Anodes - Again.

[Stemar]

Jissel's pear anode is now seven or eight years old, no more than 1/3 gone, and coated with a brown/grey deposit, so it's clearly not doing a great deal. I'm fitting a new one, (MG Duff, not a generic, on the sdvice of someone I trust) It's wired to the engine water intake, a couple of inches away, and to the engine. None of the other seacocks are in line of sight of the anode and I do have a shaft anode that needs replacing annually, sometimes a bit sooner.

So, the questions?

Just what does the anode do on a GRP boat
What should it be connected to?
Anything else I should be doing? In spite of the stern gear's appetite for anodes, the prop has a few coppery marks, which makes me think its protection is inadequate.

Thanks in advance.

 

[vyv_cox]

I only have a shaft anode and a prop anode. No others.

Anodes cannot help dezincification. Zinc trying to protect a zinc-rich phase. Paint is probably the best protection for a brass prop.

 

[Stemar]

It's supposed to be bronze. I suppose it's possible that spending a couple of hours each tide in Portsmouth mud of questionable composition isn't helping. I tried Prop-o-drev last year, but there was no trace of it by the end of the season.

 

[Teko]

It's supposed to be bronze. I suppose it's possible that spending a couple of hours each tide in Portsmouth mud of questionable composition isn't helping. I tried Prop-o-drev last year, but there was no trace of it by the end of the season.

Where in the harbour are you? Were at fareham which seems to eat anodes for fun

 

[vyv_cox]

It's supposed to be bronze. I suppose it's possible that spending a couple of hours each tide in Portsmouth mud of questionable composition isn't helping. I tried Prop-o-drev last year, but there was no trace of it by the end of the season.

Manganese bronze, of which most fixed and many folding/feathering are made , is a 60/40 brass with minor additions. Which is why they suffer dezincification. No zinc in bronze.

 

[VicS]

Jissel's pear anode is now seven or eight years old, no more than 1/3 gone, and coated with a brown/grey deposit, so it's clearly not doing a great deal. I'm fitting a new one, (MG Duff, not a generic, on the sdvice of someone I trust) It's wired to the engine water intake, a couple of inches away, and to the engine. None of the other seacocks are in line of sight of the anode and I do have a shaft anode that needs replacing annually, sometimes a bit sooner.

So, the questions?

Just what does the anode do on a GRP boat
What should it be connected to?
Anything else I should be doing? In spite of the stern gear's appetite for anodes, the prop has a few coppery marks, which makes me think its protection is inadequate.

Thanks in advance.

The anode is there to protect metal and alloy bits and pieces below the water line from corrosion ... It must be connected to them electrically.

BUT
Current advice is not to connect anodes to skin fittings. They should be made of a corrosion resistant alloy, such as bronze or DZR brass , or plastic, and therefore not need the protection of anode.

The anode should be bonded to the engine block to provide protection of the stern gear, but to achieve this any flexible shaft coupling must be bridged to complete the connection
( Or with a system of brushes running on the shaft, eg MG Duff's "Electro- eliminator")

The shaft anode may be the better way of protecting the prop if you really do have a problem .... a few pinkish marks on the surface are not a problem.

There are other reputable suppliers of anodes apart from MGDuff ..... just dont buy cheap tat from ebay which does not conform to the US military specification; MIL-A-18001

 

[superheat6k]

Jissel's pear anode is now seven or eight years old, no more than 1/3 gone, and coated with a brown/grey deposit, so it's clearly not doing a great deal. I'm fitting a new one, (MG Duff, not a generic, on the sdvice of someone I trust) It's wired to the engine water intake, a couple of inches away, and to the engine. None of the other seacocks are in line of sight of the anode and I do have a shaft anode that needs replacing annually, sometimes a bit sooner.

So, the questions?

Just what does the anode do on a GRP boat
What should it be connected to?
Anything else I should be doing? In spite of the stern gear's appetite for anodes, the prop has a few coppery marks, which makes me think its protection is inadequate.

Thanks in advance.

As described it is only protecting the water inlet, unless as Vic mentions you have a bridge cable across any plastic coupling. It also needs to be fsairly close to the metal being protected. Measuring with a low voltage ohmmeter the resistance between the anode and any metal being protected shoudl be < 1 ohm. Much more than this and it becomes completely ineffective. A distance of more than 2-3' will also see the protection rapidly dwindle.

Vyv advice as to only having anodes on the shaft and prop is also sound - you can apply too much zinc, which can create a hard rough layer on the protected metal e.g. propellor blades, that is an ideal base site for barnacles to grab onto.

But it does sound like the protection being provided is just the sea water inlet fitting.

On a motorboat like my own I have to think about a hard revving prop and shaft, which is quite a substantial mass, plus P bracket and rudder, and all times 2 shafts. As part iof this Winter's shaft gear refit I have added new double coupling straps plus shaft brushes, but will no longer be using shaft anodes nor trim tab anodes.

 

[boatmike]

I agree with those who say the skin fittings are not the problem and should be bronze anyway. Also the engine is not always the best earthing point as gearboxes often have a high electrical resistance and couplings certainly need to have continuity straps. Site the anode as near to the prop as possible and earth to the back of the gearbox casing. Then with the boat out of the water check the resistance between the anode and prop with a suitable meter. Shaft anodes are good certainly if you have room for one but again you need to clean the shaft and check continuity. so many are rendered ineffective by bolting over a dirty or corroded shaft. They also don't present the area of a larger hull mounted anode.