How long does your prop anode last?

[Nicholas123]

Mine seems to totally disintegrate in one year & I am wondering if this means it is a viable solution & practical application for protecting my prop.Maybe a different metal like aluminium would be more satisfactory?

 

[MoodySabre]

Mine used to last just a year. Then I fitted one from MG Duff which was much chunkier, at twice the weight of the cheapos. It lasted two years continually in the water and would have done another. I only changed it as it might be two years before the boat comes out.

Well at least you know it's doing its job. Stick with zinc but buy a bigger one (or fit two).

 

[Burnham Bob]

had two (didn't remove the old one just added a new one) and they seemed to last for several seasons. shaft out last year, new anode and it seems only to have lasted a year. MoodySabre is probably right about the chunkiness and mass of the anode in question

 

[VicS]

Mine seems to totally disintegrate in one year & I am wondering if this means it is a viable solution & practical application for protecting my prop.Maybe a different metal like aluminium would be more satisfactory?

The trouble is that you have a small chunk of zinc in contact with a copper alloy prop with a relatively large surface area. The electrochemistry means that the rate of consumption of the zinc will be quite high.

If alumiinium alloy anode is available it could well be worth a try.

a few quick calculations taking into account the difference in density, the difference in atomic weights and the valencies suggest that theoretically an aluminium anode could last about 1.4 times as long as a zinc anode of the same physical size. In practice aluminium anodes are a little more reactive than zinc so that's rather optimistic. I believe about a third extra life is normal.

BUT

If you are connected to shorepower do you have a galvanic isolator in the system

If you have a shaft drive could you also fit a shaft anode, a hull anode or even use a hanging anode.

Not sure about the validity of the previous suggestions to fit a larger anode or to fit two. You may be able to do that with shaft anodes but can you do that with prop anodes.

 

[prv]

What sort of prop is it?

If it's a modern Volvo folding prop on a saildrive, you're actually doing quite well to last a year. It's something of a design fault on those props; they include a rubber bush between shaft and prop to absorb the shock of the blades opening - but this also has the effect of electrically isolating the prop so that no other anodes can be connected to it. So you're left with the small ring of zinc (in two halves) attached to the prop itself, and this is basically undersized. I've heard estimates as short as four months for how long it will last, though ours thankfully hangs on a little longer.

Pete

 

[MoodySabre]

Not sure about the validity of the previous suggestions to fit a larger anode or to fit two. You may be able to do that with shaft anodes but can you do that with prop anodes.

I had assumed it was a shaft anode. Now looking at the OP's bio it refers to an old boat.

 

[prv]

I had assumed it was a shaft anode. Now looking at the OP's bio it refers to an old boat.

Would have been nice if he'd thought to mention what kind of stern gear he has, wouldn't it

Pete

 

[Tranona]

Mine seems to totally disintegrate in one year & I am wondering if this means it is a viable solution & practical application for protecting my prop.Maybe a different metal like aluminium would be more satisfactory?

A year is quite reasonable if it is just one of those little buttons on the end of the prop. Better that than your propeller dezincifying.

Best way to increase anode capacity to increase the length of time before replacement is to fit a hull anode bonded to the prop shaft, probably through the gearbox casing. You may also need to bridge the coupling if it is the flexible type. More information on fitting anodes on the MG Duff website.

 

[VicS]

Would have been nice if he'd thought to mention what kind of stern gear he has, wouldn't it

Pete

He has a Macwester 27 fitted with a DV10. ... but no phone!

So probably no room for additional shaft anodes .. possibly not even room for one of the slim collar type ones, so a hull anode might be the solution.

However if he has a decent manganese bronze fixed bladed prop it probably does not need any cathodic protection anyway!

 

[nigelmercier]

My saildrive anode seems to last a lot longer since I added a hanging anode. Last time I looked there was no apparent reduction in size after a year. I guess you could do the same with a propeller anode, bit of a faff though if the propeller is isolated.

 

[Nicholas123]

He has a Macwester 27 fitted with a DV10. ... but no phone!

So probably no room for additional shaft anodes .. possibly not even room for one of the slim collar type ones, so a hull anode might be the solution.

However if he has a decent manganese bronze fixed bladed prop it probably does not need any cathodic protection anyway!

Well done Sherlock ,you have correctly identified me & I still don't use a phone to contact people who advertise on the internet but cannot be contacted by it Sorry.

Apologies but it is a shaft anode,I did'nt know props could have separate anodes.The one I put on last year was about 1/2" broad & had about 3/4" meat around the circumference.It did'nt so much erode as completely disintegrate.
I think I can fit a neater looking one in the 1 3/4" gap I have got on my 25mm shaft directly behind the prop but will it last any longer?

Interesting to know that if my prop is good quality manganese bronze it would be ok but is it & how can I tell? I think it's good quality because it looks old & unblemished but it could be phosphor bronze or some other kind of derivative could'nt it

 

[Tranona]

The fact that the anode is eroding suggests something is going on between the shaft and the prop so would be unwise to omit an anode. A larger one will last longer for obvious reasons, but make sure you leave at least 15mm space between the anode and the stern bearing for water to flow. A prop anode is mounted on a special nut on the back of the prop, but usually they are small and expensive so not worth it if you can use a shaft anode. Do not buy the cheap shaft anodes as some of them do not have a steel liner so come loose as they erode. Make sure your shaft is exactly 25mm and do not fit a 1" anode. The proper size will be a tight fit and may need tapping with a hammer to get it seated properly.

 

[GrahamM376]

A year is quite reasonable if it is just one of those little buttons on the end of the prop. Better that than your propeller dezincifying.

Best way to increase anode capacity to increase the length of time before replacement is to fit a hull anode bonded to the prop shaft, probably through the gearbox casing. You may also need to bridge the coupling if it is the flexible type. More information on fitting anodes on the MG Duff website.

Have the above + shaft anode. Despite this, our Featherstream prop anode only used to last a year. Although the main body was still OK, it used to get eaten away around the mounting bolt holes and would have fallen off if left longer. By antifouling around the bolt holes which are through the thinnest part, it now lasts 2 years and erodes more evenly. Still good contact/bonding around the base.

 

[VicS]

Well done Sherlock ,you have correctly identified me & I still don't use a phone to contact people who advertise on the internet but cannot be contacted by it Sorry.

Apologies but it is a shaft anode,I did'nt know props could have separate anodes.The one I put on last year was about 1/2" broad & had about 3/4" meat around the circumference.It did'nt so much erode as completely disintegrate.
I think I can fit a neater looking one in the 1 3/4" gap I have got on my 25mm shaft directly behind the prop but will it last any longer?

Interesting to know that if my prop is good quality manganese bronze it would be ok but is it & how can I tell? I think it's good quality because it looks old & unblemished but it could be phosphor bronze or some other kind of derivative could'nt it

You have not said if your are in a marina and have shorepower connected I f you do, even if connected but not actually in use you should check that you have a galvanic isolator fitted unless the earth is not bonded to the DC negative etc.

Manganese bronze (actually not a true bronze at all as it is a copper zinc alloy and called manganese brass by some suppliers) is used for props because of its resistance to seawater corrosion

The fact that the anode is eroding suggests something is going on between the shaft and the prop so would be unwise to omit an anode. .

If zinc and a copper alloy are bonded together and immersed in seawater the zinc will be dissolved. They form a galvanic cell with about 0.6 volt driving reactions similar to electrolysis.
It does not follow that anything beneficial is going on

 

[Tranona]

It does not follow that anything beneficial is going on

Was not suggesting it was beneficial. The OP has already been directed to the MG Duff website so should be able to get an explanation as to why he should fit an anode.