Propeller Shaft Zinc Reality Check

[SBriggi]

Hi folks!

Just looking for a reality check! I have 3 seasons in New England on a 2001 Bruckmann Blue Star 29.9, twin Yanmar 6LP-DTE with 1-1/2" shafts with 17"x24" bronze propellers. There are no bonding cables nor transom zincs, so the engine zincs and shaft zincs are it, and the engine zincs are changed annually and 50% remains, but the shaft zincs are a different story.

The first season I ran the boat with the shaft zincs (2 @ ??lbs) that were installed and they were gone by August. 2nd season I ran X-7 zincs (2 @ 1-1/2lbs) and they were done when I hauled the boat in October. Last year I ran 1 x X7 and 1 x X7H on each shaft (4-1/2lbs), only a partial zinc left on each shaft...

As I prepare for this season I plan on running 2 x X7H on each shaft(6lbs).

Does this seem excessive to those of you that have a 30ish fast trawler powered by twin diesels?

Thanks for your thoughts.
Steve

 

[DavidM]

Seems like a bit much. You might look for a marine electrical tech who can do a corrosion analysis. If ok, then consider adding a transom zinc.

David

 

[Comodave]

I know you don’t want to hear it but I would add a bonding system and with it some transom anodes. That will take some of the load off the shaft anodes. It will be some work but not too difficult, mostly just grunt work.

 

[SBriggi]

Typically, what does a well protected 30' twin diesel generally carry on the shafts? 2lbs? 3lbs? More?

I like the idea of setting up the boat similar to last year and getting some measurements this summer, but if typically 6 lbs are used...

Thanks,
Steve

 

[Fleming]

I would use the extra zinc. No harm in over protecting your shaft, but you don't want to leave the brass (40% zinc, they're brass not bronze) prop unprotected. I don't think you need a hull anode nor do you need to bond a boat that was built unbonded as long as the through hulls are of proper material.


What I would do is verify that you have a galvanic isolator if you are using shore power. Without one you may very well be using your anodes to protect your neighbors boats, which is one reason why you would see rapid wasting of the anode. If you don't have one, install one, and you will probably be able to get by with one anode a season.

 

[SBriggi]

I like the though on the galvanic isolator...need to do some home work.

Thanks Fleming!

 

[SteveK]

I like the though on the galvanic isolator...need to do some home work.

Thanks Fleming!

I was about to ask a question and you posted you do not have a galvanic isolator. A question.
Is your AC shore power green ground bonded to battery DC?
Without a galvanic Isolator that is still working you may have a ground going through the boat into the water to ground. This I think will wear on the shaft zinc.
By experience, My previous boat shaft zincs lasted longer and I credit the shaft saver (plastic) that isolated the shaft from any bonding leaks. The shafts did have a shaft brush wired back to the transom zinc.

 

[SBriggi]

Yeah...I believe the 2 other boats on the same shore power may have something to do with the zinc wear...I will look into the galvanic isolator and a method to test the water around the dock this summer.

Thanks for the input!

 

[Snapdragon III]

Might also try one of the zincs you clamp onto ground and just hang over the side on a cable while at the dock. Would be much easier than installing grounded transom zincs, and could give you a better idea if the larger project was worth it.

 

[Steve DAntonio]

I know I sound like a broken record with these responses but, a reference cell test will tell you everything you need to know, no guess work. More here Reference Cell Testing; Know Thy Corrosion Protection Level – Editorial: Old vs. New | Steve D'Antonio Marine Consulting
and here

Feature: Busting the “Hot Marina” Myth – Editorial: A Blue-Collar Resurgence | Steve D'Antonio Marine Consulting

Adding anodes to the transom will have no effect, as the shaft and prop are effectively isolated from the bonding system. Common $20 shaft brushes will also not bridge that gap, they still have too much resistance. You'd need a read slip ring shaft brush for this to work, more here Getting to Know Your Vessel – Bonding Systems and Corrosion Prevention | Steve D'Antonio Marine Consulting

The guppy anode over the side would work, provided you clamped the wire directly to the shaft. The oil filled marine gear effectively isolates the shaft from the bonding system.

Regardless, you should have an ABYC compliant galvanic isolator installed. It will likely have no effect on the shaft anode consumption, however, they are still very valuable in preventing other nearby vessels from consuming your anodes that are connected to the bonding system. More on galvanic isolators here “Zinc” Anode Selection and the Role Played by Galvanic Isolators – Editorial: Fire Extinguisher Selection | Steve D'Antonio Marine Consulting

 

[Fleming]

as the shaft and prop are effectively isolated from the bonding system.

Steve, while I have the utmost respect for your knowledge and experience, my own experience tells me that it's not so definitive. I quite often see a good bonding connection through the transmission to the shaft. Certainly not always, but often enough that my personal view is "the shaft and prop can sometimes be isolated from the bonding system" rather than the way you word it.
I have documentation of several that I know had a good bonding connection to the shaft, but I'm going to start recording the info in my own database so I can get a better idea of the percentage.

