450: Stray current corroded saildrives

[boatpoker]

Originally Posted by skipperpete

Good question and I don't fully understand the precise definition but I would describe ordinary corrosion as occurring where saltwater simply drips onto a metal surface and eats into that surface but electrolytic corrosion in all its forms requires an external electrical input through , in these instances a salt water conductor. Bi metallic corrosion between metals is a whole other mystery but might also be described as electrolytic. In both the incidents , no reason was ever discovered for the damage so would appreciate any enlightening views on this. The propeller and shaft attached to the 4JH were both destroyed, even the nut and most of the key. Both boats were in marinas.

My posing the question was to point out that the uninformed and incorrect advice in this thread benefits no one.

As a certified Marine Corrosion Analyst I highly recommend this entire thread be ignored by anyone wishing to actually learn about these issues.

[rom]

Originally Posted by boatpoker

As a certified Marine Corrosion Analyst I highly recommend this entire thread be ignored by anyone wishing to actually learn about these issues.

We stupid cruisers would be very grateful if you could share a little bit of your knowledge explaining how such a disaster can happen.


You seem to think the cause is stray current (electrolytic corrosion) on the boat itself, not related to other boats or the marina. What scenario do you have in mind ?

[Cadence]

Originally Posted by boatpoker

Dear OP. The statement above is a perfect example of why you should not take advice from this forum.

Surely you knew that was in jest?

[mjuneau]

Now this thread is going to take a turn and go off topic

[Cadence]

One thing I hadn't seen mentioned was the possibility of DC wiring laying in a wet bilge or a terminal block severely corroded causing leakage if it is from an onboard problem. Somewhat unlikely but who knows.
Not being a certified anything, maybe a nut, there were some good suggestions as to possible causes of the OPs severe problem. I hope it does not reoccur, I'm sure it was costly. There my be a pearl or two of wisdom for those with a less severe problem. JMHO

[David B]

Originally Posted by skipperpete

My condolences to the OP re the loss of the sail drives to what appears to be electrolysis, it seems to be fairly common on boats in langkawi with no common thread linking the events. A recent SD 50 saildrive on a grp monohull was destroyed by very specific and severe corrosion that ate away the lower shaft bearing carrier but not the leg itself. Seawater had entered the saildrive so it was replaced with a complete new SD60.
Prior to this, a low hours 4JH series Yanmar was substantially destroyed by what can only have been electrolytic corrosion and required a new engine, propeller and shaft. The corrosion attacked along the raw water intake and the first casualty was the oil cooler and then the heat exchanger which leaked onto the starter motor at the rear and the circulating pump at the front, destroying them both due to simple saltwater corrosion. Attachment 179580Attachment 179581.
Hope the photos are attached

Frightening stuff. Did their insurance cover this?

[boatpoker]

Originally Posted by rom

We stupid cruisers would be very grateful if you could share a little bit of your knowledge explaining how such a disaster can happen.


You seem to think the cause is stray current (electrolytic corrosion) on the boat itself, not related to other boats or the marina. What scenario do you have in mind ?

Stray current does not equal "electrolytic corrosion" which is in fact galvanic in nature and could not possibly cause this type of corrosion in less than a few years.

The only possibilities for corrosion to act this quickly are 1. A lightning strike. or 2. Stray current corrosion which can make drives and shafts disappear in a matter of days.

[Mark Johnson]

Stray current corrosion is generated from your boat’s own DC wiring, and a sacrificial anode is no protection from it.

There can be no problem at all for years, and suddenly there is, after an event like an electrical storm.


I have had it happen to me!

[Derek Lumb]

Originally Posted by boatpoker

What is the difference between electrolytic corrosion, electrolysis, galvanic corrosion, stray current corrosion and simple salt water corrosion ?

The main issues here are galvanic corrosion and electrolytic corrosion. Galvanic corrosion is caused by the current which flows as a result of two items of different metals which are connected together and placed relatively close together in salt water. This is why we fit sacrificial anodes to protect our saildrives and propellers. Electrolytic corrosion is the situation where two metalic items are connected to a dc voltage source and placed in salt water. The mechanism is similar to galvanic corrosion but the driving voltage comes not from the fraction of a volt difference between 2 metals in the galvanic series but the driving voltage from a voltage sourse - usually the 12v battery. In the situation you describe it is only the latter which could cause the sort of damage you describe in such a short time. Either could be described as stray current corrosion.



Given the history I would be suspicious of damage caused by the lightning strike on board. Any point where the positive wiring becomes connected to a through hull fitting or to anything sitting in bilge water could cause problems.



