Prop being eaten

[AKM]

Old MD7A with 110S saildrive & fixed 3 blade prop. Ensured good contact between saildrive & anode but anode barely gets touched & prop. is being eaten away. Any ideas?

[MartinR]

Alu or bronze prop?

[Montanan]

Snipet located on the web.

"Volvo Penta Zinc Anode for 110S Saildrive.

Please note this is a Zinc anode for salt water use. A Magnesium version should be used for fresh water. In brackish water an aluminium version should be used. Anodes should not be mixed on any installation.

If your anode is worn away by 33% or more, it is advisable to change it as more of its surface area has been worn away and the remaining anode will wear away much quicker.

The anode protects the drive, engine and propeller from corrosion due to electrical currents in the water. When you change your anode, make sure that it is not painted and that it has a good contact via the bolts and metal surface to the saildrive. "

[AKM]

Aluminium prop. Saltwater marina. Zinc anode definitely isn't protecting prop.

[Sailmonkey]

Originally Posted by AKM

Aluminium prop. Saltwater marina. Zinc anode definitely isn't protecting prop.

If the prop isn’t electrically connected to the drive leg the leg anode isn’t protecting it.

Put a meter on the ohm scale and see what the resistance is between the prop and drive leg. Of course you’ll need to be out of the water to do this.

[Montanan]

Originally Posted by Sailmonkey

If the prop isn’t electrically connected to the drive leg the leg anode isn’t protecting it.

Put a meter on the ohm scale and see what the resistance is between the prop and drive leg. Of course you’ll need to be out of the water to do this.

Exactly. Sure sounds like electrical isolation of the prop from the ring anode.

References: Read the Final Check.

https://www.cruisingworld.com/beware...ed-saildrives/

https://www.yanmar.com/us/wp-content...-Corrosion.pdf

The hull potential as defined by ABYC is, “the composite potential of the electrically-connected
immersed meal parts of a hull, relative to a reference cell.” To adequately protect the aluminum saildrive
the hull potential of the boat must be maintained between -950 to -1100 mV. At no time should the hull
potential drop below -900 mV or any exposed aluminum on the saildrive will start to freely corrode. The
desired hull potential can be achieved by using sacrificial anodes that are connected to the boats’
bonding system or by using an impressed current system. Anodes should be selected based upon
where the boat is operated/moored. In salt and brackish water, zinc and high energy aluminum anodes
are recommended. In freshwater, high energy aluminum and magnesium anodes are recommended. At
no time should magnesium anodes be used in salt or brackish water.
The boats’ hull potential should be tested on a fully assembled boat that is placed in the water and
measured using a silver/silver chloride reference electrode. A Certified ABYC Corrosion or Electrical
Technician should be utilized to perform the test.

[Corrosion-Eng]

Originally Posted by AKM

Old MD7A with 110S saildrive & fixed 3 blade prop. Ensured good contact between saildrive & anode but anode barely gets touched & prop. is being eaten away. Any ideas?

You have stray current creating marine electrolysis corrosion. Do a Google search for Marine Electrolysis and click Conquer marine corrosion.

[Capt Phil]

Montanan hit the nail (or anode) right on the head!
Phil

[AKM]

Thanks for the info. Boats back out of the water next week so I'll do some testing.

[fstbttms]

Originally Posted by Corrosion-Eng

"...marine electrolysis corrosion."

This is not a thing and anybody who uses this term and claims to know what they are talking about does not.

[Montanan]

Dang metal eating barnacles munching on the prop!!!!!

FYI:

Electrolysis, Stray Current, Galvanic Corrosion on boats explained

"ELECTROLYSIS : Chemical changes in a solution or electrolyte due to the passage of electric current.

As a Certified Corrosion Analyst I'd like to clear up the "ELECTROLYSIS", issue. This term is frequently misused due to lack of understanding of the processes involved in corrosion.

ELECTROLYSIS is actually the opposite action to galvanic corrosion so the distinction is NOT trivial semantics as some people suggest.

ELECTROLYSIS is the forced introduction of an electrical current in an electrolyte (water) to cause a chemical reaction to separate the components of the water. This process produces Hydrogen and Oxygen.

GALVANIC CORROSION on the other hand is an electrochemical reaction that causes electrons to flow from one metal to another metal. One of the metals is the anode and the other is the cathode. If you put the two in an electrolyte that conducts current, and connect them with a wire, they act pretty much the same way a battery does. A current flows between the two metals. The electrons from one are "sacrificed" and plated (somewhat) onto the other metal. This happens when you have dissimilar metals such as aluminum and bronze close to each other and in "electrical" contact. The aluminum disappears.

STRAY CURRENT CORROSION is simply corrosion caused by stray (leaking) current from a bilge pump or a battery charger among other things.

