connected to the Perkins gen set, and one to the bow thruster for additional protection.

 

[boatpoker]

Bit of a misnomer to call it a "rating". The galvanic series is just a list of metals with their natural voltage potentials. Lots of examples of these "galvanic series" tables on line.