Prop moths

[Cerebus]

It is all a little complex if you haven't looked into it. Basically it is to do with the dissimilar metals and a big lump of steel near to a metal such as props and some through hulls are made from will eat them, if they are not protected by suitable anodes. Also, if the steel hulled boat has any current leakage (especially , but not necessarily, DC) to the Hull (Earth) this will also cause problems. There are two problems that can occur, Cathodic (Dissimilar metals) and electrolysis which is caused by the current leakage in association with dissimilar metals.
The above is a very basic and simple explanation. No doubt the experts will come along and rip all the above to pieces. If you do a search for Cathodic protection and Electrolysis you will find lots of info. Have a couple of paracetamol handy as well as a good hour or four free.

Thank you.

 

[Alex_Blackwood]

There are many myths and untruths about the subject. Here are some facts.
Galvanic corrosion requires two (or more) dissimilar metals to be connected together and immersed in a conducting liquid. If these requirements are met, galvanic corrosion can occur. So if a steel boat sits next to you in the sea nothing untoward will happen. If the steel boat and yours are in a marina and both plugged into the mains, the earth wires in your cable can connect your boats together, in which case the requirements for corrosion are met.

However, and this is where the myth falls down, steel is anodic to copper alloys, so it is the steel boat that will corrode, albeit very slowly as there is a size factor involved. Your anodes, being more anodic than steel, may be wasted more quickly but your prop and shaft will not.

A galvanic isolator installed will disconnect the earth wire if a galvanic current is detected.

Vyv. Thanks for expert explanation, knew I should have left it to the experts. However, just to nit pick, you don't mention electrolysis as such. Also a GI will not disconnect the earth. It will not allow a voltage of less than approx. 1.4 volts to pass. The actual value will be dependent on the diodes used. Any Earth leakage voltage above this value will pass and ensure that safety devices such as RCD's will operate correctly. As Paul rightly states. A GI is not required if the earthing arrangements do not include a connection to the sea. Apologies if being pedantic.

 

[vyv_cox]

Vyv. Thanks for expert explanation, knew I should have left it to the experts. However, just to nit pick, you don't mention electrolysis as such. Also a GI will not disconnect the earth. It will not allow a voltage of less than approx. 1.4 volts to pass. The actual value will be dependent on the diodes used. Any Earth leakage voltage above this value will pass and ensure that safety devices such as RCD's will operate correctly. As Paul rightly states. A GI is not required if the earthing arrangements do not include a connection to the sea. Apologies if being pedantic.

I did not mention electrolysis because it has nothing to do with original post.

I do not pretend to understand how a galvanic isolator works, I simply understand the name. Even in the most extreme and ridiculous case, e.g. gold against magnesium, the voltage would only reach 1.8 and in more realistic scenarios under 1.0 V

 

[Hacker]

Hopefully a final update. Decided to replace the prop, new one ordered from Clements (who happened to have all the data from the original one), circa £1900 inc Vat and delivery, bad news is that it won’t be ready till end of August. Then thought I’d better strip the spurs cutters down and replace the various bits. Found out that the port rotating cutter had a huge hole eaten on the inside; so that needed replacing at £400.

Next job was to finally find the fault. What a faff. Squeezing into the engine room resulted in some blood flowing from a scalp wound. My original theory was that there was high corrosion where the bonding wire was attached to the back of the stern gland, as there was quite a lot of surface corrosion on the stern gland. Took it all apart, cleaned everything up and rebuilt it. Tested the resistance and there was no change (still around 700 Ohm). After a lot of cursing I started trying to trace resistance across various parts of the system. It all pointed to the connecting wire between the stern gland and the grounding post on the anode. Took everything apart again only to realise that my multimeter leads weren’t long enough! Finally managed to measure the resistance and it was the culprit. Thought I might get away with cutting off a few inches to remake the connection. No such luck. The cut wire was very brown and corroded. So purchased 3m of 10mm tinned wire plus a couple of connectors (all for under £9).

Just now need to pull the wire through and remake the connections. So a final bill of around £2500 all for £9 of prevention.

All antifouling and anodes replaced (just a couple of top coats on the stern gear to go). We’ve decided to relaunch next week with the original prop so that we can get some use over the summer. Will lift in slings when the prop arrives to change it.

 

[Tranona]

Hopefully a final update. Decided to replace the prop, new one ordered from Clements (who happened to have all the data from the original one), circa £1900 inc Vat and delivery, bad news is that it won’t be ready till end of August. Then thought I’d better strip the spurs cutters down and replace the various bits. Found out that the port rotating cutter had a huge hole eaten on the inside; so that needed replacing at £400.

Next job was to finally find the fault. What a faff. Squeezing into the engine room resulted in some blood flowing from a scalp wound. My original theory was that there was high corrosion where the bonding wire was attached to the back of the stern gland, as there was quite a lot of surface corrosion on the stern gland. Took it all apart, cleaned everything up and rebuilt it. Tested the resistance and there was no change (still around 700 Ohm). After a lot of cursing I started trying to trace resistance across various parts of the system. It all pointed to the connecting wire between the stern gland and the grounding post on the anode. Took everything apart again only to realise that my multimeter leads weren’t long enough! Finally managed to measure the resistance and it was the culprit. Thought I might get away with cutting off a few inches to remake the connection. No such luck. The cut wire was very brown and corroded. So purchased 3m of 10mm tinned wire plus a couple of connectors (all for under £9).

Just now need to pull the wire through and remake the connections. So a final bill of around £2500 all for £9 of prevention.

All antifouling and anodes replaced (just a couple of top coats on the stern gear to go). We’ve decided to relaunch next week with the original prop so that we can get some use over the summer. Will lift in slings when the prop arrives to change it.

It is the resistance between the prop and the anode you need to check. The stern gland housing is nothing to do with the propeller and in fact does not need connecting to an anode at all. Go back to first principles. A galvanic circuit is created when 2 dissimilar metals are immersed in an electrolyte (seawater) . You introduce an anode into the circuit to protect the lower potential metal - usually zinc in the propeller alloy. The stern gland housing does not meet those conditions. It is not in seawater nor connected to any other metal.

As I said earlier, the first line of protection for your sort of stern gear is a shaft anode. As a back up a hull anode should be located close by and bonded to the shaft, usually through the gearbox, but better if possible using brushes on the shaft as in my post#5. You can safely remove the wire to the stern gland housing - it is not the cause of your problem with the propeller - even though it was not previously a good connection. It is a red herring. Sorry you got a scalp wound doing something that was unnecessary.

Before you launch check the resistance between the anode and the propeller/shaft. That is the only thing that matters.

BTW Spurs are notorious for galvanic corrosion as you have found because they are not made of 316 but and alloy with lower potential than the shaft they are attached to. Ideally better to replace them with a Stripper which is 316 and has no corrosion problems (as well as being a more effective cutter)