New Gori propeller corroded (pictures)

[boatbod]

I have some similarly horrific photos of bronze thru-hulls corroded in half (horizontally) at the waterline. How the boat in question didn't sink is a mystery... Just the 3M 5200 was holding things together!

[Maine Sail]

Originally Posted by Suijin

I've posted this before but it's worth it again for this thread.

This is a sail drive what was the victim of stray current. And this is off a cat and the other was just as bad. It took approximately 10 days for this to happen. Stray current off victim boat itself:

It should be noted that this is the effect of a DC stray current leak. Nine times out of ten I find the DC stray current leak originating on-board the owners vessel while at the same time the owner is claiming the "marina" damaged his/her boat. DC stray current corrosion can be extremely fast and very, very dangerous, boat sinking dangerous...

[Sailmonkey]

Originally Posted by Maine Sail

It should be noted that this is the effect of a DC stray current leak. Nine times out of ten I find the DC stray current leak originating on-board the owners vessel while at the same time the owner is claiming the "marina" damaged his/her boat. DC stray current corrosion can be extremely fast and very, very dangerous, boat sinking dangerous...

And when told about it, most owners chose to ignore it. I can sort of understand the near term desire to not spend money diagnosing something they don't understand, but this usually costs a lot more in the long run.

[a64pilot]

It's my understanding that AC current CANNOT cause electrolysis, that has to be DC current?
One possibility of getting DC stray current onto your boat is through the grounding lug of the AC shorepower cord. I believe that all the boats around you are all connected together through the common ground, and if one of them is leaking stray DC current into the ground wire, all boats connected to the shorepower are getting backfed DC current through their ground connection.
Unless they have an isolation transformer, that breaks the connection sort of?
AC current will flow though the transformer making it still a safe ground, but DC current won't flow through a transformer?
Is that the operating principle?

Never mind, apparently just diodes, no transformer

[Scot McPherson]

ac will cause corrosion, it's just different. It's called ACC. (Alternating Current Corrosion)


AC flows in both Capacitive and Resistive currents, the capacitive current causes reduction and oxidation. The Resistive current causes _severe_ corrosion. Thankfully salt water is not highly resistive, so it reduces the amount of resistive current.


As a result, I don't use isolators or any of that, they can fail. I keep my shore power completely isolated from all other systems. My inverter does not supply AC to any shipboard circuits, it only supplies power directly to appliances via a 20 amp plug (like you have in your house).


cathodic protection is the same for AC as it is for DC, you use a sacrificial anode. It's the same one you are already using, you don't need another one.

[fstbttms]

Originally Posted by Maine Sail

It should be noted that this is the effect of a DC stray current leak. Nine times out of ten I find the DC stray current leak originating on-board the owners vessel while at the same time the owner is claiming the "marina" damaged his/her boat. DC stray current corrosion can be extremely fast and very, very dangerous, boat sinking dangerous...

This prop went from looking like to new to this state of corrosion in just a few weeks. It was ultimately lost, even after the source of corrosion was discovered and corrected (broken insulation on a 12-volt bilge pump wire.)

[Maine Sail]

Originally Posted by Sailmonkey

And when told about it, most owners chose to ignore it. I can sort of understand the near term desire to not spend money diagnosing something they don't understand, but this usually costs a lot more in the long run.

This is why, after a corrosion survey, I insist we meet on the vessel to discuss the issues found. I then show them, physically using the test equipment, what is going on.

When they see the readings live, and realize that it's not the "marina" or the "other boats" they tend to get religion pretty quickly.

I even had a survey in MA that I conducted where the owner was ready to sue the marina owner. When I got to the boat it had a properly wired isolation transformer (owner did not know what it was just that it hummed) and a DC leaking bilge pump.

When I explained what the isolation transformer was, and what it did, and where the DC stray current was actually coming from, he became pretty quiet.. He genuinely seemed to feel pretty bad. I did not know why. From what I heard later he had apparently been pretty abusive about the issue to staff etc.. The funny thing is, I was paid to come in and prove it was the marina's fault that he had a major corrosion issue.

