Trim Tabs: do they really need anodes?

wongaero

wongaero

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Ok, a bit sacriligeous but can anyone explain why I shouldn't miss them off this year? (keeping the ones on the drive leg obviously)

My Trim Tabs aren't electrically connected to anything else - there's just a wire going into the electric actuator which is connected to the transom and tab by plastic fittings. I've been fitting a great big 100mm diameter anode to each tab every year and they do wear by a small amount.

Is there any point fitting an anode to this isolated bit of stainless steel, I can't see the electrical justification for it?
 
You don't need them for the tabs themselves (provided they are stainless steel) but for everything else that is metal and not stainless steel (gold excluded). If you don't have them on the tabs, the tabs will "eat" of something else such as your drive leg. If you've got a lot of anode's on the drive leg you might get away with it for a few months but I would not take that risk.
As soon as you are out of anodes your drive legs will deteriorate very quickly
 
on a slightly differant note does anyone know where the Trim Tab control box would be and what it looks like on Lenco trim tabs. saves starting a new thread.

My port side one is on and every other light stays on like its not getting a signal from the trim tab I need to check there is no wires off in this box but dont know what it looks like, or where it is
 
I didn't fit them for first two years and now have small holes in my stainless steel trim tabs. Fitted anodes last year and they fizzed away to almost nothing but trim tab corrosion arrested.
 
I'm not sure of the science behind it, but they can help simply by being in close proximity in the water to outdrive legs etc. even if not electrically bonded to them.

dv.
 
thanks all, mmm, think I'll keep fitting them...

My Lenco tabs don't have a control box as such, just the two confusingly-wired switches on the console which directly control the actuators - others may be different I s'pose
 
Am not an expert - best ask VicS - but if anodes are seen to wear down steadily then they are doing their job.
But if they don't wear at all. it's NOT good news - you've got problems; similarly if they erode overnight!
I need to replace my trimtab anodes, and the others, every couple of years.
 
All metal under water on your boat is electrically connected through the water, saltwater even more so. The closer the metals are the more "connected" they are. This is why you need a anode on all your under water metal parts.

Anodes are also called sacrificial anodes ie they are the least "pure" and will dissolve first. There is a ranking in how pure a metal is and that decides in what order they are sacrified in this context.
Stainless steel have different qualities and can be better or worse in terms of being 100% stainless steel, if your screws holding the tabs are less quality stainless then your tabs they might be attacked more and weakened. If your tabs have small portions of regular steel in them then these portions will get eaten away.
You may have noticed getting small rust stains on your so called stainless steel stuff, that's the same inpurity.


from wikipedia:
Galvanic corrosion is a process that degrades metals electrochemically. This corrosion occurs when two dissimilar metals are placed in contact with each other in the presence of an electrolyte, such as salt water, forming a galvanic cell. A cell can also be formed if the same metal is exposed to two different concentrations of electrolyte. The resulting electrochemical potential then develops an electric current that electrolytically dissolves the less noble material.

Cheers/
 
I have anodes on my tabs and they do get eaten away so I guess them must be protecting something even if it isn't the tabs themselves. For the sake of £20 I'd fit them.
 
[ QUOTE ]
best ask VicS

[/ QUOTE ] Oh dear.

The direct answer to the original question is that if the trim tabs suffer from any corrosion anodes may well help to protect them. Hugo Andreae's and Nonito's posts support that.

If the anodes are are only wasting away slowly then only replace them when necessary.
It does not automatically follow that because the anodes are wasting away that they are doing anything useful. A chunk of zinc bolted to stainless steel or any other metal with a potential significantly higher (see the Galvanic series ) and immersed in seawater will be eaten away. That's just the way the electrochemistry works. Likewise with anodes associated with bronze, brass or stainless props. I agree though that if they dont waste away at all they aren't doing anything useful and also if they disappear alarmingly quickly some serious problem probably exits.

I am wondering what effect high water flow velocities will have. High velocities in turbulent flows can remove the protective oxide films and result in what is sometimes called "corrosion erosion" I have seen (large ) copper water pipe work eaten into holes as a result of high turbulent flow rates down stream from bends. Whether anything similar will happen to stainless steel I dont know but maybe that is why stainless steel trim tabs corrode or erode

There are some comments to be made about things that have been said in this thread.

Anodes mounted on electrically isolated stainless trim tabs won't to be doing anything to protect the dive leg etc. They must either be mounted on the leg or nearby on the hull with a good electrical connection to the leg to do that. An electrical connection between a stainless steel trim tab and an aluminium alloy drive leg is likely to result in severe corrosion of the drive leg.

The wording of Michael_J's post troubles me.

For corrosion to occur between two dissimilar metals a they must be immersed, or at least in contact with, the same body of electrolyte, that being seawater as far as we are concerned, and there must be an electrical connection between them. That could be due to them being in good contact with each other or by a wire or other solid electrical conductor. Half of the electrical circuit that causes the corrosion is via the electrolyte, the other half is via the contact or the bonding between them.
True though the shorter the path through the electrolyte the greater will be the corrosion rate.

Different grades of stainless steel in contact with each other can lead to one being corroded. Fixing screws should therefore be the of the same alloy as the trim tabs.

A fairly good account of galvanic corrosion and, as you would expect, of cathodic protection by sacrificial anodes on MG Duff's website.

Some more technical reading on the National Physical Laboratory website. although the discussions on cathodic protection are mainly about "impressed current" systems rather than by sacrificial anodes

Finally don't forget the need to have a galvanic isolator fitted if your shorepower is earthed to the boats earthing or anodes and you leave it connected, in use or not.
 
If the trim tabs are 100% separated they would just eat at their own in purities if there are no sacrificial anodes attached (or the screws if they are more in pure then the tabs).
However, in practice you often have a return path through the antifoul paint, if not directly painted on the tabs and electrical ground such as the legs and bow thruster (eventhough this is often the case) it is still in very close vicinity (electrically closer or less resistance then the water path to ground) which gives you the basis (even if its slower) for galvanic corrosion.
 
[ QUOTE ]
If the trim tabs are 100% separated they would just eat at their own in purities if there are no sacrificial anodes attached (or the screws if they are more in pure then the tabs).
However, in practice you often have a return path through the antifoul paint, if not directly painted on the tabs and electrical ground such as the legs and bow thruster (eventhough this is often the case) it is still in very close vicinity (electrically closer or less resistance then the water path to ground) which gives you the basis (even if its slower) for galvanic corrosion.

[/ QUOTE ]

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