Outdrive anodes gone in 4 weeks

[Roughcut]

Any specialist marine corrosion experts out there ? I have a Botnia Targa with twin DP-G outdrives - and the (zinc) prop anodes have eroded away in just 4 weeks. Anodes on the transom bracket and extra anodes bolted to the cavitation plates show minor corrosion - nothing untoward. Its just the prop anodes that disappear at a crazy rate.

When I first got the boat a few years back, the anodes would pretty much last the whole season - and although there's been a variety of engine/outdrive work since then, I can't think what's changed that would affect this. . I've checked the battery negative isolation to engine block (ground) and the groundd bonding from block to legs. All looks ok. The two Volvo Penta impressed current devices appear to be flashing at the correct rate - but I have no idea if they're actually achieving anything.

I've used a silver chloride standard electrode and measured the voltage from ground bonding point(s) on the outdrives as -910mV on one side and -960mV on the other. I guess the difference might indicate a bonding issue - but I'm not sure if a 50mV difference is that significant?

The boat is on a swinging mooring with no mains electricty connections anywhere nearby. There's fairly fast current flow (sea water - not electric!) - but again, nothing that has changed over the years.

Does anyone know of any marine corrosion experts I could talk to or get to look at this? I'm out of ideas for now. I've just put new anodes on so I have about 3 weeks to find an answer!
Thanks.

 

[Momac]

Maybe the anodes just remove were magnesium - supplied by mistake?

 

[emmalina]

Measure from your other anodes to the ODs if you have anything like a volt that will be your problem.

 

[Roughcut]

Maybe the anodes just remove were magnesium - supplied by mistake?

Thanks - but they’re all zinc.

 

[Roughcut]

Measure from your other anodes to the ODs if you have anything like a volt that will be your problem.

Thank you - though I’m not sure how I’d measure voltages under water! The voltages I mentioned were from a standard electrode dipped in the sea and measured to the outdrive bonding point in the engine bay.

 

[emmalina]

900mv is enough to eat anodes so where is it coming from ? isolate your batteries and try measuring again

 

[Roughcut]

900mv is enough to eat anodes so where is it coming from ? isolate your batteries and try measuring again

Hi Emmalina: The -900mV is the natural emf potential of the metal outdrives (Aluminium and Zinc) in seawater (an electrolyte) - referenced to a Standard Electrode. It's unrelated to the boats batteries - and doesn't change if they're disconnected. (There's a good explanation of all this on the EDT directION website at Hull Protection from Corrosion | EDT DirectION )

 

[Portofino]

Bilge ?
How dry , can you see all the switches + pumps and there wiring connections?
How old are the switches?

Presume on a swinging mooring you leave them live when you leave it ?
Would the water level inside the ER bilge compartment ever reach the OD rear transom plate (s) ?
Might not be rain water could be a weepy skin fitting or other underwater point of entry , ie salt water making an internal circuit from a faulty electrical connection intermittently?

As you say somethings changed .

 

[A_8]

Are the props stainless steel?

 

[simonfraser]

Are the props stainless steel?

SS props need different annodes ?

 

[Portofino]

He said in post #1 “ but again, nothing that has changed over the years.”

I took that as it says in my reply .
So if they ( props ) are SS they did work over the years …..until now .

 

[BruceK]

Are the props stainless steel?

This and if they are the older variety the prop hub has anodes which are likely gone. If they are the newer variety I found a mid season change mandatory and would have to beach the boat to change. Ali props and the problem went away

 

[A_8]

If it is the C style SS that goes with the DPE drive the hub is designed to and needs to be isolated from the SS steel. Over time or "suddenly" one year this isolation stops working due to corrosion and general (mis)handling. Anyhow, very common and when it happens the anodes starts to wear down much faster. There is an easy check in that you measure the electrical resistance between the hub and the blades preferable sometime before you mount them.

This is not to say the OP may have another more serious issue but this is very common and for some reason it is typically ignored.

 

[Roughcut]

Bilge ?cupful in the
How dry , can you see all the switches + pumps and there wiring connections?
How old are the switches?

Presume on a swinging mooring you leave them live when you leave it ?
Would the water level inside the ER bilge compartment ever reach the OD rear transom plate (s) ?
Might not be rain water could be a weepy skin fitting or other underwater point of entry , ie salt water making an internal circuit from a faulty electrical connection intermittently?

As you say somethings changed .

Thanks - good thoughts, I'll double check - though there's not usually more than a cupful in the bilge - and the only nearby skin-fitting is Nylon/Plastic.

Bilge ?
How dry , can you see all the switches + pumps and there wiring connections?
How old are the switches?

Presume on a swinging mooring you leave them live when you leave it ?
Would the water level inside the ER bilge compartment ever reach the OD rear transom plate (s) ?
Might not be rain water could be a weepy skin fitting or other underwater point of entry , ie salt water making an internal circuit from a faulty electrical connection intermittently?

As you say somethings changed .

Thanks - good thoughts, I'll double check - though there's not usually more than a cupful in the bilge - and the only nearby skin-fitting is Nylon/Plastic. Measuring the emf from the Standard electrode to the outdrive bonding point, there's no change whatever breakers are activated/isolated - nor if the engines are running. But I'll run through it again and see if there are any likely looking exposed connections.

 

[Roughcut]

SS props need different annodes ?

Yes - the props are Stainless - but possibly more significant is that they've been re-bushed at different times over the past few years. I believe that VP may have changed the metal of the hub core - so I wonder if that's a factor. But still should be protected by Zinc anodes (in sea water) plus the impressed current devices.

 

[Roughcut]

If it is the C style SS that goes with the prop e drive the hub is designed to and needs to be isolated from the SS steel. Over time or "suddenly" one year this isolation stops working due to corrosion and general (mis)handling. Anyhow, very common and when it happens the anodes starts to wear down much faster. There is an easy check in that you measure the electrical resistance between the hub and the blades preferable sometime before you mount them.

This is not to say the OP may have another more serious issue but this is very common and for some reason it is typically ignored.

Thanks - I did measure the resistance from hub to outdrive when it was out of the water, and though I don't recall the exact figure, I think it was in the low megohms. The other area of concern now is the bonding continuity across the outdrive itself. The transom bracket is well bonded to the engine with v low resistance - but the leg is only electrically connected via the drive shaft (as far as I can see). In addition - the lower part of the leg is a separate component, so I suspect the bonding impedance gets higher and higher resistance as you move down the leg to the propeller area. Which might explain why the prop anodes are the most eroded. Part of the problem with doing measurements is what the acceptable measurement values are. Has anyone with a well protected leg ever logged any resistance readings? But the fact that both outdrives seem to be roughly equally affected seems to imply I have a more generic issue. I just can't suss what it is!

 

[DavidJ]

Have you changed your supplier of anodes?
(clutching at straws here)

 

[Roughcut]

3 different suppliers - all reputable dealers too!

 

[Roughcut]

I gather there's an old Volvo Penta publication dealing with corrosion in outdrives - "Marine Electrical Systems" - part no 7733534-7 . Anyone know where I might find a copy?

 

[volvopaul]

Volvo have done some strange things .
DPH used to have nibral props , they now sell stainless , the new DPI has stainless props .
They no longer supply zinc anodes , gone to aluminium to save the planet .
Im lost on how good aluminium will be against stainless or nibral in salt water .
Yes I am well aware zinc is still available from non vp suppliers .

If you have the dpi drive doest have its own protection system ?