Wiring aluminum boat

Bert...the mission is Preventing Galvanic Corrosion. A boat is a mass of dissimilar metals submerged in an electrolyte. Pumps, plumping, tanks .........great damage can result by including stray electric current to this mass of metals. Even Paint systems will fail.

 

Let me explain once more why a ground strap or cable is good engineering practice.

In the first place fully isolated DC systems exist only in the imagination of some people.
Even with all electrical equipment fully isolated, there will always be a fairly low resistant path between DC and the engine block, caused by moisture, dirt and carbon deposits. Connecting the field windings of a starter motor or alternator to a terminal instead of the metal housing provides isolation only when the part is dry and unused. True isolation is always labelled with a class identification, referring to a minimum distance between conductors and housing, materials used and a test voltage of 2500 or 4000 volts.

In real life 99,99% of all marine engines have DC negative connected to the engine block. If you want to dispute, think of the millions of Mercruiser, OMC, Yanmar and Volvo engines and all outboards, regardless of brand names.
All of these have virtually 0 ohms resistance between DC minus and any part bolted to the engine.

In a stern drive setup, it is impossible to isolate the engine block from the hull, but a conventional prop shaft at first glance looks promising. Now assume that all precautions have been taken to keep the engine isolated from the hull, like isolated engine mounts, a non-conducting shaft coupling and fuel lines without a braided shield.
A simple test with a multimeter will reveal there is still a conductive path between the hull and the engine: seawater in the cooling hose.

Is that bad? No it isn't as long as all the DC wiring is in good order and nowhere is a wire touching the hull or a wet connector.
Under such circumstances it doesn't make a difference whether there is a ground strap or not, because there flows no current.

But if a short between DC positive and the hull or a current leak develops, the circuit closes and current will flow through the raw water pump and/or the walls within the heat exchanger. Dependent on the amount of current, it may take a few weeks or less than a day before corrosion has eaten through the 0.05 inch wall of a heat exchanger. And it may also make scars on the balls, rollers or needles in a bearing.

With a ground strap, none of that will occur. There will only be a blown fuse or damage to the part that created the short or leakage.
Reasoning should tell you what is wise and what isn't, not what is written in a handbook unless the author's reasoning is explained in detail.

 

That make sense and is well written and explained. I hate it, if people tells me , I must jump of the Eifel tower, why? Well just somebody say so.

No detailed explanation why. I think CDK and also IKE, that your explanations is clear enough, that I advise fishNduck to ignore all other half baked explanations. All I like to add is to use liquid insulations #61 and seal any connection, which has different materials, even a springwasher.
Bert

 

First off -- the 16' AL DC craft seems to have been lost in the comments -- also appears to have been answered -- and so I expect to be forgiven for being off topic.

I'm here to learn, and this is not an area where I have any specialized knowledge, so bear me no malice if I conflate too much of the information I've read. That being said -- I have some observations and questions.

I sense confusion in the word "ground" -- between a ground intended to be part of a working circuit and a ground intended to provide an alternate path in the event of failure. It seems to me that you can "float" a system, meaning the hull is never a part of the working circuits -- and still connect the ground to hull (as protection to potential which might otherwise cause problems). If this is correct, a single point of ground with the loads in an isolated star configuration seems logical. That's as far as I've thought that out -- so I'll stop.

A consensus on the the mix of AC and DC seems to be missing. If I'm reading correctly, I'm reading that both the AC and DC circuits can be grounded in the same way (i.e., engine block). I find this solution confusing -- and am wondering what precautions would be appropriate to maintain critical systems when one or both of these circuits were compromised.

 

The confusion is understandable because the word 'ground' is used for two completely different things.
The DC ground is also called 'return' and is an active part of the DC circuit. It is tied to the engine block because that's the way small engines are made, it reveals their origin as automotive products.

AC ground is just a protection and takes no part in the distribution of electricity. That is done by the live and return (neutral) wire only, both of which are isolated from ground. It also plays a role in electrical noise suppression, where a high resistance and capacitive path to both live and neutral is allowed.
If AC ground is left floating, it carries half the AC voltage and so does any metallic object connected to it. Not deadly, but it stings, so we connect it to the hull.
In case a ground fault develops where current flows from live to protective ground, a safety device senses it and disconnects the AC source.

 

yes indeed. Ground and negative seem confused. When you begin electrifying a boat the first thing you install is the " Ground Plane".. EARTH system. On a metal boat the entire hull is the ground plane, on a plastic boat a copper strap running fore and aft with its discharge point into the sea water via keel bolts, thru hulls or a dedicated copper grounding plate on the outside of the hull.... would be the ground plane. This earth or ground plane is not one leg of the electric system. No electricity is transmitted thru this ground plane to power equipment. The chassis of all metallic electrified equipment like a pumps or battery chargers are bonded...wired..to this ground plane. The ground plane is a safety feature. If a piece of isolated, two pole marine equipment developed an internal fault, electrifying its chassis, the stray electricity is dumped into the ground plane via its bonding strap and into the sea...just like the energy of a lightning bolt...rather than into the operators body.


