Shaft protection

[25931]

Can someone please explain why a prop shaft should be cathodically protected ? I would have thought that the prop being more vulneravel would have protected the shaft so if the prop is protected does that not automatically protect the shaft or is it that a protected shaft increases the prop protection ?

 

[Tranona]

Rather confused here. It is not the shaft that needs "protection", but the prop. The prop is a often less noble metal than the shaft and when the two are together in salt water you can get corrosion from one to the other, weakening the prop. You attach the anode so that it is depleted rather than your prop, Rather difficult to attach an anode to most props (although you can with an anode nut) so common to use a hull anode and bond it electrically to the shaft/prop assembly - usually through the gearbox.

Google Galvanic corrosion for a detailed explanation or look on the MG Duff website.

 

[VicS]

Rather confused here. It is not the shaft that needs "protection", but the prop. The prop is a often less noble metal than the shaft and when the two are together in salt water you can get corrosion from one to the other, weakening the prop. You attach the anode so that it is depleted rather than your prop, Rather difficult to attach an anode to most props (although you can with an anode nut) so common to use a hull anode and bond it electrically to the shaft/prop assembly - usually through the gearbox.

Google Galvanic corrosion for a detailed explanation or look on the MG Duff website.

Or fit a shaft anode.

pros and cons

propnut rather small

hull anode relies on connection though gearbox bearings unless a brush contact with the shaft is used

Shaft anode not always possible and sometimes difficult to get on tightly

 

[25931]

Or fit a shaft anode.

pros and cons

propnut rather small

hull anode relies on connection though gearbox beaings uless a brush contact with the shaft is used

Shaft anode not always possible and sometimes difficult to get on tightly

Perhaps I am being obtuse but what I am trying to understand, mgduff don´t explain, is does the anode protecting the prop need to be bonded with the shaft ?

 

[fisherman]

Even MG Duff are a bit vague about bonding, and I've had lots of differing advice. For my money, connect everything to everything else and the hull anode, so any current created by the potential between dissimilar metals goes along the wires. Any isolated metal underwater, in my case keel band, rudder have their own anodes. I lost a lot of zinc from my prop after installing a R&D coupling which isolated the shaft from the engine/gearbox. That's when you need a shaft anode, or to bridge the coupling as I did.

And beware. I have suffered degradation of a steel pot hauler cabinet on deck, due to wet ropes being hauled and connecting it to the ocean, it is isolated from the underwater gear.

 

[dmayes]

Bonding

Any metals that are connected together electrically and under sea water will form a circuit where the least noble will be eaten away. This means that a shaft anode will initially protect the propeller as it is electrically connected to it via the shaft, when it has disappeared the propeller will start to be eaten away. The SS shaft wil be the last to go. Hull anodes need to be connected by wire to any metals that they are designed to protect and should be fairly close to them to be effective.

 

[Tranona]

Perhaps I am being obtuse but what I am trying to understand, mgduff don´t explain, is does the anode protecting the prop need to be bonded with the shaft ?

That is the most convenient way as explained above. The Zinc needs to be electrically connected to the prop, which it will be through the shaft and in line of sight in the water that the prop is in. So if using a hull anode it is usual to mount them on the hull close to the stern gear with a bonding wire inside to the gearbox. However, if you have a flexible coupling you will need to provide a metal bridge across the coupling. The easiest way of providing a small anode is to use the special nut for the end of the shaft that takes an anode. It is then in direct contact with the prop. Only downsides are that they are relatively expensive and not very big so may need replacing in a shorter time.

 

[25931]

That is the most convenient way as explained above. The Zinc needs to be electrically connected to the prop, which it will be through the shaft and in line of sight in the water that the prop is in. So if using a hull anode it is usual to mount them on the hull close to the stern gear with a bonding wire inside to the gearbox. However, if you have a flexible coupling you will need to provide a metal bridge across the coupling. The easiest way of providing a small anode is to use the special nut for the end of the shaft that takes an anode. It is then in direct contact with the prop. Only downsides are that they are relatively expensive and not very big so may need replacing in a shorter time.

Perhaps there is an opportunity in the market - it should be possible to produce a flexible coupling which will make electrical contact.

