Stainless Steel Duoprops

[Greg2]

We have an ongoing problem with galvanic corrosion on our aluminium duo props. We have tried improving anode protection and seem to have reached the point where the rearmost prop on one leg in particular is badly affected (new prop corroded to the point of needing replacement in approx. 7 months). There is an issue with the marina but the source of the problem has yet to be located.

I am thinking of trying stainless props to address the problem so I have two questions. The boat is a Fairline Targa 35 fitted with KAD 42's.

1/ We currently run on B6 props, giving circa 35 knots WOT and according to the VP prop chart we appear to need C4 or C5 props to achieve similar performance. Has anyone any experience/knowledge of stainless props for these engines/this boat in terms of which size will be about right?

2/ I understand that, in theory, replacing the aluminium props with stainless may move the problem i.e. the drives might corrode but we have an additional anode wired in to address this. Has anyone any knowledge of addressing corrosion issues by fitting stainless props and whether it worked or not?

Many thanks in anticipation.

 

[carlton]

7 months? Something not right there.

I have SS props and, other than the usual 12 month anode replacement I have no problem.

You mentioned an 'issue' with the marina - I'm sure you've already checked, but do you have a functioning galvanic isolator onboard?

 

[[2068]]

You have a galvanic corrosion problem. Most of the props on my S28 date from 2003 - they are getting a bit tired, but are still hanging in there.

Fitting a load of Stainless Steel below the waterline on a boat with dissolving props is a receipe for disaster, imho. What will happen is that you will have shiny SS props, your anodes will dissolve incredibly quickly, and then your aluminium outdrives will dissolve shortly thereafter.

At a first pass I would get a good marine electrician in to check your shore power / charger / isolator. It could also be other boats near to you causing a problem, dodgy marina shore power system, being too close to a load of steel etc.

dv.

 

[whisper]

7 months? Something not right there.

I have SS props and, other than the usual 12 month anode replacement I have no problem.

You mentioned an 'issue' with the marina - I'm sure you've already checked, but do you have a functioning galvanic isolator onboard?

Lucky you!! With alloy props our anodes last approx 7 months, with SS only 5-6.
No shorepower fitted, berthed alongside a quay wall and dried out for about 6hrs in 24.

 

[carlton]

Alongside a wall, eh? I've heard tales of certain walls in certain marinas being stuffed full of all sorts of bits of metal by way of reinforcement, etc.. D'ya reckon that could that be playing a part?

 

[[2068]]

The "berthed along the quay" wall could be what causes your anodes to fizz away. If the wall is made of plastic or bubblegum, then no issue. If steel piles...

 

[jimmy_the_builder]

A few years ago I had a problem with galv corrosion on one of the legs on my Targa 37. After a bit of running around with a meter, we established that there was a continuity break between the lower half of the leg, and the upper half - and it was the upper half of the leg that was bonded to the hull anode. We fitted an external bonding strap between the lower and upper halves of the leg - and hey presto! problem solved. So - as a first step - might be worth some basic checking with a meter. As others have said, fitting a galv isolator will help as well.

Cheers
Jimmy

 

[GrahamHR]

If the C series props are anything like the props fitted to the DPS drive, there is no continuity between the prop and the outdrive / anodes. On the DPS props there are tight fitting plastic sleeves between the aluminium hub and the props which effectively isolate the props from the hubs electrically. The nuts mechanically hold the prop assembly in place, but only electrically connect the hubs to the outdrive. So, anodes fitted to the drive will do nothing to protect the props if they are the same design; In which case, it soulds like the problem is external stray current related.

 

[Greg2]

Thanks all for your comments.
I think there is definitely a galvanic problem in the marina - lots of us have problems but we haven't tracked it down (it is a club). We have a galvanic isolator fitted - ironically we fitted it after our neighbour lost props and part of drives and it was after then that we had our first problems.

I think we will go through our wiring to see if any obvious issues but we only have shore power on when we are onboard, otherwise it is unplugged.

Interesting the point about top and bottom halves of the leg - I'll check that out as well.

 

[whisper]

The "berthed along the quay" wall could be what causes your anodes to fizz away. If the wall is made of plastic or bubblegum, then no issue. If steel piles...

Carlton and FP, you may be right. However the quay is stone and used to be a coal wharf. Mind you, when they built the flats I expect a lot of scaffolding bits and pieces were dropped in the water.

 

[GrahamHR]

After a quick Google search," coal wharf corrosion", I came across this:

"If your anodes are being eaten too fast, then:
1. You are moored in a vulnerable place where there are some problems - the site of an old coal wharf perhaps; or a stray electrical current either on your boat or nearby."

It also seems corrosion in the holds of ships carrying coal is a serious problem; made worse when the moisture level is high and there are chlorides present; sounds like salt water!

Your anode rate may be high because they are protecting your outdrive in a more hostile environment than normal salt water; a large quantity of coal in the vicinity, possible breaking up into a fine dispersion. The props corroding because maybe nothing is protecting them; stainless ones may last longer and not affect the anode erosion rate.

 

[enterprise]

We have a Targa 28 with Kad32's and recently changed from Alu A7's to Stainless C6's, as a result we changed our anodes to Aluminium instead of the normal zinc.
After aprox 6 months use I can at least confirm the rate of anode corrosion has decreased significantly by using aluminium anodes and are now confident that a set of aluminium will last for far longer than the zinc equivalents.

