Wiring aluminum boat

Thanks CDK for reminding me, I forgot to mention in #59 to use Star Brite or a similar product if one uses 2 different connection metals.
Bert

 

I like to throw a spanner in the discussion. I believe that as soon you transfer serious large currents through a 100 yard cable and the cable is parallel to a 100 yard ship's hull, automatically you will induce in the hull stray currents. Depending where the stray current find 2 different metals, whether a weld, or a old fashioned poprivet or rivet, corrosion will start if it is exposed to moisture. This is regardless whether the electrics is floating, double throw switched, single throw switched or having a central earth point, or whatever.

The proof is very simple. You stand with rubber boots 10 meter away from a metal mast in which a lightning strike struck. Question to the forum, do you think you feel something? You think, you will survive or is your blood so thick that you don't feel anything, or are you dead?. You did not make contact with the earth and you were isolated.
Intriguing question and the answer will tell you whether you like to isolate the electrics.
Bert

 

BertKu. You'd want to do a bit of electrical 101. Firstly static builds up on insulators not conductors, static is dissipated into the air, via insulators ( teh cables touching the hull etc etc). lightening of cource has a path through the mast and through the hull , in fact lightening is the very least of your problems on a ally boat. Lightening and DC have nothing in common.

You laptop powered via a power brick is "floating" most DC systems are floating.


The recommendations I have seen for aluminium boats are to keep the hull electrically isolated from the DC and especially the AC system While its requires some searching it is possible to get isolated engine electrics. Two wire senders, two wire starters and alternators are available. This is because you do not want the hull to have any risk of DC current in it.( see alka selzer)

ISO ( RCD) standards, allow where RCBOs are fitted that there is no connection between DC ground ( a black wire in Europe, not yellow as somebody mentioned) and protective AC ground ( AC earth, yellow/green in 230VAC) An isolating transformer should always be used on a aluminium boat with a fixed permanent AC system

 

For Bert and the original poster. Concerning metal boats and best practice there are serveral informative web sites with well written articles on a wide range of issues like electric systems, lightning protection, Corrosion, alloys, .

The Metal Boat Society and the writings of Naval architect Michael Kasten are good.

http://www.kastenmarine.com/articles.htm

http://www.metalboatsociety.com/images/MBQsample/MBQ-Jan2010.pdf

http://www.metalboatsociety.com/images/MBQsample/MBQ JanWtr_2011.pdf

 

We are not discussing AC, but DC systems for a 18 feet boat. Like I have said before, AC is a different ball game alltogether. Lets stick to DC. But thanks for your input.
Bert

 

Absolute correct that static is build up on insulators, but your voltage potential is transmitted via the copper to the other parts on your floating electrical system. You did not finish your argument or you jumped the gun. Think about a car (metal) with rubber tyres and you touch it and get a flipping shock. The insulators are the tyres. But you get a flipping big shock from the metal.
Can I have an answer on my lightning question? DOES A DC CURRENT INDUCES A VOLTAGE POTENTIAL IN A PARALLEL CONDUCTOR - YES or NO (Example of the lightning strike)
If you feel NO, good luck to you, maybe we suggest that you do also a 101 course together with me.
If you answer is YES, it means thus that in an ALU boat whatever system you have, you will induce stray currents in the hull, those stray currents will create corrosion at welds, items mounted to the hull with not the same material, rivets etc. That is the reason why IKE mentioned that one should be careful with an aluminium boat. (Think about where the aluminium is on the ladder)
But if it is YES , you have given yourself the answer, that it does not matter whether the electrical DC system is floating or it is earthed to one point.
Bert

 

Hi Michael,

Nice articles, but maybe I have overlooked some paragraphs, but there is nothing on having a DC electrical system floating. It only said, keep your AC system seperated from the DC circuit. Even my offspring will grasp that.

Bert.

 

Some convoluted logic here. Floating DC systems have no exposure to static ( well no more exposure), static is dissipated in a number of ways including the fact that the insulators have a path to ground. Hence static never builds up in the first place. I'm not sure what you are trying to say with the car analogy.

we all understand induced current, via magnetic coupling, to try and suggest that floating systems are more prone to lightening damage is nonsense

What is clear is that the best system, if you can afford it, is to ensure an aluminium hull is as electrically inert as possible. No doubt there will be eddy currents, but the key thing is not to impress current by including the hull in any ground path, inadvertent or other wise


Dave

 

no so fast Bert

quote from Kastens web site

"Aboard a metal vessel, the ideal would be to make use of a completely floating ground system. In other words, the negative side of the DC power will not permitted to be in contact with the hull nor any hull fittings, anywhere. With a floating ground system, a special type of alternator is used which does not make use of its case as the ground, but instead has a dedicated negative terminal."

he obviously overlooks the starter and the various senders, but as I said , solutions are there too.

Dave

 

Just to finish my triple wammy, What you have pedalled here is dangerous "pseudo-science. A floating DC system is no more or no less exposed to lightening then a so called grounded system. ( because actually in most fibreglass boats the DC is effectively floating). Lightening induced current will tear up both systems in equal measure. Current flows due to lightening are very complex and not fully understood either.


Spain builds its boats to EU RCD standards a far more rigorous process then ABYC and arguably Europe has the greatest experience of aluminium boat building anywhere, possibly followed by NZ. To suggest that the RCD standards are unsafe , without a shred of proof in your case, is just plain nonsense.

