Prop Shaft Electrolysis


I have just hauled Kristy out to repaint the bottom after 2 1/2 years.  She's been sitting in the water in a marina at Brunswick GA for the last 2 years where I've hired a diver to clean the bottom and replace the zincs as needed every 4-6 weeks.  I was surprised to see no zinc on the prop when we hauled out, and dismayed to see severe electrolytic corrosion of the prop shaft when I started changing the seals. I posted a pic at the KristySM243 folder
I need advice on what is involved in removing the prop shaft.  Is it necessary to remove the bottom of the drive aft of the keel?
I guess I'm going to look into isolating the boat electrically.
Currently Tiger Point Marina, Fernandina Beach, FL

amelliahona <no_reply@...>

Dear Kent:
I too am shocked at the state of  your prop shaft.  I looked for the diagram of the C-drive in my files, as I too have seen a drawing of it recently, but I can't find it.  Here is a link to a photo of the C-drive (I think you may have posted it) if that will help. 
 It appears there is access to the shaft and bearings of the C-drive from the forward end but probably this is glassed in and I am not sure how you remove the C-drive housing.  I think you are going to need to have a serious discussion with Amel as to how to proceed.  I will continue to look thru the photos and files sections of this forum as I am pretty sure the diagram is on this forum somewhere. 

As for zincs, mine last about two seasons (only in the water for 6 months at a time due to hurricane haul out annually.  Whenever I leave the boat in a marina I DO NOT leave it plugged in to shore power.  I have the caretaker plug it in once a month for 6 hours to really top up the batteries, but otherwise depend on some small solar panels to keep the batteries trickle charged.

One time I asked Amel why they used the red plastic cap on the auto prop rather than placing a zinc there.  Their reply was that it was un-necessary and that you can "over-zinc" the boat.  I wasn't really sure as to the logic behind either of those answers but seeing as they were the experts and I was the novice I didn't press for more details.

I wish you the best.

Gary Silver
s/v Liahona   currently at Jolly Harbor


Kent, I am curious how long a prop Zinc usually lasts you?
We have to change Libby's every 3 months. We have seen this rate of burn in 3 marina's over the last 3 years. No one place better than the other.
Knowing that Amel replaced the prop Zinc with a plastic cap and we burn through that particular Zinc every 3 months concerns me.
Our rudder Zincs last a lot longer. 6-8 months.
Looking for some logic here.

San Diego

amelliahona <no_reply@...>

Kent:  I think I found the diagram of the C-drive you were referring to.  It is in the Files section of this site posted by BeBe under the topic C-drive

Also, as related to your anchor chain issue, I just read about your back surgery etc, so sorry if my suggestion seemed insensitive.

Best of luck, 

Gary Silver
s/v Liahona


Hi and Happy Holidays to all!

I finally heard from Amel about my C Drive.  It was sent to the supplier in France who repaired it, but found that the bearings were too loose for them to be comfortable sending it back to me.  They are going to send me a new one which will take several weeks to build and will cost 2930 Euros plus taxes and shipping.  That's a lot less than I anticipated.  I should receive it in Feb if it doesn't get stuck in Customs again.


While it looked like something that could have been repaired in the US, Joel strongly urged me to send it back to Amel for inspection and repair.  Thank you for that recommendation, Joel, I'm not sure that anyone here would have recognized the tolerances required of this proprietary Amel drive.


I guess I'll be sailing by this Spring.


Steady as she goes,





Hi, everyone,
I hope you are all having a great sailing season.

My new C Drive has finally arrived and we are sorting out the reinstallation to be done in early Feb when I can be present.  Hallelujah!!!!

I have just started looking at the electrical system to find the source of the current leak...with the help of a British electrician who was highly recommended by the marina owner.  Here's what we've found so far:

1. With the 24v battery switch closed (on), and the 220V shorepower off (at the breaker on the switching panel on the engine room forward bulkhead), there is 3Volt DC between the bonding system and the negative battery pole.  This disappears when the 24V battery switch is opened (off).  When the 220V breaker is turned on, this voltage jumps to 6V, whether the charger is powered or not (at the 220V panel in the galley).  When the 12V charger is turned on, the voltage jumps to 9+V.  My electrician is pretty confused by this.

2. When measuring this voltage between the negative battery pole and the bonding system, if the 24V battery switch is opened (turned off), there is a spike to the full battery voltage of 27V which degrades to 0 V over a minute or so.

3. We also discovered that the bonding wires from forward of the engine room were not connected to the angle iron rails that support the engine, but rather went to the stainless "muffler" and to the engine, outdrive, and transmission.  The bonding wire from the zincs on the rudder is attached to the angle irons.

I'm going to do some more investigation this evening, but wondered if anyone had any ideas about what may be going on.

As usual, thanks ahead of time for any light you can shed.

