Prop strut zinc

[Kestrahl 6]

My yacht seems to have a wire going from one of the bolts inside the boat on the shaft strut to the gland and then to a bolt on the engine.. I presume this is using the engine zincs to save the strut and gland?

[wheels 543]

No not the engine Zincs. It is part of the bonding system so as that the main Zinc is doing the work and that all metal parts in the water are at the same potential. The engine zincs are protecting just the engine components, not suually connected to that system. Like heat exchanger and other cooling system items etc.

[rigger 47]

Wheels will give more info but the wire would be the bonding wire, it is to connect the metal bits together so that everything is earthed. Not sure if the engine zincs will protect the strut.

 

Hopefully someone that knows more will post a better response.

[Kestrahl 6]

No not the engine Zincs. It is part of the bonding system so as that the main Zinc is doing the work and that all metal parts in the water are at the same potential. The engine zincs are protecting just the engine components, not suually connected to that system. Like heat exchanger and other cooling system items etc.

 

The only zinc the boat has apart from the ones in the engine (salt water cooled volvo) is the one on the prop shaft. In connecting to the engine it is connecting everything to the negative on the battery i guess.

[PaulR 3]

What the Americans AYBA call the Common Negative Ground and want labelled as such. but also to keep all the different metal anodes (prop, shaft, P bracket etc) in a common electrolyte (sea water) at the same electrical potential. Just think of a simple chemistry experiment, two different metals in a salty solution = electical current generated = makes frogs legs twitch according to Faraday I believe.

 

Google galvanic Batteries

[wheels 543]

The only zinc the boat has apart from the ones in the engine (salt water cooled volvo) is the one on the prop shaft. In connecting to the engine it is connecting everything to the negative on the battery i guess.

Yes correct. There should be one earthing point on the engine, which is normally one of the main bolts that connects the Starter to the motor. One and only one main connection point. Then one heavy earth lead to the battery from that point and the bonding wire from that point to each metal component that is in contact with the salt water. There is one possible problem with your set up though. Possible only, so don't panic yet. The shaft can sometimes be isolated from the engine. You may have a shaft coupler, which sometimes can be a rubber isolator. That part is easy to protect by simply using a jumper wire from a bolt on oneside of the coupling to a bolt on the other. The next place of isolation is a little more difficult though. That is the gearbox. You have a lot of metal components that are all isolated by oil and even if there is an electrical connection, it is sometimes not a good one. So that can be sorted by using a "shaft Brush". This is a carbon contact that runs on the shaft and is bolted to the earth and connects the shaft to the earth system. That way the shaft anode is then protecting everything.

[SanFran 12]

I have issues re this subject also....

Can anyone enlighten me why the shaft anode would totally disappear in a year... this year, then in previous years its only been half gone.

 

The only two things I can think of is... our bilge pump has probably been working a bit more than it was used to... due to a just discovered loss of the log seal... or 2... we replaced (after finding where it used to be located) the engine anode.

[PaulR 3]

It can be all sorts of stray electrical currents around your boat from your neighbours, especially in a marina.

 

Get a very sensitive Volt meter that can read small voltages e.g. 100th of a volt, connect 1 end to shaft / prop and drop other end into water, port, starboard, aft, bow etc and you may see a voltage = electrolysis all working.

 

Another trick for prop shafts. Connect a heavy wire from P bracket, skin fittings etc to a large insulated electrical jaw clamp on the shaft flange, AFT of the gearbox and AFT of the insulating / flexible shaft coupling.

NEVER CLAMP AROUND THE SHAFT.

CLAMP at right angles to shaft so that WHEN (not if) you start the shaft rotating, the clamp will be pulled off harmlessly.

 

Then after mooring reattach clamp.

 

If the clamp is around a shaft, the shaft will spin in the jaws, metal on metal => heat & sparks => engine room fire!!

