Bonding Bow Thruster

[Iggy]

I think I know the answer? It needs to be bonded. But........... here's what happened.

I upgraded my thruster with a well known firm last winter. Thrusters is all they do, and the work was very good!. The old thruster was 12V with 3 cables. With a 2nd Neg cable going to the bolts that hold the motor and lower unit in place. The new one is 24V, works great by the way! On the install, they did not connect the 2nd Neg/bonding wire to the bolts. I questioned it and they said it was not needed.

So now its fall and I pull the boat. The zincs are barely eaten up at all. Nothing like the last thruster. That cable needs to be re-installed, yes? I just want to ask the forum, in case I am missing something.

 

[Fleming]

It doesn't "need" to be bonded for cathodic protection because it has it's own anode, but bonding is also done to reduce the risk of stray current corrosion that can happen if the motor resistance to ground breaks down over time as motors tend to do. The bonding wire provides a path for the overcurrent protection to trip/blow rather than creating a damaging stray current corrosion path to the boat.

 

[Bmarler]

I don’t bond my thruster, it has its own zinc.

 

[BB-marine]

The thruster manufacture does not want the thruster bonded. The thruster manufacture specifies a anode to protect the thruster only. This keeps the thru hull fittings and other under water metals bonded separate from the thruster metal parts. Bennett Trim tabs recommends the same. keep the planes separate from bonding and protect them with anodes fastened to the planes. The reason the anodes protecting your new thruster were still in good condition is the anode was only protecting the thruster and not the rest of the hulls metals.
Brian

 

[SteveK]

Bonding the DC negative and AC ground will remain an issue with me. It creates a path to close a circuit for stray current. Not surprised thruster zinc does not wear out as fast.

 

[Jeff F]

Maybe I'm showing my ignorance here, but what's the difference in practice? Doesn't the thruster already have connectivity between the negative cable and the housing?

I fail to see what difference the second negative cable to the housing would make. What am I missing?

 

[O C Diver]

One of the considerations for not bonding the bow thruster is an issue with the anode being the first anode to go depending on the bonding system. Depending on the number of connections and quality of your bonding system, the path of least resistance (pun intended) may be through your bow thruster.

Further, if your bow thruster has two propellers and two anodes, you already have redundancy that tying it to the bonding system would offer.

Finally, if your anodes are aluminum (as Side Thruster switched to a number of years ago) and the hull anode is zinc, there's a good chance the thruster anode will go first if tied to the bonding system.

Ted

 

[Fleming]

Maybe I'm showing my ignorance here, but what's the difference in practice? Doesn't the thruster already have connectivity between the negative cable and the housing?

I fail to see what difference the second negative cable to the housing would make. What am I missing?

The housing is not electrically connected to the negative cable.

OC Diver and BB Marine are correct in that if you bond the bow thruster you could end up making the thruster anode try to protect other metals which it isn't intended or sized for. That's the reason 'not' to bond it. As long as you maintain that anode in good shape you shouldn't have a problem. But it cuts both ways. If you don't replace the anode on the thruster in time it can suffer galvanic corrosion while a bonded thruster could be protected by the hull anode.

But galvanic protection is only one of the reasons why we bond our under water metals. If the thruster motor positive side shorts or develops low resistance to the casing on an unbonded thruster you will have stray current corrosion which will damage the thruster very quickly. If it were bonded though, the bonding wire will complete the circuit and trip the overcurrent protection, preventing the stray current damage.

It's ok to not bond the thruster, mine is unbonded, but I'm ok with the very low risk of stray current and I stay on top of all the aluminum anodes on my boat meticulously. I'm of the opinion that it's fine either way, bonded or unbonded. Just maintain your anodes as everyone should be doing anyway.

 

[Fleming]

Finally, if your anodes are aluminum (as Side Thruster switched to a number of years ago) and the hull anode is zinc, there's a good chance the thruster anode will go first if tied to the bonding system.

Ted

I don't think so, zinc is more negative on the galvanic scale than aluminum, so I would expect the zinc anode to protect the aluminum anode.

 

[Jeff F]

The housing is not electrically connected to the negative cable.

That's what I missed, thanks. Is that universal? My recollection is that on my old Vetus they are connected. But I suppose that may have been connected at installation deliberately. Or maybe I misremember.

