vyv_cox
Well-Known Member
Indeed so. I was recently contacted for assistance with a badly corroded Volvo heat exchanger/manifold. My contact advised that it is made in a light alloy, presumably an aluminium - magnesium one. Not ideal.
Through hull anode bolts
- Thread starter Tinto
- Start date
vyv_cox
Well-Known Member
Good question! It seems some builders fit them because they always have. Mine was wired to the P-bracket, presumably to protect against dezincification but this is impossible. Dezincification is an internal process known as selective corrosion in which the zinc-rich phase is consumed protecting the copper-rich phase. So a zinc anode cannot protect the zinc-rich phase as they have almost the same voltages.
I took mine off nearly 30 years ago.
bignick
Well-Known Member
many thanks.
yet another job to be added to this winter’s list, though this one will be at the bottom.
Tinto
Well-Known Member
Thanks for the information. I am going to take a closer look at what everything is connected to. Engine is a mermaid. Today I noticed that the seacock for the seawater cooling circuit had a puddle of blue/green water around it so I am wondering how good the electrical connection to the anode is
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Tranona
Well-Known Member
There is no need for any anode for the seawater inlet, not any other through hull fittings. The green water is probably just verdigris and will wash off. It is just surface discolouration on the bronze housing caused by dampness. Through hulls in a GRP boat are not connected to any other metal, unlike your prop and shaft which are, and because they have different potential the lower one, the zinc in the prop alloy wastes. To avoid this you connect an anode, usually zinc to the shaft and this wastes in preference to the zinc in the prop.
The type of engine is largely irrelevant as the only seawater (which is a weak electrolyte) is in the heat exchanger and piping. Some heat exchangers have an anode because there are mixed metals in its construction, but this is nothing to do with a hull anode connected to the stern gear.
lustyd
Well-Known Member
Sometimes they have different metals within the seacocks, surely?
vyv_cox
Well-Known Member
They do and they fail regularly. Brass balls in bronze bodies for example. There is not a great potential difference between the copper alloys but it can be enough for problems
Tinto
Well-Known Member
I plan to change as many as I can to true design ones over the winter
stuartwineberg
Well-Known Member
Ok then what is causing the hull anode on my moody 36 to completely vanish each season. We have a galvanic isolator, the sea cocks aren’t bonded and the boat has saildrive with its own anode that doesn’t much perish. I had always assumed it was for the benefit of the engine. The fuel tank, water tanks and keel cooler for the fridge are bonded as is the steering quadrant. It’s always puzzled me. Engine is Volvo D2-40
Tranona
Well-Known Member
Although as Vyv says some have metal balls running in Teflon so rarely cause problems. Spindles and handles are often steel. However these items are not in seawater although they mat be damp. Rarely do you get any corrosion on valve bodies, even plain brass ones. The failures are almost always with threaded fittings - skin fittings and tails where seawater does sit. Many of these fittings are plain brass and they do dezincify then break if subject to pressure by for example hitting with a hammer or even pulling hard. An anode will not do anything to stop dezincificatiom.
Tranona
Well-Known Member
I am afraid Marine Projects never really understood the subject and tended to bond anything metal to an anode - totally unnecessary. How can water end fuel tanks inside the boat nowhere near any seawater suffer from galvanic action that can be stopped by a sacrificial anode in the seawater outside the boat. Does not mean they can't corrode because they may well have mixed metals in their construction, or suffer crevice corrosion in welds, or simple rust if mild steel. There is an argument for steering systems having anodes because they are often a mixture of yellow metal and stainless or aluminium. However again this mix is rarely in seawater outside the boat so again an anode won't do anything. The engine does not have anodes, but the drive does as you know, but that is electrically isolated from the rest of the boat and systems. Don't know about the keel cooler but doubt it is made of mixed metals. It is probably bronze with bronze fittings.
stuartwineberg
Well-Known Member
Indeed, agree with all that so what is wasting the main anode. I have gone from a pear to one double the size and it still goes
Tranona
Well-Known Member
No real idea, but it is not protecting anything. because apart from the keel cooler there is nothing that can "see" the anode through the electrolyte (seawater).
vyv_cox
Well-Known Member
How big is the keel cooler and what material? Only immersed components can be protected. Is the keel itself connected? That is a big cathode even if painted.
stuartwineberg
Well-Known Member
It’s a pretty small spiral of bronze tubing. Good question about the keel, not sure. It’s iron. We are on a berth against the main marina walkway, lots of metal there.
bignick
Well-Known Member
This thread has opened up more questions in my mind than it has closed.