Your job as operator of the boats electric system is to constantly monitor...detect ... electrical faults that are dumping into the ground plane and fix them. A voltmeter between positive and the ground plane and between negative and the ground plane shall read Zero volts . If not..fix the problem.

This is the reason two pole breakers..or fuses...which break both positve and negative are adventagous when trouble shooting an electric leak. Throw all the breakers and pull all the fuses....the fault disappears...one by one energize the breakers and presto...you found the electric leak...refrigeration system or running lights or main engine alternator. Without an isolated, floating, two pole system a leak in one piece of equipment becomes a leak contaminating all equipment and is very time consuming to track down.

This hold true for 10 ft boats, 100 ft boats, plastic boats, metal boats.

If you are building a boat or adding a major piece of new equipment like an outdrive its important to read the product data sheet to see if its electrically isolated or two pole equipment. None isolated equipment is a poor choice for continuous immersion in sea water and will have a short service life.

http://www.volvopenta.com/volvopent...features/Pages/unique_saildrive_features.aspx

 

Thank you -- as always.

You're making a distinction that I've never recognized -- the distinction between ground and neutral. In my reading, I conflate the two.

If I understand -- neutral is part of, and intended to be part of, a working circuit -- ground is not.

 

My apology, I was in a bad mood, I burned a Brushless motor and the delivery time is long.
My criticism was directed to one person.

What I still remember from my study times. This is what I would do, if I had an aluminium 16 feet, 12/24 Volt DC boat. I would select a good position to mount the zinc ground plate to the hull. (under water level). With a very thick 9 mm (64 mm2, 0.0279 Ohm/100 meter) copper cable I would come down from the top of the mast without sharp bending’s and if possible without giving the lightning a chance to jump to the alu mast. However the alu mast has an higher Ohm resistance and thus the current flow would be restricted. At the top a very sharp point to conduct the lightning to the copper. If I can find a good unit, I would buy an ioniser, which either put a high negative voltage on that highest point or a very high positive point to that point, depending if the sensor, senses a negative lightning surge or a positive. (i.e. lightning from earth to cloud or lightning surge from cloud to earth. The reason is that if I create a higher conductive path for the lightning electrons to find its way, the lighting will be diverted to a shorter lightning path somewhere else. I will leave it over to Michael to find me a good one, he is a master in finding information on the Internet. (And spinning a near believing good story around it)
At 40.000 Ampere and approx 20 Khz bursts, the voltage potential would be for a 6 meter mast (40.000 x 0.0279 x 6 ) / 100 = 67 Volt. I would survive in the event of a strike. The heat will probably kill me. I would try to make a Faraday cage in the cabin.
Because I have all negative wires centrally connected and thereafter to the central point on the hull, I will not get stray currents, like Dave said eddy currents. Corrosion would be limited to items mounted to the hull which has different materials. I would try to seal them with # 61 from exposing to moisture.
The switchboard would be with single throw switches. All negative wires leading to one central point, that central negative point with a thick strap to the zinc ground plate. The charger I would buy , would be a charger with a good insulated primary. The approx 14.4 Volt secondary side would be earthed to the casing and connected to the boat’s negative. i.e. central point. The 220 Volt primary side, I would have with a double throw switch FLOATING connected to the offshore mains. I would not connect the off shore earth to the boat’s earth to avoid stray currents.
The 12 Volt to 230 Volt AC 150 watt inverter would be on the primary side earthed to the inverter’s casing. ( and is connected to the central point) The secondary side, 230 Volt AC, the earth (If the inverter has an earth 3rd wire) would be connected to the 230 Volt of the boat’s Microwave, fridge or whatever, but those casings would also be connected to the same central negative /earthed point.
That is what I would do.
Bert

Edited: The problem is, when in a harbour. You don't know how good your next door neighbour is. If he has a bad electrical installation, the stray currents created by him/her would affect your boat's corrosion. Anybody a good suggestion in how to measure with a simple milliVolt meter stray currents from neighbours boat's?

 

Bert and CDK.

Firstly you seem to choose to ignore acknowledged experts in the aluminium boat field. I notice Bert you choose to ignore NA Michael Kastens comments a man with a track record in design.

Secondly CDK, whether a inadvertant low resistance path exists or not, does not justify creating one directly to the hull. It is entirely possible and practical to ultilse isolated negative engine components. These are (a) alternators, starters motors and sensors.

Keeping the hull out of any DC return circuit ( there is really no such thing as DC ground) is done simply to avoid impressing any current wherever possible into the hull, either via leaks from the DC + side or just DC Return load dumping. Stray DC current can do far greater damage then AC.

As to AC firstly note that AC protective earth is not supposed to be used for RF suppression. It exists purely to trigger breakers or fuses when fault currents flow to earth ground. ( this works because mains is earth referenced. ) RCBOz have largely suplanted the role of the AC earth wire anyway.

In an aluminium boat an isolating transformer is basically a derigiour requirement. Certainly for one with any fixed AC installation. This removes any earth referenced AC return path and creates a floating AC circuit, just like you are doing to the DC one as well

What you guys are doing is reparroting conventional GRP wiring practice. Aluminium
Needs far greater care.

