Mystery of the vanishing zincs (HELP!)

John, CD28 · Post by **John, CD28** » Wed Mar 04, 2009 10:00 pm

I repowered the boat last year, and all goes well. Except for the zinc - that goes quickly. Very quickly.

Before I repowered, the boat had a bronze prop (2 blade Michigan Sailer) on a bronze shaft, and a drive saver. There was nothing to bridge the drivesaver electrically, so I'm thinking the old prop and shaft were electrically isolated from the boat's bonding/grounding system. The collar zinc just forward the prop lasted several seasons. The old prop feels like phosphor bronze, or magnesium bronze.

In comes the new motor, a STAINLESS shaft, and a new aluminum bronze prop (Variprop). A new drivesaver is in place, and again nothing to ground/bond the shaft. The new motor & drive train runs great.

After 10 weeks in the water, I haul the boat for a quick bottom wash, and the zinc tail cone is gone from the end of the new prop. It's been less than 3 months, so I figure the screws backed out (despite the locktite) and the zinc fell off. I put a new zinc on, and she goes back in the water.

I haul for the season a month later, and that new zinc is down to an eroded little mound, barely there, and ready to drop off. Now I suspect the first zinc dissolved, and didn't just fall off mechanically.

What's causing the zinc to erode so fast? Do I need to bond the shaft/prop? Is the stainless shaft & bronze prop creating a battery? Is Something else going on here?

Any and all ideas are much appreciated!

Best,
John
CD28 Tantalus
www.yachttantalus.us

David · Post by **David** » Wed Mar 04, 2009 10:13 pm

John,

Is your boat on a mooring, on a dock or in a marina? The bronze prop and the stainless shaft do create a battery, so you do need the zinc, but one zinc should certainly last the sailing season. Is the stuffing box connected to the bonding system? is there underwater metal close to the prop / shaft that is bonded? What else is connected to the bonding system (house ground, green wire AC ground, etc)? In the photos on your website it looks like you hve a bonding wire connected to the cutlass bearing mounting bolts?

John, CD28 · Post by **John, CD28** » Wed Mar 04, 2009 11:09 pm

David wrote:John, Is your boat on a mooring, on a dock or in a marina? ...Is the stuffing box connected to the bonding system? is there underwater metal close to the prop / shaft that is bonded? What else is connected to the bonding system (house ground, green wire AC ground, etc)? In the photos on your website it looks like you hve a bonding wire connected to the cutlass bearing mounting bolts?

The boat sits on a mooring (wired for shore power, but never plugged in).

The stern tube is bronze. Yes, there is a grounding wire, but the shaft doesn't actually touch the stern tube. There is no stuffing box, just the PSS shaftseal I put there, and that doesn't appear to ground the shaft.

The only other metal nearby is the rudderpost. There is no grouding wire off that stuffing box, but with cable driven wheel steering, that whole structure may well have found a ground at some point. There doesn't appear to be any signs of erosion on the rudder post, shaft, or prop (yet).

The boat has a sintered bronze Dynaplate to which all the bonding wires are attached. It seems to be in pretty good shape.

John

mitiempo · Post by **mitiempo** » Thu Mar 05, 2009 3:04 am

I remember seeing a spring clip of some sort that was mounted beside the shaft and contacted it to make a connection - this would allow you to ground the shaft. Sorry, I don't remember what it is called but maybe someone can remember it.

Post by **Tim** » Thu Mar 05, 2009 7:09 am

I have always been of the opinion that one either bonds everything, or nothing--no in between.

It wouldn't be unheard of for there to be some sort of current leak through your existing bonding system that is causing your zinc to erode; typically, when you have zincs eroding as quickly as you say, it's clear evidence of a problem along these lines. Bonding systems are notorious for creating problems where none would exist otherwise.

Since your boat's not plugged into a slip, you have to look at the boat's own electrical system for the root cause, I think. I've seen boats with all their through hulls becoming wasted because of a bad bonding system, so be glad it's just your zinc.

John, CD28 · Post by **John, CD28** » Thu Mar 05, 2009 1:54 pm

Thanks All,

I'm not sure why the zincs are now eroding so quickly, but clearly something has changed.

I'll start by running a wire across the drive saver, to bring the prop & shaft into the bonding system. I'll hang a big zinc grouper off the boat too, clipped to the backstay (bonded) to offer up more anode.

Any other suggestions?

John

David · Post by **David** » Thu Mar 05, 2009 2:01 pm

Well, I would suggest disconnecting your bonding system and see if it makes a difference in zinc life. I'm not sure that bonding the prop shaft will do anything other than eat up the zinc faster.

Popeye1865 · Post by **Popeye1865** » Thu Mar 05, 2009 10:50 pm

don't take steps to mask the problem, Fix It. as Tim said something is up find the issues. check to see if there is a continuity between the shaft and eng. is the shaft saver allowing a connection. people use SS shafts and bronze props all the time with zincs that will last a season. (you should change them every season anyway!)

John, CD28 · Post by **John, CD28** » Fri Mar 06, 2009 11:02 am

Popeye1865 wrote:don't take steps to mask the problem, Fix It. as Tim said something is up find the issues. check to see if there is a continuity between the shaft and eng. is the shaft saver allowing a connection. people use SS shafts and bronze props all the time with zincs that will last a season. (you should change them every season anyway!)

I agree - there is likely an electrical grounding/bonding problem here. I'll get the multimeter out and start poking around. I've got electrons flowing, and need to shut them off (open the circuit).

I'll check for continuity between the shaft & transmission, rudderpost, etc. Maybe take a look at the bilge pump, dynaplate, thru-hulls - try to see the circuit the prop & shaft are now part of.

A zinc really should last more than a month.

Thanks again guys.

John
Cape Dory 28 Tantalus

EDIT: Maybe the shaft IS finding a ground via that bonding wire on the stern tube. Maybe that's the bridge allowing the electrons to flow, and the zinc to give way.

Popeye1865 · Post by **Popeye1865** » Sat Mar 14, 2009 7:44 pm

even if it was bonded to the shaft with a shaft brush your still shouldn't be burning thru zincs. there is an issue the point of bonding is to tie all metal objects in contact with water to a centralized zinc if the zincs are burning up then there is a current leak or issue in the marina. figure out where the leak is coming from.

John, CD28 · Post by **John, CD28** » Mon Mar 16, 2009 9:04 am

I climbed in and put the meter on the shaft - it's not grounded. There is no continuity between the shaft and the boat's bonding system. The transmission, bronze stern tube, and rudder shaft are grounded. Even the stainless cap that covers the outer end of the cutless bearing is grounded (screws into the stern tube) but the shaft isn't.

I'm at a loss as to why I'm losing zincs so fast. I'm thinking this much zinc erosion suggests the zinc/prop/shaft are part of a larger circuit, possibly involving the whole bonding system as Tim suggests. The boat's on stands now; Is it possible the prop/shaft become grounded when the stern tube is full of sea water?

All the same, I think I need to look for some leaky electron source.

bccelizabeth · Post by **bccelizabeth** » Sun Apr 12, 2009 9:38 pm

Any updates on this issue? I'm facing similar. Thanks.

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Mystery of the vanishing zincs (HELP!)

Mystery of the vanishing zincs (HELP!)

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