Trim tabs and anodes

[wombat88]

I have Bennet trim tabs that were probably installed sometime towards the end of the last century.

I assume they are stainless. They are covered in layers of a/f. The boat has an outboard and there is no other metal under water.

The tabs are not bonded to anything as far as I can see and yet they have anodes that I religiously replace every year without thinking about it. The anodes do appear to waste a bit.

Question is: Do I really need these anodes? Does stainless need anodes?

 

[Keith-i]

Amazing coincidence. I was just contemplating the same thing before I replace the anodes on mine too. Mine also waste over a year or two but can’t see the real need on stainless. Especially as trim tabs are not mission critical.

 

[Plum]

I have Bennet trim tabs that were probably installed sometime towards the end of the last century.

I assume they are stainless. They are covered in layers of a/f. The boat has an outboard and there is no other metal under water.

The tabs are not bonded to anything as far as I can see and yet they have anodes that I religiously replace every year without thinking about it. The anodes do appear to waste a bit.

Question is: Do I really need these anodes? Does stainless need anodes?

I have been unable to find any reliable data on this and agree that Stainless Steel, not in contact with any other metal, is unlikely to need an anode. I dispensed with the anodes on my Bennet trimtabs 5 years ago (it was worth the risk as I have found they provide no real benefit on my boat and if they fail would not replace them). Checked them this week while out of the water. No sign of any deterioration.

www.solocoastalsailing.co.uk

 

[Portofino]

I use them on my bathing platforms SS structure .Doubled up and even connected the internal bolt heads ( that pass inside through the transom) to the gen boat bonding via those two round transom anodes .

Zinc is cheap . ——- well in the bigger scheme of boat maintenance and running costs .

Having said that the anodes on the SS degrade very slowly last 3 yrs .The main anodes are changed annually by comparison.



With your Bennett’s you do not really know the bolt material , If a yard bozo fitting has drilled the SS thus compromising it a little or as mentioned what other metals are in direct contact.
Its your roll of the dice disregarding fitting anodes .

Might look bad on a future survey …….in terms of what other cheap outs have been practiced entering the buyers head .

 

[wombat88]

Ta, Portofino's boat has plenty fo metalwork underwater where as I have none apart from the tabs. Engine is outboard.

I will not rush to replace them.

I once stayed in the Hotel Internationale in Portofino on expenses. Of this I am proud.

 

[ST840]

I will not rush to replace them

Erm...... so what was the point of the question?

 

[wombat88]

Erm...... so what was the point of the question?

I wanted to simply find out the opinions of others

 

[Fire99]

Firstly, having had one of my trim tabs off the boat and rebuilt by a steel fabricator, yes they are Stainless but not A4 316 Stainless. They are A2 at best.. Unpainted they are magnetic.
I have had a replacement made (for the other side) in 316, though it is noticeably heavier than the Bennett original.

Secondly, unpainted they may well corrode.. Well, correction, the rivets that hold them together (mine are two piece) and the pins that hold them to the rams will corrode. I know that because one of my trim tabs fell off for exactly that reason and the other ate through its fixing rivets and ram pins, so nearly fell off!!

Both tabs were unpainted and weren't connected to the bonding of the boat. Another vessel at the same marina had the same issue..

When mine go back on they will be well painted and I'll add a couple of anodes to be on the safe side..

 

[Tranona]

Firstly, having had one of my trim tabs off the boat and rebuilt by a steel fabricator, yes they are Stainless but not A4 316 Stainless. They are A2 at best.. Unpainted they are magnetic.
I have had a replacement made (for the other side) in 316, though it is noticeably heavier than the Bennett original.

Secondly, unpainted they may well corrode.. Well, correction, the rivets that hold them together (mine are two piece) and the pins that hold them to the rams will corrode. I know that because one of my trim tabs fell off for exactly that reason and the other ate through its fixing rivets and ram pins, so nearly fell off!!

Both tabs were unpainted and weren't connected to the bonding of the boat. Another vessel at the same marina had the same issue..

When mine go back on they will be well painted and I'll add a couple of anodes to be on the safe side..

Unlikely the corrosion between rivets and stainless plate is galvanic, but depends on the metal used in the rivet. 316 plates with stainless or monel fastenings will not set up a galvanic circuit. If there is any aluminium in the construction that would be different.

 

[Frederick911]

My trim tabs bennet date back to 1989, didn’t have anodes fitted when we purchased her 6 years ago and still doesn’t now. To be fair the stainless is a little pitted but structurally sound. Spends 11months in the sea. Just scrap off old AF each year, prime and recoat AF

 

[Fire99]

Unlikely the corrosion between rivets and stainless plate is galvanic, but depends on the metal used in the rivet. 316 plates with stainless or monel fastenings will not set up a galvanic circuit. If there is any aluminium in the construction that would be different.

Well it was certainly the retaining rivets and metal pivot pins that had eaten away. (2nd trim tab had separated into two pieces but fortunately were both dangling in the hinge groove on the transom.

 

[jon and michie]

On some bennet trim tab the rams are each held on by 2 screws to the stainless tab - the the actual ram is plastic

 

[DavidJ]

I guess the same applies to Eltrim tabs
Every year, without even looking, I tell the yard to replace the anodes.
Next year I will look first to see if there is any anode corrosion.

 

[Fire99]

On some bennet trim tab the rams are each held on by 2 screws to the stainless tab - the the actual ram is plastic

True.. But the pivot pin that the ram moves on in the little plastic bracket (that's attached by the screws, is also metal. But it's likely it's the fixing screws that failed.

 

[Parabordi]

I have stainless tabs too, and within a year of moving from Fresh Water to Saltwater there are now pin holes in the Stainless plates. So i have now fitted donut anode to each one. I was surprised as i also did not think Stainless would need protecting.

 

[Tranona]

I have stainless tabs too, and within a year of moving from Fresh Water to Saltwater there are now pin holes in the Stainless plates. So i have now fitted donut anode to each one. I was surprised as i also did not think Stainless would need protecting.

Not sure your anode will make any difference. Suspect your "stainless" plates are not 316. Anodes only work where there are 2 different metals with different potential connected together in seawater such as a bronze propeller on a stainless shaft where the zinc in the bronze is lower potential than the shaft.

 

[Parabordi]

Not sure your anode will make any difference. Suspect your "stainless" plates are not 316. Anodes only work where there are 2 different metals with different potential connected together in seawater such as a bronze propeller on a stainless shaft where the zinc in the bronze is lower potential than the shaft.

Well there are two different metals now, zinc anodes and stainless tabs? If no good, why is it happening?

 

[Tranona]

Well there are two different metals now, zinc anodes and stainless tabs? If no good, why is it happening?

What happening?

The plates are corroding because they are inferior metal - my guess. Anodes will not make the slightest bit of difference. As i said they are only effective when there is a galvanic circuit between two different metals. by definition stainless is one metal.

Just to demonstrate. in the photo the fitting at the bottom of the ruder (dark blue because it has just had an extra coat of AF) is 316 stainless attached with 316 bolts. It has been there since 1979 and is in perfect condition. 316 underwater does not need anodes. However if it is connected to other metal such as aluminium a galvanic circuit will exist and the aluminium will corrode (not the stainless). An anode wastes in preference to the aluminium. Sterndrives and saildrives are examples where this works.

 

[Fire99]

Not sure your anode will make any difference. Suspect your "stainless" plates are not 316.

I know they weren't 316 (see my original post). One fell off and the other had separated into two. I took that one to a steel fabricator that identified it as a lower grade Stainless Steel.. (Probably A2)