Dave

 

CDK. AC ground does not carry half the AC voltage. You are confusing AC neutral with ac protective earth
Depending on the code in place AC neutral and AC protective earth get connected together or AC earth is fed to a earth rod. In a ground referenced system ( mains ) AC neutral is not floating and unless high resistance paths are present neither it nor AC protective earth carry half the voltage.

In floating AC systems ie the secondary side of any transformer ( invertors etc) the AC is floating and AC neutral is symmetrical with AC live.

By the way Bert. It's curious that you want to keep AC stray currents away from the hull but choose to ignore DC ones. dC is far worse for galvanic corrosion

note your comment that the Zinc should be connected directly to the DC - is a vey dangerous thing to do. On a metal boat particular there is an strong argument to not bond at all. Where there is bonding the zi c is connected to the bonding circuit. That circuit should never be nor should the zinc directly connected to DC - doing so would dramatically increase the opportunity for stray current corrosion. ( an issue you completely ignore)

Dave

 

Thank you Dave, It is always nice that people have different opinions. It makes life interesting. I am not ignoring the fact that DC is trickier for corrosion. By having ONE central point for the actual negative DC circuit, will already avoid potential problems. In electronics, it has always been a good practise. Making the chassis or the boat hull at two or multiple places part of the negative return is a very bad practise and I am not doing that. All negatives wires are returned to one point and therefore there is no electron flow from the central point via the strap to the hull. Why?? Because the strap is not part of the closed electron circuit. Except if there is a fault along the Plus DC in the DC circuit. A good practise is always to measure any leakage current through the strap, either via a uV meter or current meter with over current protection. A good circuit must NOT have any electron flow via the strap. By floating the DC circuit, you create Eddy currents because of the capacitive working. By having the DC circuit connected at one point to the hull, you have less build up of stray current electrons and therefore less corrosion.
I challenged you in #73, but had no response from you.
The outside of the hull is normally painted, but not the inside. By having the DC circuit floating, you create stray or eddy currents. Because the hull is normally not painted inside, you will have places which will start corroding.
Not if the Central Negative DC return, is connected with a strap to the hull. You need a certain voltage potential over two different materials to create corrosion. i.e. weld, stainless steel bolt/aluminium etc. with moisture, which you will always have inside the hull.

With respect to the Zinc, you may or maybe not correct. If you can give me proof that by centralizing the Zinc plate for the central point of the lightning protection and the DC circuit, I may re-consider. But as soon I will have two connection points created to the hull, I am violating any basic electron good practise.

The best would be if two identical aluminium boats does a test.
a) The one with a lightning protection to the water and zinc plate ( isolated from the hull ??? How do you do that ??? the hull is a conductor, except if 100% painted and you can prevent the metal hull from making contact with seawater. Which is an impossible task, after you have bumped, after a few harbour visits, to any object.) Also the DC negative is connected to the same earth point.

b) The second boat will have everything floating like you suggest.

My money is on the first option, that I will have less corrosion.
You have your money on the last option.

Bert



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Michael, you are a master in finding information on the internet. Can you find me a good ioniser, which is made (preferable) for yachting. The sensor is mounted high up, senses any change in activity or just measuring lightning flashes in which direction it goes (cloud to earth or earth to cloud) and thereby changing the potential at the top of the mast to such a potential that a lightning strike rather finds its way to another place to offload the negative excess electrons.
Bert

 

Dave, if you go and stand on a square in New York and you point with you finger and shout an UFO in the air. I guarantee you that after a while many people will confirm that such a UFO hangs in the air.
Like I have said before, I hate it, if people tells me, that I must jump of the Eifel tower, because somebody says so, without good sold reasons why I should jump, or proof that it is better for me.

 

Bert now your talking eddy currents. Eddy currents do not cause corrosion, as the impressed value is miniscule. Equally even if the hull is strapped to DC- small eddy currents still exist due to characteristics of the material , hull construction and welding.

Bert don't believe me, I've already pointed out Michael kasten comments. Let me leave you with "Enerst H Sims, Aluminium boatbuilding. Alard Coles nautical. The installation of a DC current system should be fully insulated with an earth return wire and not earthed through the hull" page 84

I don't know your qualifications Bert but you have advanced no reason to strap the hull to DC ground, yet you persist in voodoo electrics and comments about UFos. Best practice insulates an aluminium hull to prevent any fault currents either AC or DC flowing through it, causing significant corrosion.

You or the other hand can rabbit on all you like about " Eiffel towers".

Dave

 

My aplogy that I can't speak Irish. I have repeated stated that the DC installation must be having one central point, whereby all negative wires are returned to that point. If it is done properly, there will be no current to earth/hull, should you connect the negative central point to the hull via a strap. In view that most designers are concerned that leakage may get unoticed for a long time, and therefore creating corrosion. They recommend a floating system. Unfortunately I am an electronics qualified person , and consider problems from the point of electrons view. I don't like floating systems. Just a personal view.

Bert