The black art of bonding, anodes etc.

[Allan]

We have a Starlight 35 with a Volvo 2030 engine.
Reading the Volvo manual it states that the gearbox is isolated from the engine. It shows a simple drawing with a battery -ve connected to the engine and a keel bolt. Our boat, built by Bowman, has this and another connection to an anode just above the propshaft.
However, we have large round anode on the hull which is connected both to the engine intake seacock and the gearbox. These are electrically isolated from the rest of the -ve system.
Can anyone give a simple explanation of whether this is correct or not and if so why.
Many thanks in advance.
Allan

 

[Tranona]

Most of that is unnecessary. I believe it is a shaft drive so the isolation is largely irrelevant - that is for saildrives where the drive needs isolating from the engine and the boat's DC system. The only thing that needs protection is the propeller and this can be done in 2 or 3 ways. The simplest is a shaft anode, but your boat does not have sufficient shaft exposed for that unlike boats with a P bracket. So the normal way is to use a hull anode such as you have and bond it to the shaft via a bolt on the gearbox side of the bell housing with a strap across the coupling if it is a flexible type. The electrical circuit is then anode to shaft to propeller. The hull anode should be sited close to the propeller as the complete "circuit" is then through the electrolyte (seawater) from the anode to the propeller. If you have a folding or feathering propeller it will almost certainly have its own anode for added protection for the bronze of the prop against the stainless gears and pins.

There is no need for the connections to the keel or the anode for the water intake as there is nothing to protect. The keel one wont do any harm but does not serve any useful purpose. You can safely remove the one to the seacock - it is doing nothing as seacocks are not connected to any other metal. Don't understand why they also connected it to the gearbox.

Seems like your anode close to the propshaft is there for the propeller so it is just a matter of checking continuity between the anode and the propeller. If the anode is "wearing" and the propeller is not dezincifying then it is working.

Hope this helps.

 

[vyv_cox]

Sadler seem to have had confused ideas about anodes, my 34 had an arrangement similar to yours. I removed the hull anode and connections to the P-bracket and inlet seacock years ago. I only have a shaft and a prop anode. Painting the P-bracket and prop offer far better corrosion protection.

 

[VicS]

Some advice on positioning a hull anode in this video. It also shows the use of an "electro eliminator" as an alternative to bonding via the gearbox

 

[Allan]

Most of that is unnecessary. I believe it is a shaft drive so the isolation is largely irrelevant - that is for saildrives where the drive needs isolating from the engine and the boat's DC system. The only thing that needs protection is the propeller and this can be done in 2 or 3 ways. The simplest is a shaft anode, but your boat does not have sufficient shaft exposed for that unlike boats with a P bracket. So the normal way is to use a hull anode such as you have and bond it to the shaft via a bolt on the gearbox side of the bell housing with a strap across the coupling if it is a flexible type. The electrical circuit is then anode to shaft to propeller. The hull anode should be sited close to the propeller as the complete "circuit" is then through the electrolyte (seawater) from the anode to the propeller. If you have a folding or feathering propeller it will almost certainly have its own anode for added protection for the bronze of the prop against the stainless gears and pins.

There is no need for the connections to the keel or the anode for the water intake as there is nothing to protect. The keel one wont do any harm but does not serve any useful purpose. You can safely remove the one to the seacock - it is doing nothing as seacocks are not connected to any other metal. Don't understand why they also connected it to the gearbox.

Seems like your anode close to the propshaft is there for the propeller so it is just a matter of checking continuity between the anode and the propeller. If the anode is "wearing" and the propeller is not dezincifying then it is working.

Hope this helps.

Many thanks for all the replies. I've now been to the boat and checked the continuity and confirmed what I thought above. The large round anode is connected, via the gearbox to the shaft and prop. The small anode above the shaft is connected to the engine/ boat -ve. As the hull anode is about 1.2m from the prop etc. it does not degrade very much but the cone anode on the Darglow definitely works very well. I'm now surprised that small anode degrades so much as it's not connected to anything nearby. At present I'm inclined to leave things as they are, although I will check regularly that the large anode is connected as the engine seacock could suffer if it's not. I should perhaps add that we rarely use shore power.
Many thanks again.
Allan

 

[Tranona]

The seacock will not "suffer". It is really not necessary to do anything with seacocks on GRP boats. As I explained the seacocks are not connected to any other metal so how can there be any galvanic action? Also most are now DZR which do not dezincify but even if they are plain brass that can dezincify an anode does not help as it is galvanic action within the alloy rather than between the seacock and another metal under water. Modern sailing boats have as many as a dozen seacocks and none use anodes.

