Stainless Duo props getting eaten away

[dypcdiver]

I am on my third set of Duo props, galvanic corrosion I belive is the cause. I suspect there is no continuity between the hub and blades because of the plastic shear protection, leaving the blades to act as an anode!

Has anybody found a solution to this problem?

Volvo Penta replaced my first set after the first season, claiming that it was a faulty set, but only after first checking that there was a gap between the antifouling and the transom shield, Zinc anodes were fitted and that there were no stray electrical currents in the area of my mooring.
My boat is kept in a Spanish marina on the Med for 5 months each year.
My boat is a Cranchi Ellipse 21 fitted with a VP 5.0Gi and a DP-SM outdrive. The outdrive is fitted with the Active Protection System.
Below are photos, the one on the left was new last year and has only been wet for 4 months, the right hand one has done 8 seasons.

 

[dypcdiver]

Re: Stainless Duo props getting eaten away

I'll reply to my own thread.
Not sure why this was moved from VOLVO PENTA section as it is not really a prop subject. It is to do with the corrosion control on VP.

 

[Don S]

Re: Stainless Duo props getting eaten away

I moved it here because I really don't think it's a corrosion problem at all. The aluminum housings would show signs of corrosion long before the SS props would.
To me, those look like metal shrinkage cavities from the casting process. I would like to see pictures of those holes and include the blades of the prop as well.
I worked in a stainless steel foundry many moons ago, and that is not corrosion.

Was hoping some other Volvo prop owners had seen similar.

 

[Thalasso]

Re: Stainless Duo props getting eaten away

I moved it here because I really don't think it's a corrosion problem at all. The aluminum housings would show signs of corrosion long before the SS props would.
To me, those look like metal shrinkage cavities from the casting process. I would like to see pictures of those holes and include the blades of the prop as well.
I worked in a stainless steel foundry many moons ago, and that is not corrosion.

Was hoping some other Volvo prop owners had seen similar.

Sure looks like bad casting.If that was galvanic corrosion there wouldn't be a outdrive.

 

[dypcdiver]

Re: Stainless Duo props getting eaten away

Ok Don I understand where you are coming from.
Perhaps I kept my text a bit short. I have had to replace both my trim/tilt rams because of pitting on the rods, these again have no electrical continuity with the leg. My paddle wheel log has had several new SS pins that have been eaten away.
Below are pictures of the both prop's blades and a nice one of the gimble bearing grease nipple (zerk to you guys)

 

[Don S]

Re: Stainless Duo props getting eaten away

Seems your SS has become the anode. I would be wondering if you are over protected with zinc anodes and the Active Protection System.
Are your zinc anodes staying in like new condition by any chance?

 

[Thalasso]

Re: Stainless Duo props getting eaten away

Seems your SS has become the anode. I would be wondering if you are over protected with zinc anodes and the Active Protection System.
Are your zinc anodes staying in like new condition by any chance?

Wouldn't being over protected harm the outdrive also by paint peeling ( bubbling) off the aluminum.

 

[Don S]

Re: Stainless Duo props getting eaten away

Wouldn't being over protected harm the outdrive also by paint peeling ( bubbling) off the aluminum.

I have no idea what all can happen when over protected. It also depends on HOW much over protected it is, or even if it is. Personally, I have never seen it happen before and why I make a comment for the OP to look into.

 

[dsiekman]

Re: Stainless Duo props getting eaten away

I just noticed similar damage to one of my props. It is not nearly as bad though and seems to be on the prop hub at the thickest part of the metal. My mechanic thought it was just a casting flaw, but I had my props tuned last season and it seems odd that it would just suddenly fall out. Then again, if it was corrosion I would expect it to eat the thinnest parts first. BTW, none of my other parts are having this issue.