Other have mentioned problems in the same marina which could point to stray currents flowing through the earth wire of your shore power connection.Yanmar recommend the installation of a galvanic isolator (or isolation transformer) because the saildrive is not isloated from the engine and hence the boat negative wiring. A galvanic isolator is essentially one or more diodes in series, in parallel with the same thing with the diodes connected in the opposit direction. A lightning strike could easily have fused the diodes into short circuit thus providing a direct connection and exposimg yout boat to any problems of stray currents in the marina.

[Mark Johnson]

Originally Posted by Derek Lumb

The main issues here are galvanic corrosion and electrolytic corrosion. Galvanic corrosion is caused by the current which flows as a result of two items of different metals which are connected together and placed relatively close together in salt water. This is why we fit sacrificial anodes to protect our saildrives and propellers. Electrolytic corrosion is the situation where two metalic items are connected to a dc voltage source and placed in salt water. The mechanism is similar to galvanic corrosion but the driving voltage comes not from the fraction of a volt difference between 2 metals in the galvanic series but the driving voltage from a voltage sourse - usually the 12v battery. In the situation you describe it is only the latter which could cause the sort of damage you describe in such a short time. Either could be described as stray current corrosion.



Given the history I would be suspicious of damage caused by the lightning strike on board. Any point where the positive wiring becomes connected to a through hull fitting or to anything sitting in bilge water could cause problems.



Other have mentioned problems in the same marina which could point to stray currents flowing through the earth wire of your shore power connection.Yanmar recommend the installation of a galvanic isolator (or isolation transformer) because the saildrive is not isloated from the engine and hence the boat negative wiring. A galvanic isolator is essentially one or more diodes in series, in parallel with the same thing with the diodes connected in the opposit direction. A lightning strike could easily have fused the diodes into short circuit thus providing a direct connection and exposimg yout boat to any problems of stray currents in the marina.

Neither is stray current corosion. The third sourcs, or “stray current corosion” is as I said, usually from a glitch in your DC ststem, and can eat your drive train in a couple of days...


Delphys Marine

[Cadence]

Originally Posted by Mark Johnson

Stray current corrosion is generated from your boat’s own DC wiring, and a sacrificial anode is no protection from it.

There can be no problem at all for years, and suddenly there is, after an event like an electrical storm.


I have had it happen to me!

Probably a lightning strike causes hi-pot shorts as the result of making pin holes in the insulation. In his case probably wiring only in a damp area. I hope that isn't his problem, that is a bitch to find and generally requires a mega. Trial and error sacrificing a drive each time doesn't seem plausible.

[Gundy]

Google up a "SeaBis Master System" and have a read. This may help get your head around some possible causes and solutions.

Cheers Gundy

[Peeew]

I might have missed this if it was posted earlier. But Yanmar now says never to use copper bottom paint on a boat with saildrives due to corrosion issues. Not just on the drive but the whole bottom should be free of copper paint.

[mjuneau]

Shouldn't a 20 amp leak be easy enough to find with a clamp meter?

[David B]

Originally Posted by Peeew

I might have missed this if it was posted earlier. But Yanmar now says never to use copper bottom paint on a boat with saildrives due to corrosion issues. Not just on the drive but the whole bottom should be free of copper paint.

Incorrect advice in regard to Coppercoat anyway. We have a saildrive with Coppercoat right up to it (and I know several who have actually painted their saildrives with Coppercoat). Not a scrap of corrosion on ours. The cone anode (Flexofold) typically lasts a couple of seasons, and the ring anode, typically lasts 3-4 seasons. We have a galvanic isolator fitted (www.safeshoremarine.com) and it also has a monitor 'panel' to indicate if there is any stray current it is blocking (I have yet to see any in all the marinas we have visited over the past 5 seasons or so).

As to how we treat the saildrive, it has 4 coats of epoxy, then 3 coats of hard antifouling. Under all of that is the original black factory finish, and althought quite a lot of that epoxy had flaked off over the last couple of seasons (this season I removed what I could and then re-did as above), there was as I said, not a scrap of corrosion under there, even though we left the boat in the water permanently connected to shore power for a full year when we were in North Africa.

I know there is this 'thing' about Copper and aluminium, but there are a whole bunch of boats out there with aluminium hulls and saildrives etc that have been done in Coppercoat.

I hope the OP gets to the bottom of this, and I hope he can post some images. There are a lot of us here I am sure, wanting to know what caused this, and therefore to ensure others do not suffer this problem one day.