There are many more types of corrosion however, the two main issues on boats involving electricity are either GALVANIC CORROSION or ELECTROLTYIC CORROSION (commonly referred to as "stray current corrosion".

Punch line ........ Do not take advice from or hire a marine electrician who uses the term electrolysis."

1. Galvanic corrosion : Corrosion that occurs at the anode of a galvanic cell.

You may remember high school chemistry class where a battery was created by connecting two dissimilar metals with wire and immersing the whole contraption in salted water thereby activating magnetic fields and starting an electrochemical process causing current to flow.
On a boat with bronze, aluminum, galvanized and stainless steel that are connected with bonding wires or simply touching each other and immersed in the water...... you accomplish the similar thing. The more noble metal is the "cathode", the less noble, the "anode". In this process the less noble metal gives up electrons to the more noble thus weakening the metal, otherwise known as "galvanic corrosion".

The "sacrificial" anodes on your shafts, trim tabs etc. are supposed to sacrifice themselves thereby protecting expensive metal parts. This is why it's important to keep your anodes in good condition and never paint them. let's never refer to anodes as "zincs" as anodes come in three basic materials for different water conditions i.e. Aluminum alloy, and magnesium for fresh and brackish water or zinc for salt water. This topic deserves a little more attention on it's own so take a look at Zincs, Aluminum and Magnesium Anodes.

A vessel suffering from galvanic corrosion is usually the source of it's own problem, although two vessel's linked by shore power grounds can create a galvanic cell between two very close boats.

2. Electrolytic Corrosion (commonly referred to as stray current corrosion) : Corrosion that results from an electrical source causing a metal in contact with an electrolyte (water) to become anodic with respect to some other metal in the same electrolyte.

In simple terms a wire touches something it shouldn't, like a faulty bilge pump float or degraded wiring lying in the bilge sending current into the water, causing one metal to give up electrons and corrode. Again any vessel suffering from this type of corrosion is likely the master of it's own disaster but the culprit could also be a neighboring vessel. This type of corrosion can can eat metals at an alarming rate. I know of one 42' motoryacht that lost both shafts, both rudders and both propellers in a space of less than two weeks.

Complicating this picture somewhat is the fact that DC can be super-imposed on your AC wiring through the common ground on board or the ground in the shore power pedestal we all share on the dock. As all vessels in the marina are connected through shorepower grounds there is potential for widespread damage. Aside from concerns of corrosion there is also potential for electrocution if shorepower cords are allowed to lie in the water let alone the fools that leave their shorepower cord plugged in at the dock while they go out for an afternoon cruise.

Recent tests have shown that AC current from shorepower in the water can also cause corrosion to underwater parts although at a much slower rate than DC. Ground fault protection systems, galvanic isolators, isolation transformers and impressed current systems are some of the various methods attempting to combat corrosion.

Salt water is generally regarded as a more serious breeding ground for marine corrosion as the salt makes the water more conductive however, polluted fresh water can be even more conductive with the right contaminants."

[senormechanico]

Originally Posted by fstbttms

This is not a thing and anybody who uses this term and claims to know what they are talking about does not.

[Lepke]

A good chance some electrical item is installed with the pos & neg reversed. The radio, etc., doesn't always need to be turned on to cause the electrolysis.
Commercial fishing in the old days, I saw a harbor mate have his 36" prop loose it's blades in 3 weeks. A radio tech had installed a radio internal repair component with the poles reversed. Sitting at the dock with the radio off was draining the battery, but the charger kept in up. Later the owner went to leave, went into reverse and nothing happened except for a little turbulence near the stern. A diver came back with the bad news and a very thin blade found on the bottom.

[Toccata]

An old thread, but I think my question best sits here.
Setup:
Savage Oceanic (grp) in salt water. Yammer with SD60
Old alloy prop being eaten by some sort of electrolytic activity.
Mixture of stainless and bronze through hull fittings, not bonded.
Rudder has a ss shaft sitting in a "non metallic" bush, housed in a bronze casting at the bottom of the skeg, this has a disc style anode either side.
Current situation (not yet relaunched)
Yanmar SD60 (with new ring style anode) A newly installed Flexofold prop (with its own new anode)
The boat was also setup with a "tear drop" anode separately bonded to the engine (only)
I have replaced all through hulls with Marelon.
Questions:
Yanmar don't have pencil anodes in the heat exchanger on this model, I presume because the ring anode is doing the protecting?
The separate tear drop anode bonded to the engine would seem to be unnecessary to me?
There must be some potential for the different metals in the anodes themselves to be the cause of activity between the anodes, particularly the 2 on the sail drive being so close together?
Does the bronze casting at the rudder 1.5m away from the engine need anodes?
Thanks in anticipation for collective wisdom and experiences