 

[dypcdiver]

Re: Stainless Duo props getting eaten away

Thanks for all the replies. Time difference means I do not see them until the morning here!
Anodes are getting eaten at what I consider is a normal rate, I can get 2 seasons out of a set if I clean them before launching. There is some corrosion on the leg but I have put this down to the salt water enviroment it lives in for about 5 months a year.
All the items that are suffering are not electrically connected to the leg, ie. the grease nipple sit in a plastic tube, the stainless part of the prop is insulated by the shear bushing. The trim/tilt piston rods are also isolated via the plastic bushings on the pivot pins and the oil seals on the pistons. About 5 seasons ago I added 2 small 3/4" zinc annodes to the rods of the trim cylinders to stop the pitting, these get about 20% eaten in 5 months, I renew these every year.
The boat is out of the water just now, but I should be launching her in a couple of days. Is there any test procedure for the active corrosion control? I know about the LED flashing between 1 and 5 seconds but that requires the boat in the water, any dry tests?

 

[Scott Danforth]

Re: Stainless Duo props getting eaten away

the salt water is your conductor. you have zinc, aluminum, stainless, and in the oil coolers, prop nuts, etc. copper. something is going to disappear in that environment. Stainless is on the opposite end of the galvanic spectrum from aluminum, zinc, and magnesium. anything that is free (not bonded/grounded to block/battery/drive) will suffer.

the worst place for a boat is in a marina in a salt environment. all the stray electrons play havoc, and with everyone plugged into shore power. if your neighbors boat is more cathodic, your boat will become more anodic.

you can add a grounding tether to your trim cylinders to electrically bond them to the drive. not sure how you would bond the props

does your boat have a galvanic isolator in your hull ground? is recommended for boats on shore power

 

[Thalasso]

Re: Stainless Duo props getting eaten away

Thanks for all the replies. Time difference means I do not see them until the morning here!
Anodes are getting eaten at what I consider is a normal rate, I can get 2 seasons out of a set if I clean them before launching. There is some corrosion on the leg but I have put this down to the salt water enviroment it lives in for about 5 months a year.
All the items that are suffering are not electrically connected to the leg, ie. the grease nipple sit in a plastic tube, the stainless part of the prop is insulated by the shear bushing. The trim/tilt piston rods are also isolated via the plastic bushings on the pivot pins and the oil seals on the pistons. About 5 seasons ago I added 2 small 3/4" zinc annodes to the rods of the trim cylinders to stop the pitting, these get about 20% eaten in 5 months, I renew these every year.
The boat is out of the water just now, but I should be launching her in a couple of days. Is there any test procedure for the active corrosion control? I know about the LED flashing between 1 and 5 seconds but that requires the boat in the water, any dry tests?

You should be changeing your anodes every year. If your getting two seasons out of a set and moored in water 5 months a year there is defiantly something wrong. You need to get a qulified marine electrican. To test anodes take a multimeter ( digatal) and put the neg on battery ground and touch the anode or what ever you want to check with the positive end. It should read 0 on the meter.

 

[dypcdiver]

Re: Stainless Duo props getting eaten away

you can add a grounding tether to your trim cylinders to electrically bond them to the drive. not sure how you would bond the props

does your boat have a galvanic isolator in your hull ground? is recommended for boats on shore power

Thanks for the reply, the trim cylinders are already grounded, but the problem is the piston rod which floats on its seals and makes no metal to metal contact.
For the first 8 years I was bearthed on pontoons without power and had the same amount of corrosion.

 

[dypcdiver]

Re: Stainless Duo props getting eaten away

You should be changeing your anodes every year. If your getting two seasons out of a set and moored in water 5 months a year there is defiantly something wrong. You need to get a qulified marine electrican. To test anodes take a multimeter ( digatal) and put the neg on battery ground and touch the anode or what ever you want to check with the positive end. It should read 0 on the meter.

Thanks for your reply, for the first 8 years I did change the anodes before launch. I presume I should put the digital multi meter on the lowest Ohms scale, the reading I get fluctuates between 0.4 to 0.8 of an Ohm

 

[Thalasso]

Re: Stainless Duo props getting eaten away

Yes to the OHMs. If you get a reading your system isn't tied together. Start checking all the bonding wires on the drive.What does your drive look like? Corroded or just paint peeling off with no corrosion?
Here are some web sites to look at, Especially the firs one. Hope these give you a better understanding on how things work and how to maintain better. Like what was said before, i don't think that is a corrosion problem on your props. The aluminum drive would be gone first baseing it on the condition of the props.

