Galvantic Corrosion

[M. R. Bober]

Prior to the replacement of the shaft on our 330, we had never experienced any galvantic corrosion. Now there is obvious erosion of the bottom paint surrounding all through hulls.
I have verified that the AC & DC grounds are electrically common, but don't know if it was always so. Does anyone know if this was standard. Other thoughts on the problem would be warmly received as I will be launched in a few days.
Thanks.



thebobers@erols.com

[Joe Valinoti]

Everything I always read said to isolate your AC and DC grounds from each other. Don't understand why changing the shaft would change that anyway.
Joe



vwman@coastalnet.com

[M. R. Bober]

The replacement of the shaft should have had nothing to do with the onset of the galvantic corrosion however the yard disconnected the bonding to the housing of the cutlass bearing. I reestablished the bonding. No other work had been done and the erosion took place in 4 months following the shaft replacement.
I have run all of the circuitry and believe that all is OK, but before I launch, I want to be certain that I haven't missed anything.
M



thebobers@erols.com

[John]

The replacement of the shaft should have had nothing to do with the onset of the galvantic corrosion however the yard disconnected the bonding to the housing of the cutlass bearing. I reestablished the bonding. No other work had been done and the erosion took place in 4 months following the shaft replacement.
I have run all of the circuitry and believe that all is OK, but before I launch, I want to be certain that I haven't missed anything.
M

The A/C and DC grounds should both be brought to a common ground point and that ground point should go to the engine block OR to a ground plate (like a Dynaplate). If the grounds are grounded at different points you set up a battery in essence because each of the two ground points would have different voltage potentials. That would create the galvanic corossion. The bonding wire circuit and any other grounds should all go to that same "common ground point". The only time this changes is if someone has an isolated a/c system. You don't mention the condition of the boats zincs. Do you have a shaft coupler with a hard rubber isolator between the coupler halves or is it all steel. If it has the rubber you need to connect a braided copper strap between the two halves to complete the ground. You don't mention what metal your old and your new shaft are made of. Our CD30 has every single ground wire (bonding included) on the boat terminated at a ground buss in the engine compartment with a #2awg ground running from the buss to a Dynaplate on the keel and another #2awg leading from the Dynaplate to a hull zinc ( Camp B12 ). The engine is grounded to the buss as well. She has absolutely no galvanic problems at all. We are talking the last 15 years. She is in the water (salt) year round with the A/C power on, battery charger in use on standby. Maybe that info will help you.

[Catherine Monaghan]

You've probably done this, but just in case ... I hope you didn't forget to install a new zinc collar around the prop shaft. Don't forget the the engine has zincs as well and they should be changed twice per season.

catherine_monaghan@merck.com
CD32 Realization
Raritan Bay

Prior to the replacement of the shaft on our 330, we had never experienced any galvantic corrosion. Now there is obvious erosion of the bottom paint surrounding all through hulls.
I have verified that the AC & DC grounds are electrically common, but don't know if it was always so. Does anyone know if this was standard. Other thoughts on the problem would be warmly received as I will be launched in a few days.
Thanks.

catherine_monaghan@merck.com

[M. R. Bober]

Good points. The replacement shaft was bronze (as was its predecesor), and the zinc was replaced at the same time. The bonding wire was knocked loose from the bolt to the cutlass bearing housing, apparently during the replacement.
I did find that there was almost no resistance between any of the throught hulls, except for the cutlass bearing housing. It was was reading a fraction of a ohm to each of the other through hulls.
I reconnected the bonding to the cutlass bearing housing, and I am hoping that all is now OK. BUT....



thebobers@erols.com

[John]

Good points. The replacement shaft was bronze (as was its predecesor), and the zinc was replaced at the same time. The bonding wire was knocked loose from the bolt to the cutlass bearing housing, apparently during the replacement.
I did find that there was almost no resistance between any of the throught hulls, except for the cutlass bearing housing. It was was reading a fraction of a ohm to each of the other through hulls.
I reconnected the bonding to the cutlass bearing housing, and I am hoping that all is now OK. BUT....

Take milliamp readings. I use a piece of copper plate with a #6awg wire soldered to it and a copper clamp soldered on the other end. I connect the clamp to one lead of a digital multimeter and connect the other lead from the meter to any thru hull or any other ground on the boat and place the copper plate in the water and take a reading.
You don't say if your problem is worse while being connected to shore power. You mention something about AC and DC in "common" in your initial post. I hope you don't have the AC neutral (white) conductor running to ground (that is okay if the AC system runs through an isolation transformer). It happens all to often causing significant electrolysis problems. It must be connected only to AC outlets and the shore power inlet. Also double check your ground at the shore power connection on the dock and make sure there is no voltage leaks to the water. Use the digital meter for that test as well, one probe connected at the ground terminal on the receptacle and the other probe in contact with the water using the AC scale on the meter see if any voltage is read, if so you've got a problem. As you are probably aware any voltage leak through any piece of equipment, switch, wire conductor can create the problem you are experiencing. One thing to keep foremost in mind is that voltage will seek the path of least resistance to ground. Make sure all connections are very clean and tight. Ground connections should all be soldered. By the way what were your ohm readings? If under 10,000 ohms you've definately got a ground leak problem. Turn everything off in the boat including the battery switch, disconnect the shore power, then connect one multimeter probe to the negative battery terminal and the other probe to the DISCONNECTED positive battery cable. See what kind of ohm reading you get. You want it 10,000 or above, anything else confirms a ground leak. Just some additional thoughts you might want to consider.