I have a 1973 410 Commander. I boat in Lake Erie. Pulled the boat yesterday for the  season and found a severe corrosion problem with the trim tab extensions and through hill for the piston rod that drives them on the starboard side. Port side no problems.

The original built in tabs were extended with a stainless steel plate. Actuation is by hydraulic cylinder. I have a photo of what the corrosion did to the plate and all but one SS Screw that held it on. Destroyed. Then I looked at the through hull and found it crumbled to white powder if scraped with a screw driver. Photo below.

I assume the through hull is suspect and should be replaced. Anyone know of a source for one that would be like original or a suitable replacement if original is not available?

The trim tab cylinder and through hill have no wire on them that might be bonding wires, In fact I have not seen any of the through hull fittings bonded. Should they be bonded?

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I've been wondering about bonding and sacrificial anodes on our Commanders.  My '74 31', and my buddies similar '69 31', have bonding points on the thru-hulls, but no wires and no sacrificial anodes.  They have been around a long time and no corrosion.  Both lived in the Great Lakes, and seems like they would have been set up from the factory to be wet-slipped.

My previous boat was much newer, built in and lived in the Great Lakes, and had factory bonding to everything and sacrificial anodes in the usual places.  Seems like that is conventional wisdom.

I found some pictures of the inside of the trim cylinder installation. Looks like a green wire is indeed attached to the cylinder bracket.

Where is the packing gland? Looks like a hose connects the actual through hull to something. I am guessing the packing is at the top end of the hose?

John Brock



The packing is right where the rod connects. If you look, there is a cotter pin there that allows you to turn it tighter and apply more pressure to the packing. I have to say that part of that assembly looks like mine on the 42. Mine actuator is driven by electric motors tuning a worm gear. Some of yours does not look original. I see aluminum bar, cast iron brackets. Brass and others. This all leads up to Galvanic corrosion. Looking at your tab extension, it seems to look like it was acting as the sacrificial anode. I would be bonding it, putting zincs on it and checking your vessel for reversed polarity and my neighbors. Good luck.


Everything in your pictures points to an electrical problem. That stainless steel trim tab extension should not exhibit that kind of degradation over a summer season.

The corrosion on the trim tab is not what you would see with fresh water and stainless steel. That corrosion is likely the result of stray electrical current.

What was located on your starboard side. dock or a neighbors boat? If it is the dock, then the problem is likely in your boats electrical system or possibly the shore service. I would be looking at the battery charger in particular as the corrosion looks like a DC leakage current through the AC side. If you have an inverter, that is also a probable problem area.

A bad or broken ground wire in your service cord, plugs or boat service wiring could also be the culprit.

I would also be looking at the boat that was on your starboard side for similar corrosion signs.

No matter what the actual source of current is/was, it is a very serious safety issue and possible electrocution hazard.

Your thru hull for the trim tab should be replaced as it is likely on the verge of failure. It appears to be a standard bronze thru hull with a stuffing box attached on top via a hose.

Technically. it appears that your trim tab is actually bonded as that is the reason you got the degree of corrosion you did. It is interesting that the port side is normal which once again, indicates serious DC leakage current in the water on your starboard side. Zincs might have helped slightly but it is very probable that they would have disappeared in a very short time and still left you with severe trim tab corrosion.

John Brock said:

I found some pictures of the inside of the trim cylinder installation. Looks like a green wire is indeed attached to the cylinder bracket.

Where is the packing gland? Looks like a hose connects the actual through hull to something. I am guessing the packing is at the top end of the hose?

John Brock

I cleaned up the bronze thru hull and tab mounting bracket. Photo attached. Seems like the thru hull is still pretty thick. I would replace it if I can find one. Ron, you mentioned it is a standard part. Do you have a recommendation on a source for it? If so please let me know.

The battery charger is a marine specific model only a couple years old. Nothing else changed on the AC or DC side in over a year. I could believe a bad connection in the shore power connector or power cord. Will check connections on inside of shore power cord. Would a bad neutral connection result in current going back thru the ground connection cause this ?

I am powered up on the hard thru a ground fault breaker. No problems but the only thing running is some lights and battery charger.

Any tests I can make to track this down when I launch next spring? I can get a silver half cell to make some measurements of voltage in the water relative to onboard metal components. Look at that with various circuits operating.

No reports of any issues with other boats....so far.

Thanks for you input.


1973 410


Added photos



I may have discovered the problem. When looking at the cylinder mounting bracket there were three small gauge wired with push on connectors that were laying loose on the bracket metal. No green ground connection. These are undoubtedly from a trim tab position indicator that went away long ago. Probably 12v, common, and signal. Could have been the 12v was in contact with the bracket.

John Brock


Here is the Groco web page for the thru hull that is probably a very good match.


The HTH series has an included hose barb on the end.

If the 12V was always hot and in contact with the bracket, that is a very likely answer to your problem. It would likely be a good idea to check the other side also, likely same scenario on that side.

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