 

[O C Diver]

I don't think so, zinc is more negative on the galvanic scale than aluminum, so I would expect the zinc anode to protect the aluminum anode.

Take your boat to brackish or freshwater where zinc doesn't work as well to not at all, and you will find the aluminum anode becomes the sacrificial anode.

Ted

 

[Iggy]

Understood, and thank you!

 

[Comodave]

Keep in mind that you actually want the anodes to be eaten up. That means that they are working. When the anode has not been eaten up much it may not be working and the metals that you wanted protected may be undergoing corrosion. And that isn’t good.

 

[BB-marine]

I don't think so, zinc is more negative on the galvanic scale than aluminum, so I would expect the zinc anode to protect the aluminum anode.

Aluminum alloy anodes are slightly more negative compared to zinc alloy anodes in salt water. I read that aluminum alloy anodes work well in salt water. When I did the Loop I made the decision to change all anodes to aluminum replacing the zinc anodes. During the Loop trip we spent about 6 months in fresh water. My reason for the change was, zinc doesn't work well in fresh water. The fresh water causes the zinc anode to oxidize forming an insulating coating reducing the anodes ability to be sacrificial. The aluminum alloy does not do this so it remains working in fresh or salt water and anything in between. When used in salt water the aluminum being close to the zinc negative potential (Zinc - 1.03V Aluminum -1.1) This makes aluminum protecting force slightly better -.07V more. Another mistake when using zinc anodes is relaunching. Once a zinc anode is submerged in water and removed from water and is exposed to air it oxidizes. This too can form an insulating coating that reduces the anodes sacrificial ability. If you see a white film on the anode use a SS wire brush to knock the film off. Check continuity.

Keep in mind that you actually want the anodes to be eaten up. That means that they are working. When the anode has not been eaten up much it may not be working and the metals that you wanted protected may be undergoing corrosion. And that isn’t good.

I know what you are saying. An anode that is not being sacrificed may not have a good electrical connection or in my explanation above may have a insulating coating on its surfaces limiting it's ability to sacrifice. You do not want this. If you see the anode not being "eaten up" a continuity test should be done. The resistance between the anode and the metal it is protecting should be no more than 1 ohm of resistance. If after tested and the resistance is less than 1ohm and the anode is not being sacrificed. Congratulations, You have a anode installed to protect when and if needed. Your electrical system in your boat is sound, the marina shore power is clean.
Brian

 

[Iggy]

Keep in mind that you actually want the anodes to be eaten up. That means that they are working. When the anode has not been eaten up much it may not be working and the metals that you wanted protected may be undergoing corrosion. And that isn’t good.

That why I was thinking that the bonding wire needs to be re-installed. But the feeling from the forum is, it is not needed.

 

[Steve DAntonio]

It depends on the thruster manufacturer, SidePower/Sliepner specifically says to not bond, ABT specifically says to bond. https://www.imtra.com/DocumentManager/ProductDocuments/Sleipner_Bonding_sheet.pdf

More on bonding here Getting to Know Your Vessel – Bonding Systems and Corrosion Prevention | Steve D'Antonio Marine Consulting

If you bond the thruster, the bonding wire needs to be large enough to carry fault current from the DC positive cable, more on that here Feature: Risks of Undersized Bonding/Grounding Wires – Editorial: You Have Enemies? | Steve D'Antonio Marine Consulting

 

[twistedtree]

I would do what your particular thruster manufacturer says to do.

 

[C lectric]

I'll second Twistedtrees comment.
Second guessing the mfgr. is often a recipe for trouble unless you are an expert.

 

[JDCAVE]

This thread raises two issues: 1) whether to “bond” the bow thruster or not and 2) whether to use aluminum sac anodes or not.

1) Having gone through 12 years with my boat, I notice the thruster anodes, whether aluminum or zinc tend to need replacing more frequently than other anodes. I’m beginning to think it is not beneficial to bond, if those anodes are protecting other areas that are protected by their own anodes.

2) I have been using Aluminum anodes for the past number of years (home dock is freshwater) but the halo on the bottom paint around the bronze through hulls is convincing me that I am over protected whilst in salt water for 2-3 months. The aluminum anodes are quite spent after 2 months in the marine environment.