They are made by Performance metals to US military spec, might be worth trying if you cannot locate the source & cause of the rapid anode wear.

 

[Greg2]

We have a Targa 28 with Kad32's and recently changed from Alu A7's to Stainless C6's, as a result we changed our anodes to Aluminium instead of the normal zinc.
After aprox 6 months use I can at least confirm the rate of anode corrosion has decreased significantly by using aluminium anodes and are now confident that a set of aluminium will last for far longer than the zinc equivalents.

They are made by Performance metals to US military spec, might be worth trying if you cannot locate the source & cause of the rapid anode wear.

Interesting. I have heard that dropping a prop size is about right when moving from ali to stainless so perhaps C5's will be right for us.

Our main issue is to ensure that the anodes take all the corrosion whereas at the moment our props, or at least the rearmost, seem to be bearing the brunt.

 

[GrahamHR]

You have an aluminium alloy outdrive, the anodes are corroding but (presumably) the outdrive isn't. So, the anodes are doing their job.

You have aluminium alloy propellers which are corroding.

Are they electically connected to ( protected by) the outdrive and anodes?

If so, very strange.

If not, what can you do about it?

Volvo's active corrosion protection unit ? ( which is an applied potential unit, sensing the potential and likelihood of corrosion and applying a corrective potential)

I have had 2 boats with DPS drives. Both aluminium alloy drives,.Both with "F" series stainless props.

The first, without ACP needed the zinc anodes changing every year (6 month immersion), not disastrously eroded but they wouldn't have done another season.

The current one with ACP; well, I feel almost ashamed to say, I used the same zinc anodes in 2008 and 2009, and to be honest they look good enough for at least another season. I have no corrosion whatsoever on the aluminium alloy outdrive. or the aluminium propeller hubs either.

In my situation the ACP works very well.

The stainless props themselves; well, they do suffer from crevice corrosion where they meet the plastic sleeve attaching them to the aluminium boss!

I have no connection whatsoever with Volvo Penta.

Graham

 

[Greg2]

You have an aluminium alloy outdrive, the anodes are corroding but (presumably) the outdrive isn't. So, the anodes are doing their job.

You have aluminium alloy propellers which are corroding.

Are they electically connected to ( protected by) the outdrive and anodes?

If so, very strange.

If not, what can you do about it?

Volvo's active corrosion protection unit ? ( which is an applied potential unit, sensing the potential and likelihood of corrosion and applying a corrective potential)

I have had 2 boats with DPS drives. Both aluminium alloy drives,.Both with "F" series stainless props.

The first, without ACP needed the zinc anodes changing every year (6 month immersion), not disastrously eroded but they wouldn't have done another season.

The current one with ACP; well, I feel almost ashamed to say, I used the same zinc anodes in 2008 and 2009, and to be honest they look good enough for at least another season. I have no corrosion whatsoever on the aluminium alloy outdrive. or the aluminium propeller hubs either.

In my situation the ACP works very well.

The stainless props themselves; well, they do suffer from crevice corrosion where they meet the plastic sleeve attaching them to the aluminium boss!

I have no connection whatsoever with Volvo Penta.

Graham

ACP is something to consider then. Some research required methinks.

Spot on re the anodes corroding and the legs not corroding. I think we may have two issues. One is the corrosion problem in the marina and the second is possibly the absence of an electrical connection between the rearmost props and the legs.

 

[whisper]

After a quick Google search," coal wharf corrosion", I came across this:

"If your anodes are being eaten too fast, then:
1. You are moored in a vulnerable place where there are some problems - the site of an old coal wharf perhaps; or a stray electrical current either on your boat or nearby."

It also seems corrosion in the holds of ships carrying coal is a serious problem; made worse when the moisture level is high and there are chlorides present; sounds like salt water!

Your anode rate may be high because they are protecting your outdrive in a more hostile environment than normal salt water; a large quantity of coal in the vicinity, possible breaking up into a fine dispersion. The props corroding because maybe nothing is protecting them; stainless ones may last longer and not affect the anode erosion rate.

Blimey, that's interesting.
As it happens we have moved off an isolated pontoon where we had the fastest anode "wear", as referred to earlier. The pontoon was supported by mild steel tanks and held in position by heavy chains. Our leg was lying approx. 1.5m from the former and 2m from the latter.
We'll see if it has slowed down at our present mooring. If not then the problem of the proximity to the tanks/chain has been replaced by the fluke of now being on an ex coal quay.
There's always bloody somat!!

 

[jparham]

Coal = carbon = an anode sucker.
My outermost ali props on KAD32s always bubble paint. They do this because they are not electrically linked to the shaft (big plastic shock absorber as mentioned above).
One fix is to drill and tap a hole for a bolt between outer (blades) and inner (splined shaft). But said shock absorber does tend to spin a bit if you wrap something around it, so bolt may shear.
Innermost props are almost always perfect (been there since 2002) with annual touch up (Hammerite all metal primer and carpaint) + antifoul.
I hang a couple of MGDuff's finest pearls over the transom, linked to boat main earth point.
Just off the props, but far enough away that if I forget to haul them in, they don't ping the blades.
FYI voltage between marina earth and boat earth is 700mV. Big pile <1.5 metres from stern. That's about 800mV .
And the feeling in our marina is that you just get more problems with S/S....(heresay - no evidence)