Metallurgy/chemistry question (anodes) for Vyv and Vic and other experts please!

[Plevier]

This follows on from someone else's recent thread, I'm trying to get some extra responses on a more theoretical basis rather than experiential.

The current Flexofold 2 blade saildrive propeller has anodes (tiny expensive buttons).
If you get one a few years old they don't have anodes.
Going by pics on the website, the 2 blade shaft version does not have anodes, but of course you can put one on the shaft.
The three blade ones do have an anode, a much bigger one.

Flexofold say that the 2 blade SD unit does not need anodes because it's all made of nickel aluminium bronze (no spec given), except bolts and pivot pins which are ANC4. The hub includes isolation so there is no electrical connection with the SD output shaft. They only started fitting them because of customer pressure (I hear by grapevine that it was Yanmar who insisted on having them but can't vouch for it). They say there is no point in adapting an older unit to take anodes, and they imply that it's not necessary to replace exhausted ones on newer units.

Darglow used to offer a machining service to fit them to older hubs, but have stopped doing it.

My prop is 2008 but was in fresh water all its life until March this year at which time it was perfect. In 3 months time I'll see how it looks after a year in salt. What's worrying is someone 20 miles from us with an identical boat (this prop is factory standard fit), new in 2010, apparently had to have a new prop after a year because of corrosion. It was done under warranty but never explained.

So, expert views please! Does theory say that it should it be OK without anodes?

 

[Tranona]

Not sure that "theory" is going to help as the evidence is that that prop on a saildrive does not need an anode. Does not explain why the boat you refer to had its prop replaced under warranty, but you will find many examples of 2 blade FlexoFolds on saildrives for many years in salt water without any problems. Mine is now coming up 3 years in the water all year round and prop is perfect. Sprayed on Trilux from new, so just wash it off, chip the odd barny off and re-spray.

 

[vyv_cox]

Nickel aluminium bronze is about as good as you can get in propeller materials. Most ships' propellers are made from it and typically they are not fitted with anodes. ANC4 is a high grade austenitic stainless steel with more nickel and molybdenum than 316. There is a potential difference between the two of about 0.2 volts with the NAB the more anodic of the couple. However, the NAB has a far greater surface area than the ANC4, which would result in a very low corrosion rate. I would be surprised if there was significant corrosion but it might be worth inspecting the contact points between the NAB and ANC4 quite closely for any signs.

 

[Plevier]

Not sure that "theory" is going to help as the evidence is that that prop on a saildrive does not need an anode. Does not explain why the boat you refer to had its prop replaced under warranty, but you will find many examples of 2 blade FlexoFolds on saildrives for many years in salt water without any problems. Mine is now coming up 3 years in the water all year round and prop is perfect. Sprayed on Trilux from new, so just wash it off, chip the odd barny off and re-spray.

Anecdotal reports do generally seem favourable but there is the odd one not so good.
Digressing a little, do you find the Trilux stays on? We did the leg with Trilux in March before going into the sea but not the prop. Any primer?

 

[Plevier]

Thanks Vyv that's pretty reassuring.

 

[Tranona]

Anecdotal reports do generally seem favourable but there is the odd one not so good.
Digressing a little, do you find the Trilux stays on? We did the leg with Trilux in March before going into the sea but not the prop. Any primer?

Guess FlexoFold worked out the low potential differential when they decided not to use anodes. Always a problem trying to draw a conclusion from the "odd one" that does not seem to fit with the norm when you only have part of the story.

I sprayed the Trilux straight on the new saildrive housing and then later on the new shiny prop when it was fitted. Did not use the primer as did not have any and the boat was only held in slings so pressure to relaunch. With an existing prop would clean and polish before using primer and then spray.