E35-3 Prop Shaft Strut - a look inside - and a zinc question

[Pete the Cat]

I've been curious about this too.

The idea of zincs is to allow galvanic action to affect the sacrificial part. Especially import to have a continuous path from the engine to a zinc.

So the idea of a zinc on the strut has always confused me - the only contact that the strut has with any dissimilar metals is the stainless bolts that hold it in place. There's nothing grounded to the strut inside the boat (in fact, the top end of the strut is glassed in and out of contact with anything but the fiberglass and the bolts), and the shaft (as you note) is insulated from it.

I have a zinc on my strut but... I've never really understood why.

I believe that it is common, but not universal, practice in some production boats to ground the engine block (which is normally connected to the ground side of your battery on most boats) with a copper bond to the strut. The reason to have a zinc on it would be to bond the boat/engine and its electrical system to something other than just the prop and shaft zincs. It would be a safety measure to ensure you do not get electrolysis on your delicate engine and drive train parts by providing another electric ground route and some sacrificial galvanic protection on that route. This was my own interpretation from working on boats--I have never read or been told this. I note that my Ericson came with no such bonding cable and has no zinc on the strut, is 30 years old and shows no pink on the strut. My Tartan 37, as I mentioned previously, is bonded to the engine, has a zinc on the strut and needed a new strut after its first 10 years (might have been neglect of PO), but has been fine for the last 20 years with a zinc replaced annually. The zinc on the strut is generally more wasted than those on the shaft and the propeller at haulout, so I would say it is needed in that application. Perhaps others have more experience with galvanic corrosion. BTW, I thought aluminum was for freshwater and zinc was for salt--is that right?

 

[Frank Langer]

I believe that it is common, but not universal, practice in some production boats to ground the engine block (which is normally connected to the ground side of your battery on most boats) with a copper bond to the strut. The reason to have a zinc on it would be to bond the boat/engine and its electrical system to something other than just the prop and shaft zincs. It would be a safety measure to ensure you do not get electrolysis on your delicate engine and drive train parts by providing another electric ground route and some sacrificial galvanic protection on that route. This was my own interpretation from working on boats--I have never read or been told this. I note that my Ericson came with no such bonding cable and has no zinc on the strut, is 30 years old and shows no pink on the strut. My Tartan 37, as I mentioned previously, is bonded to the engine, has a zinc on the strut and needed a new strut after its first 10 years (might have been neglect of PO), but has been fine for the last 20 years with a zinc replaced annually. The zinc on the strut is generally more wasted than those on the shaft and the propeller at haulout, so I would say it is needed in that application. Perhaps others have more experience with galvanic corrosion. BTW, I thought aluminum was for freshwater and zinc was for salt--is that right?

I understand that magnesium is for fresh water, aluminum for either fresh or salt water, especially brackish water, and zinc mainly in salt water. I've had this confirmed by a number of boat yards and chandlery staff, though I understand that like so many other aspects of boating, it's still controversial.
Frank

 

[Prairie Schooner]

Zinc Follow-up
Conclusion: We’ll probably put an anode on the strut, too.

One of the subjects that started this thread was whether to put an anode on the strut. Below are pictures of what the prop gear looked like after being in the water August-October. We put PropSpeed on all the gear and two aluminum anodes on the shaft. (The zinc configuration that the boat came with.) The shaft, prop, and nut all look great other than one very small fleck of PS that came off the shaft. However, the PS on the strut had bubbles forming under it, all over. The forward anode had more corrosion than the aft. The strut is not connected to anything metal inside the boat.






I thought there was an off chance there was an incompatibility between the strut bronze and the PropSpeed. I wrote PropSpeed and their assessment is that the bubbles are caused by galvanic action. That seems consistent with what I see, but I thought I’d at least ask. I take their response to mean that PropSpeed will not prevent galvanic corrosion.

Just for background, we are in Rhode Island bay waters, mostly salty, on a mooring which gets regular complete flushing during tides. Block Island Sound and the Atlantic are four miles away. Our boat is 1/3-1/2 mile away from the nearest dock.



When we got the boat the strut was painted with bottom paint. It had no zinc. The boat spent most of it's time 2-3 miles up a tidal river, in an electrified marina. There was a small amount of corrosion, but it was actually inside the fiberglass. This is detailed in post #39+ of this thread.

Based on what we see here, I think we’re going with one anode on the shaft and adding one to the strut. Both will be aluminum. I’ll strip the strut PropSpeed and reapply. Any observations from y'all are welcome.

 

[peaman]

Both will be aluminum.

What were your considerations in choosing aluminum over the more common zinc?

 

[Prairie Schooner]

What were your considerations in choosing aluminum over the more common zinc?

