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Rank: Advanced Member
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Location: Phillips, WI
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I'll be making repairs to the dc wiring this winter and I've been doing some reading on the subject. The Marine Electronics Bible states that in a 2 wire system, for a ferrous or alloy hull, you should never ground the DC negative to the hull. The engine should be isolated and directly grounded to the battery. All other DC paths should not be grounded to the hull but grounded to a buss bar.
In my boat most of the DC electronic wiring is brought to a negative bus bar but the DC battery Ground is connected to the engine and the hull. Is this the standard original wiring scheme or did someone decide to add the hull ground?
How are your systems DC grounded?
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There has been a lot of discussion of this recently. The Marinette engine is grounded to the hull. Mine were grounded to a stud thru a floor beam at the front of the engine compartment, and the batteries went to the same stud. This is OK because the engines are electrically isolated from the shaft and prop, which must not touch the hull. My trim tab pump and head, as well as a few other things, were also grounded to the hull. This is bad, there should only be one point of contact between power returns and the hull. You can't make a reliable connection between copper and aluminum, anyway. It is best to have few connections and shorter wires in the starting circuit, and to accomplish that you can wire with heavy wire directly from the battery negative to a stud on the starter, and then take a smaller wire from the same stud to the boat's ground. This is the way most trucks and cars are wired - the battery negative goes to the block directly, and then to the frame. If you have two engines and two batteries, and can start either engine from either battery using switches, then you will need to either put heavy wire between the two battery negatives, or connect battery negatives and starter negatives all to one stud. This is so the starting current can get back to the correct battery. There is a pdf on my website (32' Marinette Wiring) that shows how I did it, perhaps it will be useful. I have diesels, but they start the same. There is also some 'generic' wiring that is all I know about what the factory did, under Marinette Wiring, Generic. www.geocities.com/dougmrose/Before you go back in the water, make sure you have no connection between hull and shaft. When I got my first Marinette, someone had assembled the flange insulator improperly and there were bolts from the shaft flange thru the transmission flange.
"I remember when welfare was for poor people..."
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Rank: Advanced Member
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Thanks for the reply. Is there a case that can be made for floating the DC above the hull?
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No.
Jeff
I'm STILL waiting for my bailout!
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The older submarines used a center-ground system, +14 V and -14 V. The "middle" was tied to hull at the generator through a resistor.
There was one big advantage: troubleshooting a ground from either bus to hull was easy. If the +14 V was shorted to the hull, then the two busses, relative to the hull, would read 0 V and -28 V. If the -14 V was shorted to the hull, then the busses would read +28 V and 0 V. Neither kind of short produced fireworks nor tripped breakers. A "leak" from either bus showed up as a proportional voltage imbalance.
I considered doing this on my Marinette, but I discarded the idea because I do not anticipate a threat environment that includes torpedoes with stray-current-sensitive detonators.
I did, however, put a shunt between my ground plate and the hull, as shown on the schematic. This allows me to connect a voltmeter to the shunt wires to check for current between the wiring system and the hull. There should be none.
I found several things that were grounded to the hull by watching the voltmeter while flipping switches. In addition to the head and trim pump, there were several cabin lights with only the hot hooked up, grounded to the hull through mounting screws.
Don't forget that all the current going out through the ignition switch returns to the engine block, not back to the ground stud directly. I am finishing up a drawing that shows how my power distribution works, and I will put it out when done.
I realize that this is not as succinct as Jeff's answer, but it is the same answer. :-)
"I remember when welfare was for poor people..."
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The resistor idea is a good one for preventative maintenance checks. I purchased my 'M' this spring and before I splashed I had to go over the electrical system which resulted in snipping and clipping nearly a bushel basket full wiring that went nowhere. It was surprising that there hadn't been an electrical fire at some point.
I'm pulling the engines and will strip all wiring. I want to install a breaker panel, rerun all AC/DC wiring and build in some diagnostic/monitoring capabilities, should be fun. Have you considered or do you use mutilcore cabling? At the minimum I plan on running all wiring through conduit.
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Be sure you use multi-strand wires, not single-strand "house" wiring.
Diesel locomotives are wired with the ground of all components separated from the chassis--there's a separate ground wire for every item. The reason? Less destruction if a short occurs somewhere.
Jeff
I'm STILL waiting for my bailout!
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Not only use multi-strand wire, but use Ancor marine wire - the strands are tinned to prevent corrosion.