 

Bert: No I would not have a floating system. But we need to define what a floating system is. Just because a electrical system is not connected to the hull of a metal boat does not mean it is a floating system.

A DC floating system is one in which the zero voltage potential is not referenced to earth. It is it's own reference. Simply put it has no electrical connection to the earth. Thus it has to establish it's own zero potential. Theoretically (rarely the case though) the zero for a floating system could be anything compared to earth, i.e. + or - 100 volts. who knows. But from the floating systems viewpoint it is zero.

Systems that are grounded to earth, either through a shore connection or through the water are not floating systems. If the negative is connected to the engine block it will most likly also be connected to the earth through the water to establish a zero voltage potential. I know, there are engines that are isolated by having closed cooling and insulators between the engine and the shaft and other means of isolating the engine from the water. But most boats are not, so there is an electrical connection through the engine, gear box, shaft and propeller to the water. This establishes the ground potential at the same zero as the earth (that's why in Europe they call it earthing, not grounding).

But this does not mean it is electrically connected to the hull.

What are stray currents? Stray currents are leakage currents into the water, usually from inadvertent positive connections to the hull, or metal fittings in contact with the water. How does this occur? There are several ways, probably the one most people think of is shorts. A bare positive wire is accidentally in contact with something that is in contact with the water. But another, more insidious (by that I mean harder to detect) way is high resistance connections. Connectors in a marine environment corrode. Corrosion creates a high resistance connection. This can result in lots of heat and may cause a fire. But if there is another path to ground through what you may think is an insulator, but is really just a high resistance, but not as high as the corroded connector, then that's the path the current will take. It may have a high voltage drop due to the resistance, but it will still result in some leakage current. If it is through a fitting that is in contact with the water, or the metal hull, you then have leakage into the water.

That is why I said, way back when, that the equipment should not be grounded to the hull. You are just providing the current a convenient path that you really don't want it taking.

As for lightning, that is a whole other topic, and lightning protection systems are highly controversial. My opinion, for what's worth, is if you get a lightning strike or even a near miss, all the lightning protection will do is provide a convenient path to earth. It will probably still fry all of your electronics, and may still blow a hole in the hull through your ground plate. The best protection against lightning is stay away from it if you can. If you live on the west coast of Florida, fat chance of that happening.

 

Ike, can I ask why not, like what is the actual technical reason ( and were talking about aluminium hulls here). Lets leave the static mumbo jumbo to one side for a minute.

Dave

 

Thank you Ike, your English is better expressed then in my examples. It is what I try to get over to the fishnduck reader. He must be able to make his mind up from whatever is explained and whatever different opinions are expressed.
It is not proven to me, that a floating system as per Dave's and Michaels statements are less prone to the hull having spurs of corroding, than a wiring harness which has somewhere a central point to the hull/earth.
My opinion.
We have two identical aluminium boats. Dave and Michael have the cable harness flat onto the bottom of the aluminium hull, while I have the cable harness 10 cm above the hull in a non metal conduct and have ONE central earth point making contact with the hull and seawater.
The fact that it makes contact with the water and hull is not proven to me that I will now have suddenly more corrosion. I haven't heard any logical arguments.
While I know for sure that in the case of Dave and Michael, the currents induced, how small they may be, will create more corrosion on their boat.
I know that I am a stupid old man and still have to do the 101 course, but I haven't heard any good arguments, why I should not make contact with the metal hull. Either direct or via a 100/1000 Ohm resistor. What makes a prone to static floating cable harness not connected to ONE central earth point making a hull less corrosive. We all know what makes 2 metals corroding , moisture and voltage potential. Created due to stray currents and not having a connection sealed from exposure to moisture.
With regards of the example of lightning , we have here the second highest lightning incidents in the world. There is one country, which has more and is somewhere in South America. One golden rule with lightning is that you RE-ROUTE the lightning surge, not to try to suppress it, the energy is too high. I have a fair experience with lightning surges and have a high respect for strikes.
Bert

 

Lightning, flashing, side arching.... is a different topic and not well understood. The conventional strategy is bond all metalic equipment to the hull, making the metallic equipment on board a single interconnected mass and encouraging the lightning energy to exit the metallic mass and hull as fast as possible with the least resistance. . For lightning protection you are best advised to follow best practice and comply with USCG , MCA or your relevant classification society rules.


Concerning Ikes statement.... "That is why I said, way back when, that the equipment should not be grounded to the hull. You are just providing the current a convenient path that you really don't want it taking." All Marine equipment is two pole... electricaly isolated and must be " bonded"..... to the hull.

"Bonding " a dc system does not imply having the negative power supply on the hull. Marine equipment is two pole...isolated.



Bonding equipment to the hull via a bonding strap is standard practice with both ac and dc systems to prevent electrocution , reduce fire hazard and reduce lightning strike damage.. If a floating...free standing.... piece of electrically isolated marine electrical equipment developed an internal electrical fault it must have a way to dump this fault to earth via the bonding strap.

On a boat, The two legs of a dc electric system..positive and negative... shall be isolated from the hull at all times. The hull must not be used as one leg of the dc power system as an automobile would.

 

Again, just a statement without substantiating. Give me something WHY , not just a statement.
bert