I'm using 'aluminum' sort of generically. Specifically we use Navalloy which is a combination of aluminum, zinc, and indium. Apparently this is based on more advanced science. I met a company rep at Defender's big spring sale last year and he convinced me. The loose grip the science had on my mind as since let go. This link has quite a bit of helpful information:
Navalloy https://performancemetals.com/pages/sacrificial-anodes-faqs
If I recall, you're down in Westerly. Our boat lived for decades at Stonington On The River. If you have any questions about our specific experiences, I'd be happy to respond. The thing that was most interesting to me was how differently the two anodes degraded even though very close to each other. Most of my life seems to be a work in progress guided by working theories. Very few dogmatic certitudes.

 

[peaman]

I'm a bit confused why your source is saying that aluminum is more active than zinc, when all other sources I looked at say the opposite. In any case it isn't a great difference, and your boat apparently has a history with the aluminum. I'll plan to look into it further.

Our boat lived for decades at Stonington On The River.

This is a hoot. My boat is presently hauled out at Stonington on the River. My summer berth is closer to open water, but SotR is super convenient for winter work. It would be cool to hook up sometime in 2023.

 

[Loren Beach]

A small data point: On our river (all freshwater, this far upstream) we now use aluminum anodes, having switched over from zinc. Surveyors and boat yard mgmt advised the change.

 

[Prairie Schooner]

. . .
This is a hoot. My boat is presently hauled out at Stonington on the River. My summer berth is closer to open water, but SotR is super convenient for winter work. It would be cool to hook up sometime in 2023.

That would be fun. A mini-rendezvous or beers on the Block or something.
Jeff

 

[dhill]

@Prairie Schooner , do you have a galvanic isolator on your boat? I would be interested to know whether you are seeing the bubbles in the PropSpeed with a galvanic isolator.

Thanks!
Dave

 

[Prairie Schooner]

@Prairie Schooner , do you have a galvanic isolator on your boat? I would be interested to know whether you are seeing the bubbles in the PropSpeed with a galvanic isolator.

Thanks!
Dave

This is a little embarrassing, but I'm not sure Dave. I don't think so. We're still getting to know the boat and there may be things in the bowels that I haven't discovered yet.
The boat was in a marina for years, in a river 1-2 miles up from the ocean (Pawcatuck). Though it spent months on a mooring on the Vineyard. The strut was covered with bottom paint and had no anode. It's discussed in a bit more detail earlier in this thread, showing the level of corrosion that had developed (little-none).
Jeff

 

[Loren Beach]

I installed a Galvanic Isolator on our boat. Easiest place was inside/behind the main AC/DC distribution panel. This was not an option, AFAIK, when the boat was new. I would look there for a small rectangular item with green ground wiring in and out of it.

 

[dhill]

Thanks @Prairie Schooner ,

I had to trace through things on my boat as well - it has been a learning process! After going through the electrical system, I concluded there wasn't one on my boat either, so I installed one this year. Fortunately, the previous owner installed 350w of solar panels on the bimini, so I rarely have to plug into shore power, which should reduce corrosion. I installed it in the aft lazerette on the forward bulkhead near the AC intake. The trickiest part of the installation is getting around the port propane tank compartment, so installing it behind the main panel may be a better choice. It may also be a better location there since it is more protected from water... Hmm...

@Loren - Has heat from the isolator been an issue when installing it behind the AC/DC distribution panel? The reason I ask is that mine has lots of cooling fins on it.

Dave

 

[dhill]

Thanks @Prairie Schooner ,

I had to trace through things on my boat as well - it has been a learning process! After going through the electrical system, I concluded there wasn't one on my boat either, so I installed one this year. Fortunately, the previous owner installed 350w of solar panels on the bimini, so I rarely have to plug into shore power, which should reduce corrosion. I installed it in the aft lazerette on the forward bulkhead near the AC intake. The trickiest part of the installation is getting around the port propane tank compartment, so installing it behind the main panel may be a better choice. It may also be a better location there since it is more protected from water... Hmm...

@Loren - Has heat from the isolator been an issue when installing it behind the AC/DC distribution panel? The reason I ask is that mine has lots of cooling fins on it.

Dave

Here is the advice fro Pacific Yacht Systems on the choosing the location of the galvanic isolator: https://www.pysystems.ca/resources/boating-tech-talk/galvanic-isolator-install-location-rick/

Dave

 

[Prairie Schooner]

Here is the advice fro Pacific Yacht Systems on the choosing the location of the galvanic isolator: https://www.pysystems.ca/resources/boating-tech-talk/galvanic-isolator-install-location-rick/

Dave

Hi Dave,

Thanks for the info. We've got this gizmo buried back there. I don't think this has any galvanic isolation function.