 

[Mainecoast]

The shaft zinc lasts about 2-3 months on my boat too...GB36 with single screw. I'm installing a transom zinc this year with bonding to a shaft brush. We'll see how that works.

 

[SteveK]

Unable to copy paste from the Steve D articles. (which are good BTW)
"In most, but not all, cases the vessel suffering the damage is the source of the problem;"
Stray current is from DC sources usually into the bilge water.
In my mind any current must be part of a closed circuit, otherwise how can there be a flow from one metal to another. so how does a positive DC current from the bilge water get into the sea water completing a circuit back to the battery through the sacrificed propeller? DC current flows negative to positive. There must be a metal thru hull also in that same water which is bonded to negative. Since everything is bonded why is the current not travelling back thru the bonding to the battery, but instead out into seawater and back thru the propeller, eating it up.

What am I missing?

 

[RickyD]

At the risk of co-opting this I have also a question about zincs. When I bought my 55 it had two zincs per shaft. Someone suggested I may be over zinced. My shafts are so long they are in two segments connected and I have two struts per shaft. My zincs are not wasting away quickly so I'm not worried about that but I just decided to try one zinc per shaft. Are there any comments or suggestions?

 

[Steve DAntonio]

Unable to copy paste from the Steve D articles. (which are good BTW)
"In most, but not all, cases the vessel suffering the damage is the source of the problem;"
Stray current is from DC sources usually into the bilge water.
In my mind any current must be part of a closed circuit, otherwise how can there be a flow from one metal to another. so how does a positive DC current from the bilge water get into the sea water completing a circuit back to the battery through the sacrificed propeller? DC current flows negative to positive. There must be a metal thru hull also in that same water which is bonded to negative. Since everything is bonded why is the current not travelling back thru the bonding to the battery, but instead out into seawater and back thru the propeller, eating it up.

What am I missing?

One of several reasons to bond is to mitigate or prevent stray current corrosion. If bonded, there is a more direct, lower resistance path back to the source, which will hopefully cause a fuse to blow, or circuit breaker to trip.

 

[Steve DAntonio]

One of several reasons to bond is to mitigate or prevent stray current corrosion. If bonded, there is a more direct, lower resistance path back to the source, which will hopefully cause a fuse to blow, or circuit breaker to trip.

I realize copying is convenient, however, I grew weary of sending emails to individuals and websites that were lifting my work, sometimes with attribution and sometimes without, but always without permission. Therefore, I chose to make copying from the site more difficult.

 

[SteveK]

One of several reasons to bond is to mitigate or prevent stray current corrosion. If bonded, there is a more direct, lower resistance path back to the source, which will hopefully cause a fuse to blow, or circuit breaker to trip.

Then how does a bonded boat have stray current?
Or are we saying a broken bond or no bond is how this circuit is completed. Then I read your comment that a prop shaft is not mechanically connected to the engine ground, shaft brushes don't work to the anode. Still missing what a prudent boater should be looking at with preventative maintenance to avert stray current (besides testing for it).

 

[Steve DAntonio]

Bonding is no guarantor against stray current corrosion, it just makes it less likely. Current is still traveling through water, which is high resistance, and thus enough current flow may not be present to trip a breaker or blow a fuse.

Stray current corrosion is statistically rare, much rarer than galvanic corrosion, however, in my experience it is frequently caused by sloppy wiring in and around bilges, as well as from faulty bilge pumps and float switches. There are more complex scenarios, but those are even less common. Inspect your wiring in and near bilge water.

At rest, most marine gears, which are oil-filled, are not good conductors, certainly not achieving the less than 1 ohm threshold. The cheap shaft brushes don't achieve that threshold either. If you must bond the shaft, because there is no room for anodes, then you need a good quality solver slip ring shaft brush, details and photos for which I provided in an earlier link. Most shafts can accept anodes, so the brush isn't necessary.

 

[Steve DAntonio]

Steve, while I have the utmost respect for your knowledge and experience, my own experience tells me that it's not so definitive. I quite often see a good bonding connection through the transmission to the shaft. Certainly not always, but often enough that my personal view is "the shaft and prop can sometimes be isolated from the bonding system" rather than the way you word it.
I have documentation of several that I know had a good bonding connection to the shaft, but I'm going to start recording the info in my own database so I can get a better idea of the percentage.

I'm not saying it's impossible, just unusual to have reliable, consistent and long-lasting continuity of less than one ohm, which you can rely on for cathodic protection. If you test it shortly after a vessel has been run, in a haul out for instance, it might have continuity, but does it last for days or weeks? In my experience the answer is no. You can't test for continuity while the vessel is afloat, however, you can use a reference cell to make this test while afloat. When I carry out those reference cell tests, it is extremely rare to find a shaft/prop at the exact same voltage as other protected underwater metals, and in many of those cases the economy style shaft brush is present. I've seen it, but rarely. However, please share your future findings, I'm interested to see what you encounter.

For those who don't want to install a silver slip ring shaft brush, you can clamp a bonding system wire to your shaft when it is at rest. Of course you just need to remember to remove it before getting under way.