Google galvanic corrosion on boats. There is a ton of info there.

www.powerandmotoryacht.com/hull/confirming-continuity
www.boatus.com/boattech/MarineCorrosion.htm

seagrant.uaf.edu/bookstore/boatkeeper/corrosion.pdf

 

[Thalasso]

Re: Stainless Duo props getting eaten away

I am on my third set of Duo props, galvanic corrosion I belive is the cause. I suspect there is no continuity between the hub and blades because of the plastic shear protection, leaving the blades to act as an anode!

Has anybody found a solution to this problem?

Volvo Penta replaced my first set after the first season, claiming that it was a faulty set, but only after first checking that there was a gap between the antifouling and the transom shield, Zinc anodes were fitted and that there were no stray electrical currents in the area of my mooring.
My boat is kept in a Spanish marina on the Med for 5 months each year.
My boat is a Cranchi Ellipse 21 fitted with a VP 5.0Gi and a DP-SM outdrive. The outdrive is fitted with the Active Protection System.
Below are photos, the one on the left was new last year and has only been wet for 4 months, the right hand one has done 8 seasons.

Curious to know if you found anything wrong that could be effecting your props? Did any of the web sight reading help?

 

[dypcdiver]

Re: Stainless Duo props getting eaten away

Curious to know if you found anything wrong that could be effecting your props? Did any of the web sight reading help?

Not been able to really find anything specifically wrong. A couple of years ago at the Barcelona boat show I spent a half hour talking to VP tech guy, who had sanctioned the original replacement of the props (€1400). He, whilst not addmitting that there was a problem with VP did say that Mercruiser also were having problems with their Bravo 3 drives too. I said that the only solution I could see to get the blades included in the protection system was to drill and tap a hole through the stainless and plastic shear bushing and fit a stainless grub screw contacting the alloy hub to complete the circuit, I aso said that as I had been taking photos each year I realised that the blades creep on the plastic bushing and that was why I hadn't done it yet. His comment was that a grub screw MIGHT just fix it. boat is back in the water now for the season and I suspect I'll take the risk with the grub screw when I take here out for the winter next November.
Thanks for your interest.

 

[tkrfxr]

Re: Stainless Duo props getting eaten away

I moved it here because I really don't think it's a corrosion problem at all. The aluminum housings would show signs of corrosion long before the SS props would.
To me, those look like metal shrinkage cavities from the casting process. I would like to see pictures of those holes and include the blades of the prop as well.
I worked in a stainless steel foundry many moons ago, and that is not corrosion.

Was hoping some other Volvo prop owners had seen similar.

I did have this problem and have come to the conclusion that this is a casting defect....I corrected it as shown in this post from another forum several years back.....

http://www.marineengine.com/boat-forum/showthread.php?334169-Stainless-duoprop-electrolysis

Here is how my props looked in 2007, before the cavities were cleaned and filled with epoxy putty:




and in 2009 after scraping the antifouling paint at the end of the season:






-Frank.

 

[dypcdiver]

Re: Stainless Duo props getting eaten away

Many thanks for your reply, as you can see from my photos, the problems are very similar. Have you looked at the grease nipple for the gimble bearing and other non bonded stainless fittings. If it is a casting problem then why does it not show any signs of it on the freshly machined props? As I am on my 3rd set I naturally have closely inspected them (by eyeglass) before mounting.

At one point I thought that the frequently faulty trim sender which is always caused by the insulation cracking on a wire and allowing water into the sensor and shorting it out, might be the cause of stray current. However the new sender I fitted before last season at the same time as the new props is still in good condition.
I suppose the only way to find out if the casting is the problem would be to do a sonic and x-ray test of the prop, as we used to have done to the welds on pipelines before setting them on the seabed!