The small anode that is connected to the DC negative is not necessary. It would be sensible to use that as a ground for the 240v system as is now recommended practice, although not compulsory for boats built pre 2013. I have just used this method for the 240v system I have installed. It is the same method used by Bavaria who built my previous boat.

As in post#3 your boat was built by people who appear to have a hazy understanding of the use of anodes, but they were not alone at the time.

It is only your propeller that needs an anode. The hull anode it is connected to is only going slowly because the one on the prop goes first because it is closest. The hull anode is essentially a back up. 1.2m is fine for distance from the prop. I am on my second boat with a Featherstream and neither have back up anodes. The first was a wooden boat and hull anodes are not friendly to wood and the anode on the prop lasted typically 2 seasons. The current one is GRP hull but the propeller was only fitted in September. There was no anode with the previous fixed prop so I have not fitted one, relying on the prop anode. The boat will be lifted in May and I shall monitor the rate of wear on the prop anode and if it looks like it won't last a year in the water I shall fit an additional hull anode as a back up.

 

[Allan]

The seacock will not "suffer". It is really not necessary to do anything with seacocks on GRP boats. As I explained the seacocks are not connected to any other metal so how can there be any galvanic action? Also most are now DZR which do not dezincify but even if they are plain brass that can dezincify an anode does not help as it is galvanic action within the alloy rather than between the seacock and another metal under water. Modern sailing boats have as many as a dozen seacocks and none use anodes.

The small anode that is connected to the DC negative is not necessary. It would be sensible to use that as a ground for the 240v system as is now recommended practice, although not compulsory for boats built pre 2013. I have just used this method for the 240v system I have installed. It is the same method used by Bavaria who built my previous boat.

As in post#3 your boat was built by people who appear to have a hazy understanding of the use of anodes, but they were not alone at the time.

It is only your propeller that needs an anode. The hull anode it is connected to is only going slowly because the one on the prop goes first because it is closest. The hull anode is essentially a back up. 1.2m is fine for distance from the prop. I am on my second boat with a Featherstream and neither have back up anodes. The first was a wooden boat and hull anodes are not friendly to wood and the anode on the prop lasted typically 2 seasons. The current one is GRP hull but the propeller was only fitted in September. There was no anode with the previous fixed prop so I have not fitted one, relying on the prop anode. The boat will be lifted in May and I shall monitor the rate of wear on the prop anode and if it looks like it won't last a year in the water I shall fit an additional hull anode as a back up.

Again many thanks for taking the time to reply. I've not checked to see if the second system, that's the small anode etc. is connected to the 240v ground. I've not heard of that being a requirement before. I understood that the only earth should come from the shore power socket. I know I've read that but it was probably written before 2013. My concern that the seacock may suffer is only if the connection with the main hull anode is lost. Although it's over 2m from the prop, it would in effect replace the anode. A moot point I'm sure.
I've often said exactly the same as you about isolated items like the seacock not requiring to be connected. I did wonder if the concern was due to the connection being made by the salt water but as the indirect cooling goes via the gearbox I don't think that can be the case.
Allan

 

[Tranona]

Yes, the grounding of 240v came in with the 2013 revisions to the RCD, although some builders were doing it before. I did my new installation to the latest standard. On seacocks it is irrelevant what happens to the water after wards from the seacock point of view although some seawater circuits do require anodes like my Beta, but it is for the cooling stack against the housing and the anode is in the heat exchanger NOT in the seawater outside. As I said there is nothing in the seacock and valve that will set up any galvanic circuit.

 

[trbt]

there is nothing in the seacock and valve that will set up any galvanic circuit.

And if you do actually connect an anode to the seacock, you have now created a galvanic cell. And the anode erodes and folks say 'hey, it's working, so it must be protecting the other thing!' when in fact if you had not added the anode in the first place, there would be no galvanic cell and no galvanic action

 

[Tranona]

May well do. My latest project has mild steel galvanised (originally) bilge plates with anodes attached by a previous owner. Totally unnecessary although over the years they had worn a bit and probably caused some of the zinc coating to fail. Removed the anodes, had the keels shot blasted and epoxy coated. Hopefully nothing more required for a few years.