My limited (accurate?) knowledge is that the GI is for when you're hooked up to AC. Our boat lives on a mooring. We may only hook up to shore power 4-6 days a year. I've set the bigger priority for now as a minimal solar panel to keep our three batteries topped off. One starter, two house. Does that sound about right? Electrical stuff is all dark mystery to me.

Thanks,
Jeff

 

[Nick J]

Hi Dave,

Thanks for the info. We've got this gizmo buried back there. I don't think this has any galvanic isolation function.
View attachment 45448

My limited (accurate?) knowledge is that the GI is for when you're hooked up to AC. Our boat lives on a mooring. We may only hook up to shore power 4-6 days a year. I've set the bigger priority for now as a minimal solar panel to keep our three batteries topped off. One starter, two house. Does that sound about right? Electrical stuff is all dark mystery to me.

Thanks,
Jeff

If that's still your main battery charger, it's worth while replacing it even if it isn't used often. The one on my boat was charging above 16 volts and boiling batteries. There's lots of other members with the same experience, so even if it's working you're probably on barrowed time.

 

[Jerry VB]

My limited (accurate?) knowledge is that the GI is for when you're hooked up to AC.

That is correct - it is to isolate your boat from any stray current that may be conducted from the shore power through your boat into the water, "eating" the metal on your boat that is in contact with the water. The current conducted through the water would be due to nearby boats that have wiring faults or due to marina wiring faults.

 

[dhill]

Hi Dave,

Thanks for the info. We've got this gizmo buried back there. I don't think this has any galvanic isolation function.
View attachment 45448

My limited (accurate?) knowledge is that the GI is for when you're hooked up to AC. Our boat lives on a mooring. We may only hook up to shore power 4-6 days a year. I've set the bigger priority for now as a minimal solar panel to keep our three batteries topped off. One starter, two house. Does that sound about right? Electrical stuff is all dark mystery to me.

Thanks,
Jeff

Hi @Prairie Schooner ,

If you never connect to shore power, you do not need a galvanic isolator. If you do connect to shore power, particularly in a marina, every boat becomes like a cell in a large battery and stray currents can occur between the "cells". The galvanic isolator prevents those stray currents from going through your boat and the corrosion those stray currents can cause.

This video also explains why many Ericsons may not have galvanic isolators, since old designs failed open and therefore would eliminate the pathway to ground, creating a safety hazard. Newer "fail-safe" galvanic isolators fail closed, essentially becoming a wire like what was there before the galvanic isolator was installed. The galvanic protection is no longer there, but at least the safety that the ground circuit provides remains intact.

I have found the Jeff Cote series of seminars on boat electrics to be very helpful. I don't know him or have any affiliation with him.

https://www.youtube.com/playlist?list=PLcKJy0Frwc1idEdTkoEaoxMaJDWda_hZ8

https://www.youtube.com/playlist?list=PLcKJy0Frwc1jVWztG2apSbzS42ggLptN4

Hope that helps!
Dave

 

[Kenneth K]

If that's still your main battery charger, it's worth while replacing it even if it isn't used often.

I agree. I had a similar charger (Newmar 25) that would only charge at about 14V (measured at the charger terminals). After the long run of 10ga charging wires (from the charger--under the starboard cockpit coaming--to the back of the Main Battery Switch) the resultant voltage drop meant the batteries never saw much more than 12.7 V and they never fully charged.

 

[Loren Beach]

Looks like the OEM Newmar 1988 charger that was on our boat. Probably a "ferro resonant" design. Also know in the trade as a "hummer" and a "battery cooker"......
Overdue for replacement with a modern multi-stage solid state charger, IMHO.
In 1995 I replaced the (very heavy) one that was installed on our boat and had boiled generations of batteries dry before we bought the boat.

 

[Kenneth K]

That is correct - it is to isolate your boat from any stray current that may be conducted from the shore power through your boat into the water, "eating" the metal on your boat that is in contact with the water.

But, to muddy the waters further....

I know my DC system is grounded to the engine block, and thus, to the prop and shaft via the metal shaft coupling.

However, I've never worked on my AC electrical system so I don't know if the AC side has a similar path to ground (water). I've never seen it if it's there.

Further, until 2 years ago, I had one of these nylon drive-savers installed:
.
As this eliminates the metal-to-metal connection between engine and shaft, I've wondered if this, in itself, would prevent the hot-dock galvanic problem.

However, I believe marine electricians like to see grounding straps/brushes installed with nylon drivesavers to re-establish the engine-to-sea ground path. My boat had the drivesaver but no grounding strap when I bought it in 2016.

Hmm...