Alan Leslie

Hi Kent,

Congratulations on the receipt of the C-drive...hope the installation goes well.

Regarding your electrical sounds like there's a capacitance between the earth bonding system and the negative battery connection.
If the boat was in the water that could be the seawater if the negative power to your SSB and tuner are connected to the SSB ground plate. But I guess you're not in the water so that doesn't work.

But somewhere there is some kind of capacitive leak from the 24V negative circuit to the earth bonding.
It could be a faulty appliance which has an earth bonding connection and 24V supply?

Regarding the engine beds, the engine is isolated from the beds by the mounts so I guess it doesn't matter if the beds are bonded or not, the engine parts that come in contact with seawater are bonded, that's the important thing.

Hope there are some gurus out there that can help you further.

SV Elyse SM437
Gulf Harbour

Jim Anderson

Kent, I have been following your saga and am wishing you the best. Thank you for keeping us all in the loop.
Others be advised: When I had the generator on SM384 serviced at Amel Caraibes/Martinique in 2011 the first thing Amel's sub-contracted technician did was ask me if I wanted him to remove the Onan ground isolation relay. I had no idea what it was or why it was there at the time and it was not causing a problem. The technician told me it was "not a necessary part" and sometimes caused problems and recommended I have him remove it. Not knowing any better (from you experience and postings), and since he was an agent of Amel, I agreed and let him take it out. Priority#1 when next aboard will be to find it and have it re-installed. Hopefully I will have a prop shaft and C-Drive remaining on my next haul-out. One more lesson for me in never un-Amel-ing anything and trusting that everything is there for a reason.   - Jim  SM384 Sirena Azul, Seattle


Follow-up on the electrolytic damage to the prop shaft on Kristy:

The new CDrive is working fine, much less vibration than with the old drive and motor mounts.  No water getting into the drive oil.

After getting the boat back in the water, I have checked the hull potential at least once a week, and found good readings every time.  I did NOT check it with the engine or Onan running.

I dove on the bottom to look at the zincs and found that the prop zinc was over half gone and it's bolts nearly completely was about to fall off.  The rudder zincs were at least 1/3 gone.

I hauled out in Deltaville to look at the prop shaft and have the topsides polished.  The shaft is OK.

I have motored/motor-sailed about 30 hours and run the Onan some 50 hours since putting her back in the water 6 weeks ago.I  am certain that there is still something creating a stray current, and suspect the alternators on the Yanmar as it has not been electrically isolated yet.

I'm going to have the Yanmar isolation completed, and find the problem before I set sail again.  I'll let you know what I find.  At the very least, I know now that  hull potential must be checked with engines running, charger running, AC running, and while head pumps, bilge pump, and anchor wash pump are running.  Perhaps it should also be checked while winches and other 24v gear is operating.



Hi everyone,  Happy Holidays!

My electrical issues keep making me think I don't have my prop shaft electrolysis problem figured out yet.

Everything seemed fine all summer and fall, I was checking hull potential regularly, and it was always right where it should be, with shore power, generator, all 24v equipment on/off. It still reads -935mv which is right on the money.

My zincs are still going way too fast...half gone in 2 months.

Recently I picked up some debris that broke the impeller in my Marche AC Pump.  I pulled it out and ordered a new impeller.  When putting it back together today, with all 220V AC breakers OFF, I got a pretty good shock when I touched the brown wire going to the pump.  I was sitting on the engine bed.  I checked with my multimeter and found 124V AC between the brown wire and the bonding system.  I was hooked up to 220V 50A shore power that was reading 244V AC on my 220 monitor.
Then I disconnected shore power and cranked up the generator.  Still with all 220V AC breakers OFF, I get 72V AC between the brown wire and the bonding system.  The AC pump housing is all plastic with a magnetic impeller, so there's no water in contact with the pump motor.  It is NOT connected to the bonding system.

To complicate things further, the Water Heater just started tripping the main 220VAC breaker in the cabinet above the sink.  I talked to the manufacturer and checked the resistance of the heating element, and it is 41 Ohms, just what it should be.  The water heater is NOT connected to the bonding system.

Anyone have any ideas?  It seems to me I have a hot 220V AC connection to the bonding system somewhere?  How do I find it if it's there with everything turned off?

Thanks for any thoughts.


Hi Kent,

If you are plugged into a 50 amp US outlet at the dock i assume you did not hook up the white lug in the plug to anything.

I believe the brown wire should go to the red lug and the blue to the black lug. the yellow green to the green lug.

If I am not mistaken you will still have about 10 volts between the ground and the power lugs in the plug--as the ground and the white neutral are essentially the same.

I think somewhere on your boat there is leakage between the green yellow A/C wire and one of the hot wires.

I am doing this from memory as I am not near the boat.


sm 376 kimberlite.