 

I had to collect / deliver a 46ft UK ketch for repair after the owner used a teak block clamp to stop the shaft rotating. The teak block was charred but ok but the 40mm shaft had melted to a thin 5mm pencil size

[Kestrahl 6]

The only zinc the boat has apart from the ones in the engine (salt water cooled volvo) is the one on the prop shaft. In connecting to the engine it is connecting everything to the negative on the battery i guess.

Yes correct. There should be one earthing point on the engine, which is normally one of the main bolts that connects the Starter to the motor. One and only one main connection point. Then one heavy earth lead to the battery from that point and the bonding wire from that point to each metal component that is in contact with the salt water. There is one possible problem with your set up though. Possible only, so don't panic yet. The shaft can sometimes be isolated from the engine. You may have a shaft coupler, which sometimes can be a rubber isolator. That part is easy to protect by simply using a jumper wire from a bolt on oneside of the coupling to a bolt on the other. The next place of isolation is a little more difficult though. That is the gearbox. You have a lot of metal components that are all isolated by oil and even if there is an electrical connection, it is sometimes not a good one. So that can be sorted by using a "shaft Brush". This is a carbon contact that runs on the shaft and is bolted to the earth and connects the shaft to the earth system. That way the shaft anode is then protecting everything.

 

Thanks Wheels, makes sense. I recall seeing probably what used to be the jumper wire over the flexi coupling when I took it apart a year or so ago.

[wheels 543]

Can anyone enlighten me why the shaft anode would totally disappear in a year... this year, then in previous years its only been half gone.

Well actualy that could be a good thing. "No wear, no work". But what can change that is several factors. One yoiu have no control over is Water temperature. Another which is related is the salinaty. Both these alter that voltage. Anther could be where you have had the boat.If you have been in different places more or less than other years, then Temp and Salinity will also be different.

Opther area's are connections. As they get older, the connections may conduct less and perhaps need a clean up. Another is the Anode itself. It could be that the mix of alloy in the Anode has changed slightly and it has altered the rate of being used. And as Paul said, other boats connected in a Marina may alter things as well. It could well be that you anode is working harder to protect you because some other boat has arrived close byu and altered things.

Get a very sensitive Volt meter that can read small voltages e.g. 100th of a volt, connect 1 end to shaft / prop and drop other end into water, port, starboard, aft, bow etc and you may see a voltage = electrolysis all working.

Firstly, any Volt meter will do the job, because we are working in quite high voltages really. As in upto 1.6V as an extreme case. Do don't ever want to be seeing that kind of reading though, or the boat will be on the bottom witin the day

It is important to understand here, that we actually need a voltage. The anodes is sacrificial. It errodes away at a controlled rate to protect other metals. Hence why it is called the "Sacrificial Anode". It is also important that there is a voltage so as a set polarity is achieved. Hence why it is called the sacrificial "Anode". The Zinc is the anode of the "chemical battery" that is your boat. What is taking place is that the Current flows to the anode, but the actual electrons are flowing the otherway and they carry along the Atoms of the metals. The first metal to go being the Zinc. You can have too much current flow(not enough zinc) and thus the zinc errodes away too quickly and you can have too much Zinc and take the current flow to zero or even the opposite way and the Zince will not wear away and thus is not prtecting. Too much zinc can cause several issues. It can damage paint and it can also corrode to the White Zinc Oxide, which then covers the anode and stops it from conducting. If you haul out and find a Chalky white all over a not very worn anode, two things you need to check. Is the anode making a good connection to the metal on the boat, or is it too big. The perfect size needs to be one that is about half eatin away in 12months. I will finish here because this is getting big and maybe too complex. Better to keep short and have questions asked, rather than make it confusing.

[PaulR 3]

Voltage. My 2 el-cheapo meters are not sensitive enough.

 

My digital clamp one only measures whole volts but goes up to 999V, knot that I've tried that. Mainly it's great because it can LOCK any reading and has an audio "beeeeeep" for continuity testing.

 

My small analogue multi-meter smallest voltage scale is 0 to 10VDC and so is knot sensitive enough.

