Zinc issue/question

My 38 foot trawler has zincs on both prop shafts and a zinc on each trim tab (why it has trim tabs, I don't know!).
The zincs on all parts, except the port trim tab, last 9 plus months. The port trim tab zinc wears out after about 4 months. My question is why just 1 zinc is failing this often?

One reason might be the connection to the boat's bonding system. As was explained to me by the former director of engineering at a major boat manufacturer, the current flowing to the zincs is miniscule. So it takes very little resistance to slow or stop it.

We had a similar problem, one transom zinc going away normally and one not at all. I checked the continuity of the boat's bonding system to and between the zinc mounting studs (inside the laxarette) and found that all was fine with them. That's when I mentioned this to my friend.

His suggestion was to first clean the mounting studs for the zincs with bronze wool to remove any corrosion, coating, scum, whatever, that could create resistance to this tiny current.

I did that, and that was the end of the problem. Today, when I change the transom zincs (I can do this from the dinghy, for the shaft zincs we use a dive service) I clean the bronze mounting studs with bronze wool along with the nuts and washers that hold the zincs in place. And we have never had the problem of a zinc not going away normally again.

So you might try this before moving on to checking the boat's bonding wiring, connections, etc.

I have slowly gone over the bonding system, replacing terminal ends, etc, since I purchased the boat. However, I am going to have to look again though, as I do not recall a bonding wire to either trim tab. How it would attach to the trim tabs, I'm not sure. Thanks for the input.

When the trim tab zincs are replaced do you clean/sand the mounting area on the tab?

Do you sand/clean the mounting surface of the zinc?

Do you then check the resistance level of the connection with an ohmmeter.

If you answer no to the above that may be your problem.

The ohms should be virtually zero. Your meter may not register less than 0.01 or 0.1 ohm so that can be taken as virtually zero. If the reading is much more then the surfaces were not clean enough.

The two mounting surface MUST be clean and TIGHT. As explained the voltage and currents are tiny and almost any resistance will stop the protection.

When using the ohmmeter you may have to scratch a clean spot on the tab and the zinc that you can touch the probes to.

Most trim tabs are not bonded, but rather zinced individually, since there is little inside the hull to attach a bonding wire to.

Are the tabs painted? If so then the surface area exposed to seawater may be limitd which will also reduce zinc loss.

Just be sure to CLEAN those mounting faces and check the resistance after.

As for the tabs on a trawler they may not help it plane but they can be used to reduce the big stern hole, they do on mine, and also level out a boat, while running, that has a list.

C lectric said:

As for the tabs on a trawler they may not help it plane but they can be used to reduce the big stern hole, they do on mine, and also level out a boat, while running, that has a list.

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I have a sister-in-law that I use the trim tabs for.

I second the cleaning mentioned by Marin & C lectric...

I'd also add - shaft & some tab zincs have a "contact point" on the mating surface w/ the shaft or tab. To help ensure good contact - in addition to cleaning first - the correct way to mount these is to tighten attachment bolt(s) and then tap w/ a hammer and re-tighten bolt(s). This helps "seat" or mate the contact w/ the surface the zinc is being mounted to.

C lectric said:

As for the tabs on a trawler they may not help it plane but they can be used to reduce the big stern hole, they do on mine, and also level out a boat, while running, that has a list.

Click to expand...

obthomas said:

I have a sister-in-law that I use the trim tabs for.

Click to expand...

Is it because she induces list or because she is a "big stern hole"?

Are your trim tabs electric? If so, maybe a problem is allowing a small amount of dc power to get to the tab itself?

The trim tabs are hydraulic. I will try the installation, sanding and seating instructions posted with the new zincs this weekend.
Interestingly, I was home yesterday and while sitting out back, a dive service came by who said he remembered this boat as he cleaned it years ago and even knew the boats name although no longer on the transom. He was also aware of the port trim tab zinc failing prematurely. It seems it has done this for years now.
I have to imagine there may be some other underlying issue. So, I will keep after it!

Goldenstar38 said:

The trim tabs are hydraulic. I will try the installation, sanding and seating instructions posted with the new zincs this weekend....
It seems it has done this for years now.
I have to imagine there may be some other underlying issue. So, I will keep after it!

Click to expand...

I'd suggest starting w/ the bonding system attachment to the problem trim tab - ensure good clean connection - wiring / terminal crimps - maybe even reattach terminals & make sure crimps are A++ - Compass Marine How To Articles on wire terminations, crimpers, etc are very enlightening.

Any possibility that the elec motor on the hydraulic pump has some leakage to ground & is attached to the bonding system at/near the tab making that tab see a voltage others do not??? Just a guess but maybe worth a check if all else fails?? A good digital multi meter may be able to detect something at the tab bonding vs other points in the bonding system?



Please Keep us posted on what you find.

Trim tab zincs are going on this weekend.
I will have to go back and check the hydraulic pump and wiring. The pump is behind a bulkhead and another 3 feet forward of the tabs. Perhaps an issue with the wiring. I'll find out!
Thanks for the input.