Since we’re talking about potential differences between metals, would a bog-standard multimeter be sensitive enough to register the voltage differences? I’m thinking that it might be interesting to temporarily disconnect some of the straps and measure the PD.
is this viable? Or would a multimeter not be sensitive enough?
I'm not after a calibrated/accurate result, just to know what (and maybe when, if I turn stuff off and on) is causing my pear anode to deplete before I remove it.
Thanks, Nick
Since we’re talking about potential differences between metals, would a bog-standard multimeter be sensitive enough to register the voltage differences? I’m thinking that it might be interesting to temporarily disconnect some of the straps and measure the PD.
is this viable? Or would a multimeter not be sensitive enough?
I'm not after a calibrated/accurate result, just to know what (and maybe when, if I turn stuff off and on) is causing my pear anode to deplete before I remove it.
Thanks, Nick
Tranona
Well-Known Member
No need to measure if you know the metals. You can use the tables such as here corrosion-doctors.org/Definitions/galvanic-series.htm
Generally speaking there are very few exposed mixed metals on sailing yachts, usually just the propeller (yellow metal alloy containing zinc) attached to a stainless shaft. The large differential between the stainless and the zinc means that the prop will eventually dezincify and become mainly copper. Adding an anode means that wastes in preference so delaying or hopefully eliminating dezincification. Another way of voiding it is to use a better alloy for the prop, but this is expensive. The other area where anodes become important is with folding and feathering props because they are invariably constructed of stainless (for gears and sometimes blades) and bronze alloys. Most therefore, like my new Featherstream have their own anodes which will protect both the prop itself and against the shaft. These often last less time than hull anodes to the shaft because they are fairly small by comparison.
Generally speaking there are very few exposed mixed metals on sailing yachts, usually just the propeller (yellow metal alloy containing zinc) attached to a stainless shaft. The large differential between the stainless and the zinc means that the prop will eventually dezincify and become mainly copper. Adding an anode means that wastes in preference so delaying or hopefully eliminating dezincification. Another way of voiding it is to use a better alloy for the prop, but this is expensive. The other area where anodes become important is with folding and feathering props because they are invariably constructed of stainless (for gears and sometimes blades) and bronze alloys. Most therefore, like my new Featherstream have their own anodes which will protect both the prop itself and against the shaft. These often last less time than hull anodes to the shaft because they are fairly small by comparison.
VicS
Well-Known Member
A digital multimeter should be OK. Both of mine measure to 0.1 mV on the 2 Volt range.
You won't be looking at high impedance sources , so no problem on that score either.
Be on the lookout for readings significantly higher than you'd expect from the published galvanic potentials. They might indicate a leakage from the DC system which could cause electrolysis.
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superheat6k
Well-Known Member
Electrolytic corrosion is not just galvanic - i.e. the corrosion caused by dissimilar metals, but also stray current, which in many cases will be a lot more severe, especially AC high voltage leaks.
Here is a list of things to check.
1 Disconnect the +VE terminal at the battery/ies. Turn off all services that would be off when the boat is left idle. This normally just means the bilge pumps and battery charger are left on.
Measure for voltage between the loose terminal and battery post from whence it was just disconnected. The result should be 0.0v. If you can measure a voltage then you have a DC leakage, and this could easily be eating your anode.
2 So following on check any flapper style pump switches. Disconnect each in turn and repeat check at 1, then dod the same for the bilge pumps. If the reading drops to 0v you have found the culprit.
3 Do you have a live / neutral reversed polarity indicator for the shore supply ? If so is the indicator a neon or a diode - if the latter this could be bypassing your GI. However, disconnect the power reversal lamp and purchase a test plug and use it each time you plug in to shore power.
4 Do you have any steel piles or steel hulled boats nearby.
5 Do you have any rather dodgy looking boats nearby. Either of these can emit stray currents that hitch a ride along your bonding circuit, causing havoc to your anodes.
Tranona
Well-Known Member
The boat in question does not need a bonding circuit. It has a saildrive with its own anode which wastes slowly. If he removed all the existing bonding and the anode he would have nothing to create a circuit through the water.
I had 2 similar boats with saildrives and no other anodes moored right by a huge steel pile for over 10 years with no problems, even on shorepower. With the last boat only one anode change in 6 years in the water all year round.
See this thread forums.ybw.com/index.php?threads/galvanic-isolator.587219/