 

So a scale deflection / range of 0 to 2 or 0 to 1V should show a reading, but always start with a HIGH scale and work downwards.

 

Google as there is quite a lot of info available e.g. metal comparision charts with expected voltages and as Wheels stated, comments about Zinc purities, props "pinking" and photos.

[wheels 543]

Arrr OK Paul, I get ya. Yes you need a Digital Multi, and most cheapies should be capable. Actually I had placed a lot of that info in our technical Archive. But not really sure what all happend to that Forum.

[Guest]

A word of warning with "common" (or technical) earths. OK, two words, voltage drop. The earth leads need to be of VERY LOW Resistance so HEAVY gauge wire (most of the current flows on the outside so greater the surface area the less Resistance). Hence most earth leads are BRAIDED copper and the shorter the better. Any potential between bonded components will setup an electrolytic action.

[idlerboat 116]

...um dont put a "clamp" on the prop shaft....

the clamp is most likely to be bronze or similar....

Also be very carefull of wipping brushes on stainless shafts.

 

Yes you electricaly bond...but at the same time you may be introducing disimilar metals at a very large range of the galvanic scale.

 

Grafite impregnated stuffing box material has caused huge pits in stainless shafts....including my old boat.

 

Isolate or bond ?

There seems to be no perfect way. So the best thing seems to be DIRECT straight to your annodes...yep add a hull annode.

 

Bond your engine to that. Unless you have a totaly isolated engine (very unlikly) Your starter motor and altinator will earth to your engine....even if you have all plastic mounts (like i do) and have a plastic gear box to prop shaft damper......

 

you will sill be connected through your throtle and gearbox cabbles...

 

plastic boat ?...a very small film of salt water on the inside of your hull will conduct electricity just fine.

 

I would rather go with noble metal than my ability to keep all galvanic and electrolytic action at bay...

 

just me..

[wheels 543]

so HEAVY gauge wire (most of the current flows on the outside so greater the surface area the less Resistance).

No not quite correct. Surface conduction is for RF only. DC and low frequency AC all flow through the cable equally. The reason and only reason a fine braid is used is purley for flexability and the ability to withstand vibration without fracture. Cross sectional area is pure and simply current capacity. Resistance is not a factor of cross sectional area. Resistance is the ability of any conductor to allow a current to flow through it. I was trying to find a word to explain, but the best word is in fact, Resistance. If I explain this differently, Copper has a natural Resistance of X(without looking it up) per metre.

 

In general

Also, remember that Galvanic corrosion, or corrosion between dissimilar metals, Must have a third component, that being anm electrolyte and in our case, water. By the way, pure water does not conduct electricity. it is the impurities in water that do the conduction. So in our case, it is salt and the other minerals in Salt water. So bonding inside the Hull, and in theory all the wire and connections should be protected from water, the dissimilar metals is not an issue. What is trying to be done is to produce an electrical path that is a better conductor than that going through the Salt water. This results in us affectivily shorting the "cells" of the "battery" and causing any stray voltage that is left, to use the Sacrificial anode as just that, the anode.

The anode is the Positive of our "battery" and the electrons flow away from the anode toward whatever is the cathode in the entire "battery". With these electrons, Atoms of metals are carried along. In our case, hopefully those particles are Zinc and they flow to the cathode. In a completely still electrolyte, they would actually plate themselves to the cathode. Exactly the same process as Electroplating. But the water the Boat floats in has too much movment and so carry away the Zinc Atoms and they simply dissapear into the mix of the many that make up saltwater.

 

Yes there are two differing oppinions of Bond or not to bond, which is the answer? well it really depends on the situation, what the boat is built from, what metal is exposed in the water and so on and so on.

 

However, here is a new twist. This comes back to our new electrical laws. Because...if you have an Electrical installation for connecting to a Marina, then by law, you are supposed to have an Earthing plate attached to the external of the Hull and all metal must now be bonded back to that plate along with the Electrical Earth. Question, How many have that Huh????? How many have had that tested or even asked for in their Electrical Test huh????