 

[fisherman]

Perhaps there is an opportunity in the market - it should be possible to produce a flexible coupling which will make electrical contact.

http://www.asap-supplies.com/marine/r-and-d-marine-flexible-shaft-couplings

Para 3.

 

[Tranona]

Perhaps there is an opportunity in the market - it should be possible to produce a flexible coupling which will make electrical contact.

Rather defeats the object of a flexible coupling - but as you see from the link fisherman gives you can get an electrical connector to add to the coupling. Otherwise a braided metal strap across the coupling connected to the bolts on either side does the same job.

Seems to me you are trying to see problems that don't exist, or if they do there are well established solutions to the problem. So, your prop may not need an anode, but probably worth fitting a small one on the end of the shaft and monitor how it gets on. If it goes quickly fit a hull anode.

 

[Slow_boat]

I have been advised to take a heavy wire, maybe multi-core HT lead or jump lead, bolt to anode take-off, lead to prop shaft astern of the coupling, bare the last 6-9'', and twist it in a loose loop round the prop shaft to act as a brush type contact.

Does that sound reasonable?

 

[Tranona]

I have been advised to take a heavy wire, maybe multi-core HT lead or jump lead, bolt to anode take-off, lead to prop shaft astern of the coupling, bare the last 6-9'', and twist it in a loose loop round the prop shaft to act as a brush type contact.

Does that sound reasonable?

Perhaps not the best way of doing it. That principle is used by MG Duff with brushes that contact the shaft, but the bridge across a flexible coupling is the easiest and most common way of doing it. If you have a solid coupling you obviously don't need a bridge.

 

[tidclacy]

Had the same problem two years ago. Now have shaft anode and one on the end of the prop. Eroded well so doing their job. Hull anode basically redundant see below.

http://coxengineering.sharepoint.com/Pages/Electrochem.aspx

 

[fisherman]

To bridge the coupling I flattened a 15mm copper pipe to make an external strap, next time I had the coupling off I drilled the centres of the two metal straps inside and put in a small stainless bolt with locknuts.

 

[cimo]

Perhaps there is an opportunity in the market - it should be possible to produce a flexible coupling which will make electrical contact.

....or indeed, a prop-to-shaft arrangement that doesn't.

 

[25931]

....or indeed, a prop-to-shaft arrangement that doesn't.

Would there not still be an electric field ?

 

[cimo]

Would there not still be an electric field ?

I'd expect a potiential difference would still exist, but the current circuit would be broken, significantly reducing the galvanic effect.

My anode wear has become negible since I isolated underwater dissimilar metals - apart from the shaft anode that is - a relaible method to isolate prop from shaft would be an interesting development I reckon.

 

[captainboo]

There is the Electro eliminator available for connecting the prop shaft to the anode but it is pricey (but not as much as a new prop!).

http://www.mgduff.co.uk/leisure-craft/fitting-instructions/electro.html

I am going to try something similar with washing machine brushes!

Bridging the flexible coupling works but it relies on good conductivity through the gearbox across oil covered bearings and gears.

 

[Tranona]

My anode wear has become negible since I isolated underwater dissimilar metals - apart from the shaft anode that is - a relaible method to isolate prop from shaft would be an interesting development I reckon.

It is already dome on saildrive props where there is a rubber bush in the hub. However the shaft is splined and it is not so easy to isolate a prop that is on a taper to provide the drive.

On small installations like a yacht auxiliary where the volumes of metals are small, the solutions offered by anodes are usually adequate. Bit different when you get to large MOBO props and mixed metal rudders and shafts.

 

[GeorgeTina]

Isolated shaft

I'd expect a potiential difference would still exist, but the current circuit would be broken, significantly reducing the galvanic effect.

My anode wear has become negible since I isolated underwater dissimilar metals - apart from the shaft anode that is - a relaible method to isolate prop from shaft would be an interesting development I reckon.

I fitted a R and D coupling and did not use the bridging foil so shaft and prop isolated. Could not see the advantage of earthing the shaft to the engine and possibly the marina earth via shore power. If there is no circuit there is no possibility of galvanic corrosion. Use a shaft anode which I replace each year although there is always plenty of "meat" left. Seems to work for me.