Trim tabs anodic protection & how much zinc to apply

[superheat6k]

My boat has stainless trim tabs which are isolated from the main bonding circuit. Since purchase I have replaced the dome anodes as they wear, but I am not sure they are necessary.

So should I ...

1 Continue with the dome anodes - 1 each side

2 Dispense with the anodes as the tabs are stainless and are not electrically connected to anything else.

3 Attach them internally via their mount bolts to the stern gear bonding circuit. In their natural position they do not face the main anodes which are some 1.5m away.

The rest of my bonding circuit is now removed because I have changed all the seacocks to Forespar Marelon. I have fitted new strap links across the R&D couplings (2 on each) plus a shaft brush, because I do not want another pink propellor ! I have also replaced all the bonding circuit cables with 6mm2 wire with crimped and soldered lugs. All external fittings (props / rudders / shafts / P brackets) now measure barely above base meter resistance relative to each other.

I found last year that having shaft anodes as well led to a coating of what seemed to be a zinc like substance on top of the velox on the props. Whatever it was the barnacles loved it ! The main anodes are large oblong block anodes sited close to the propellors, between the bronze rudders & P Brackets. I don't wish to over anode again.

 

[TheOrs]

I removed mine.
My set-up is similar trim tab wise; stainless tabs, isolated. I have a Merc outdrive though.
My tabs are anti-fouled.

 

[VicS]

My boat has stainless trim tabs which are isolated from the main bonding circuit. Since purchase I have replaced the dome anodes as they wear, but I am not sure they are necessary.

So should I ...

1 Continue with the dome anodes - 1 each side

2 Dispense with the anodes as the tabs are stainless and are not electrically connected to anything else.

3 Attach them internally via their mount bolts to the stern gear bonding circuit. In their natural position they do not face the main anodes which are some 1.5m away.

The rest of my bonding circuit is now removed because I have changed all the seacocks to Forespar Marelon. I have fitted new strap links across the R&D couplings (2 on each) plus a shaft brush, because I do not want another pink propellor ! I have also replaced all the bonding circuit cables with 6mm2 wire with crimped and soldered lugs. All external fittings (props / rudders / shafts / P brackets) now measure barely above base meter resistance relative to each other.

I found last year that having shaft anodes as well led to a coating of what seemed to be a zinc like substance on top of the velox on the props. Whatever it was the barnacles loved it ! The main anodes are large oblong block anodes sited close to the propellors, between the bronze rudders & P Brackets. I don't wish to over anode again.

Stainless steel trim taos should not need the protection of anodes therefore option 2 would be my preference, but if there is any sign of corrosion of the trim tabs refit the anodes.

Bonding the SS trim tabs to the stern gear bonding circuit might take the lid off a whole new can of worms. I definitely would not do that

 

[vyv_cox]

Stainless steel trim tabs should not need the protection of anodes therefore option 2 would be my preference, but if there is any sign of corrosion of the trim tabs refit the anodes.

Bonding the SS trim tabs to the stern gear bonding circuit might take the lid off a whole new can of worms. I definitely would not do that

I agree with all of that. Look out for crevice corrosion where the tabs meet their supports, although an anode will not do much to prevent it.

 

[superheat6k]

Thanks Vic & Vyv - sound advice as usual, so it looks like I will save myself ~£30 on unnecessary anodes this year.

There are no signs of corrosion on the tabs. Looking closely shows these are affixed to a strip of wood with wood screws, not through bolted so affixing to the bonding circuit would not be viable, and I agree with Vic's comment about not creating a new world of pain.

 

[vyv_cox]

Thanks Vic & Vyv - sound advice as usual, so it looks like I will save myself ~£30 on unnecessary anodes this year.

There are no signs of corrosion on the tabs. Looking closely shows these are affixed to a strip of wood with wood screws, not through bolted so affixing to the bonding circuit would not be viable, and I agree with Vic's comment about not creating a new world of pain.

Stainless screwed to wood is even more liable to crevice corrosion, so keep a good eye on it.

 

[Momac]

I have just replaced the anodes on my stainless trim tabs. The anodes had depleted so they must have been doing something - perhaps they protect the trim tab actuators (which are electrical in my case) .

 

[vyv_cox]

I have just replaced the anodes on my stainless trim tabs. The anodes had depleted so they must have been doing something - perhaps they protect the trim tab actuators (which are electrical in my case) .

Zinc is anodic to passive stainless steel, so will corrode if connected in seawater. But not necessarily providing any benefit.

 

[vas]

interesting thread Trevor,

I'm also on ss trim tabs, on wood obviously with the extra hassle of having half of the bathing platform supports welded on the ss "base" of the trim tabs
Tabs are longish at 60cm each by approx 20cm and they take two anodes each.
They all wear out nicely each year.
Shaft anodes are normal ones close to the p-bracket and need replacement each year for sure!
hull anodes are two massive (relatively) disk/dome things on the transom between the two trim tabs.
All bonding properly replaced, measured, etc, transom ones don't wear much (have the same ones now 3-4 yrs!)

Thought about removing the trim tab ones, but I'm not keen on having any nasty surprises so will keep them on for now, maybe just put one on each side and see, but last post by Vyv above suggests I'm wasting my money if I read it correctly?

confused!

V.

 

[TheOrs]

My understanding...

The anode is one metal protecting another. The protective metal (anode) has to be less noble than that which it is protecting.
Stainless steel is pretty noble, so it will quickly cause an anode to dissolve. The stainless is protected by the anode but decent stainless steel shouldn't really need protecting. It is hardy enough to look after itself. If you paint your tabs; they are further protected and have even less requirement for an anode. As the tabs sit under the water at rest then it seems to make sense to anti-foul them.
My trim tabs are anti-fouled. My tab anodes didn't deteriorate from one year to the next - There was nothing for them to do. I removed the anodes several years ago and it made absolutely no difference to the condition of the tabs.

Where this might be a problem is if your tabs are electrically connected to the rest of the boat's underwater gear (& act as a large cathode). Mine are screwed into fibreglass & have hydraulically driven, plastic bodied actuators.

 

[Bandit]

Use the small round dome anodes bolted through the Tab part of the Trim tab.

Connect a stainless cable from the anode bolt to one of the screws going in to the transom on the otherside of the hinge to provide protection to both sides of the Tab unit. I often see corrosing damage to the bit on the transom or the screws in to the transom rusting away. I do this to my boat I use Volvo Penta outdrive earthing cables they are a coule of quid each about 5 inches long and have a stainless ring connection at each end.

You posted some photos of corroded shafts and I think props?

My advice would be to check or get someone to check all of your bonding from first principles to see what is connected to what. Check all of your bonding wiring if green strands and terminals replace with tinned wire not domestic household wiring and look in to fitting shaft brushes.