As a result I’m leaning towards using zinc anodes. The alternative is to put on zinc immediately before the marine cruising season and switch to Aluminum at the end of the season. Lately, I have been diving down to replace the anodes and it works well, but I do that when I have an opportunity in warm marine areas.

Jim

 

[SteveK]

From what I have read and if I had thrusters I would either use a dedicated anode or bond, but not both. Bonding creates a circuit from say a thru hull to the thruster zinc.

 

[JDCAVE]

From what I have read and if I had thrusters I would either use a dedicated anode or bond, but not both. Bonding creates a circuit from say a thru hull to the thruster zinc.

And currently I have both, installation by a PO. And as I say, I am thinking of removing the thrusters from bonding system.

Jim

 

[Bmarler]

This thread raises two issues: 1) whether to “bond” the bow thruster or not and 2) whether to use aluminum sac anodes or not.

1) Having gone through 12 years with my boat, I notice the thruster anodes, whether aluminum or zinc tend to need replacing more frequently than other anodes. I’m beginning to think it is not beneficial to bond, if those anodes are protecting other areas that are protected by their own anodes.

2) I have been using Aluminum anodes for the past number of years (home dock is freshwater) but the halo on the bottom paint around the bronze through hulls is convincing me that I am over protected whilst in salt water for 2-3 months. The aluminum anodes are quite spent after 2 months in the marine environment.

As a result I’m leaning towards using zinc anodes. The alternative is to put on zinc immediately before the marine cruising season and switch to Aluminum at the end of the season. Lately, I have been diving down to replace the anodes and it works well, but I do that when I have an opportunity in warm marine areas.

Jim

Significant wasting of aluminum anodes in just three months is a bit alarming. Many people use aluminum year round in salt water with no halo around the fittings.
Have you checked your fittings with a silver half cell to see if you are not leaking a small amount of current?

 

[JDCAVE]

No. I have not. I am normally moored in Freshwater, where (I understand) a silver half cell is not useful. Any thoughts most definitely appreciated. Shore power is isolated using a Charles Isolation Transformer.

Jim

 

[DWJensen]

Interesting comments on this. We have two thrusters (Vetus), bow and stern and neither are connected to our boat-wide bonding system. Both have their own anodes and both anodes are still going strong after 3+ years with no changes needed so far. Our shaft anodes generally last about 6 months and the large transom anodes go for about a year. Mostly saltwater, from the FL Space Coast to the upper end of LIS and back each year.

 

[Fleming]

As a result I’m leaning towards using zinc anodes. The alternative is to put on zinc immediately before the marine cruising season and switch to Aluminum at the end of the season. Lately, I have been diving down to replace the anodes and it works well, but I do that when I have an opportunity in warm marine areas.

Jim

Zinc anodes disintegrate pretty quickly in freshwater. Here's one I saw last week. If you touched this anode with your finger the texture is mushy, like thick mashed potato. This boat was sitting about a mile from the ocean but where a freshwater creek runs past the marina and the water is on the freshwater side of brackish every time the tide runs out.
This anode wasn't just wasting away, it was falling apart.

 

[Bmarler]

No. I have not. I am normally moored in Freshwater, where (I understand) a silver half cell is not useful. Any thoughts most definitely appreciated. Shore power is isolated using a Charles Isolation Transformer.

Jim

The time to check would be when you’re in the salt water. I wonder if you have some dc leakage causing your anode wasting. A halo around the through hull fittings would suggest this is happening.
If your bonding is in good condition, ideally, your aluminum anodes should last a year.
Are all of the through hull fittings displaying the same activity?

 

[Bmarler]

Zinc anodes disintegrate pretty quickly in freshwater. Here's one I saw last week. If you touched this anode with your finger the texture is mushy, like thick mashed potato. This boat was sitting about a mile from the ocean but where a freshwater creek runs past the marina and the water is on the freshwater side of brackish every time the tide runs out.
This anode wasn't just wasting away, it was falling apart.

Usually the problem with zinc in fresh water is that the zinc forms an oxide layer that prevents it from doing its job.
That said, lots of people use zinc in fresh water, it just depends on the area, mineral content, etc.
A mushy anode suggests high activity. Different suppliers may have different alloying materials too, changing supplier may change the way an anode wears.