Originally Posted by fstbttms

This prop went from looking like to new to this state of corrosion in just a few weeks. It was ultimately lost, even after the source of corrosion was discovered and corrected (broken insulation on a 12-volt bilge pump wire.)

Absolutely, poor bilge pump wiring is perhaps the number one cause I see of DC stray current. I see and find others too, but bilge pumps are the major culprits. I've seen improper bilge pump & installation practices cause thousand of dollars in damages, including sinkings.

I know of one boat owner who used the frame/thru-hull fitting of his new underwater eBay sourced lights as part of the DC neg circuit. The aluminum thru-hull/frame, yes you read did that correctly, corroded right through. This only took a couple of days, the boat sank, and it was totaled. Sad part was it had just undergone a $45,000.00 twin 250HP Yamaha outboard motor re-power a few weeks earlier. To save some money, after the 45k bill, he decided to tackle the underwater lights himself.

[Sailmonkey]

That brings up another point. What is the worlds fascination with the stupid submersible bilge pumps?!? Use a diaphragm pump for day to day maintenance and leave the submersible up a little higher for flooding.....stray current from the always wet electrical motor goes away!!

[Maine Sail]

Originally Posted by Sailmonkey

That brings up another point. What is the worlds fascination with the stupid submersible bilge pumps?!? Use a diaphragm pump for day to day maintenance and leave the submersible up a little higher for flooding.....stray current from the always wet electrical motor goes away!!

Bingo! If you have the bilge space to do this it is a terrific option..

[buzzstar]

Originally Posted by Maine Sail

Bingo! If you have the bilge space to do this it is a terrific option..

The motor is always wet? I do not think so, which is not to say electrical leaks do not occur from submersible pumps. they can and do, but if the motor were always wet my understanding is they would not operate as pumps because the motor would fail.

I have no objection to being wrong if I am. It would change a piece of incorrect "knowledge"I have had in my head for years.

[senormechanico]

Originally Posted by Maine Sail

Bingo! If you have the bilge space to do this it is a terrific option..

Or have a dry bilge with a PSS shaft seal.

[Sailmonkey]

Originally Posted by buzzstar

The motor is always wet? I do not think so, which is not to say electrical leaks do not occur from submersible pumps. they can and do, but if the motor were always wet my understanding is they would not operate as pumps because the motor would fail.

I have no objection to being wrong if I am. It would change a piece of incorrect "knowledge"I have had in my head for years.

The motor is not always wet, but the very nature of the construction of those stupid little pumps makes them failure prone. You have a cheaply constructed case with a motor stuck in it relying on low quality rubber grommets on the wires and a tiny little shaft poking out the bottom to keep water from entering the housing and creating a ground path in the water. Also, by having the pump in the water means that you need to run the electricity into the water.....how many times have you seen a gooped up mess of wire laying in the bilge leading to the pump?

I concur with seniormechanico that the dry bilge is the ticket also.

[buzzstar]

Originally Posted by Sailmonkey

The motor is not always wet, but the very nature of the construction of those stupid little pumps makes them failure prone. ....

I concur with seniormechanico that the dry bilge is the ticket also.

Thanks. Being correct is little better than the knowledge gained via being incorrect.

[boatpoker]

When I took my Marine Corrosion Certification course the first thing said was "If you use the word"electrolysis" in your exam it will be an automatic failure as you clearly have not understood the issue". If you hire someone and they use that term, hire someone else !

This looks like low grade DC stray current to me. AC stray current can cause corrosion but at an extremely slow rate. 99% of the time stray current corrosion is caused by your own boat. Neither a galvanic isolator nor isolation transformer will protect you from current leaks in your own vessel.

I vote with Mainesail. Check your bilge pump for stray current first.

[Maine Sail]

Originally Posted by boatpoker

When I took my Marine Corrosion Certification course the first thing said was "If you use the word"electrolysis" in your exam it will be an automatic failure as you clearly have not understood the issue". If you hire someone and they use that term, hire someone else !

Absolutely.. Nails on a chalk board & similar to "amps per hour"...