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I second Ancor Marine. Its more expensive but well worth it. Marine strand wire is finer, allowing more strands for flexibility and bending etc. And as Bpboater stated, its tinned.
The wiring Marinette used was strand but some if it is thick strand. Too stiff, imo.
And from the factory only the ground wires to the hull were from the negative batteries.
I have noticed the trim tab ground went to the hull but I suspect it was in an effort to save time/wire. It is small and momentary so it shouldn't have a constant negative effect.
I was told by Althouse the reason you DON'T ground all your electronics throughout the boat to the hull is to prevent clusters of circuits around the hull. Apparently its "healthier" to ground everything to a single/compact location.
Something else I noticed a while back are the anodes for the skeg are somewhat close if not directly under where the batteries are grounded to the hull. Not sure if that is coincidence or design.
I can say w/ a properly grounded system with everything leading to central (negative terminal of battery) ground the boat remains very safe from hull blistering.
This is not the same as dissimilar metal corrosion like most of us see where the bow rail meets the deck. That is a result of poor isolation, allowing water to get between the paint and Aluminum. And, unless you have removed and drilled out the weep holes or installed nylon spacers under the railing flanges they are full of stagnant mud water in constant contact w/ the Al deck and the stainless flange. Perfect little battery.
-Karl
1986, 32' Sedan, twin 360ci, 275hp Chrysler's w/ K&N flame arrestors
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It is much easier to wire with 2-wire cable, red for power and yellow for return. The proper return path sort of takes care of itself. You might consider getting http://skycraftsurplus.com/index.asp?PageAction=VIEWPROD&ProdID=1542 which is 14-ga marine (tinned) wire for about $50/100', a good price. I use wire from these guys all the time. Be sure to use stranded, tinned, 3-wire cable for AC, so you can carry the green ground as well as the neutral. 14 AWG is the right size.
"I remember when welfare was for poor people..."
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The Navies of USA, England - all major countries have been using 2 wires isolated systems for over 50 years. I know - I was there from 1961. The reason is it stops electrolysis of engines, hulls etcetera via the cooling water path to the seabed and other vessels. Prove it for yourself:
If you have any doubt set up an experiment. Use 2 of 6 volt torch batteries wired in series to get 12 volts. On the remaining 2 terminals connect 3 meters of house earth connection wire - the green or green yellow multi strand single insulation type. On the 2 remaining ends connect 150 milli meter, 6 inch, length of ordinary copper tube. The 12 milli meter, half inch diameter type. Best to solder that connection. Drop each copper pipe in top 150 milli meters water at a total 5 meters distance and measure current flow with a multimeter. You will find it its quite high at 1.8 amps. (Fresh water is virtually the same as salt).
0.002 amp will cause electrolysis you can see in 3 months. To prove this run power from a very small car battery with a battery charger.
Designer.
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I have posted the way I have handled the power wiring on my boat, it is at www.geocities.com/dougmrose, select '32 Marinette Wiring' to get the schematics in .pdf form. The diagram of the power wiring is Sheet 6. This is all from scratch: I removed miles of old zip cord, odd bits of automotive wire, and weird rusty things that didn't seem to be connected. I mounted five breakers on the top of the kickpanel under the wheel, so that they would be easy to reach. I ran a 10 gauge two-wire cable from each to the system to be powered. Note that I powered the Accessory Panel (eyebrow) in two halves: Port half has mostly lights, starboard half has mostly pumps and electronics. The Accessory Panel grounds are also in two halves. It is important that any load wired to a portside switch have its return back to the portside power, and so on. If the grounds get interchanged, or a ground is 'borrowed' from something near by, troubleshooting will be a nightmare. Note that "trim tabs" are powered by a switch on the port side of the accessory panel. I ran new 10 ga cable from the panel down the tube and all the way back to the hydraulic pump near the transom. This lets me power the pump through the panel switch, and brings the return back to the panel. See sheet 14. Power to the various trim tab switches all comes from the pump, which I have not shown. Engine wiring is completely separate from the boat's, and each engine is separate. This way a fault in the boat's circuits can't stop an engine, and a fault in one engine can't stop the other. Don't know if all this is necessary, but it is the way I was taught. Stray currents are probably not a big problem in the hull, because the hull is welded and has very low resistance - still it is bad practice to use it as a power return, and a connection to aluminum can't be trusted anyway.
"I remember when welfare was for poor people..."
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