Converting Diesel to Floating Ground

[Tashtego]

Leaving the starter motor as is grounded. To start the engine the block must be grounded which is done with a large relay. - This is a question I have. Relay properties and supplier recommendations?



thanks[/QUOTE]

Of course you already have a relay on your motor of the appropriate size and configuration - the starter solenoid. You just need another starter solenoid in the ground wire leading back to your starting battery. Run it from the same wire that energizes the starting solenoid in the hot side. It will connect the engine block to the starting battery's negative post when the starting circuit is energized and will disconnect once you let go of the starter switch/button just like the starter solenoid. Naturally you won't run any other ground wire to the block. Right - - - RIGHT!

You should run separate hot and ground wires from the alternator (the hot wire actually runs from the regulator/charging relay/duo-charge etc.) to the starting battery It would be a good idea to run normally closed relays in these wires. Set them up to open when the starting circuit is energized. This will separate the house bank from the starting bank during starting. If you run a two-bank battery monitor you will need two shunts, one for the house bank and one for the starting bank. Remember that the shunt connects to the battery negative with absolutely nothing else connected between the shunt and the battery. I am building the same setup for my Perkins 4-108 this spring.

[msponer]

Oh, I am confused, so I'll duck out. I thought the intent of the 2-wire senders and isolated alternator was to isolate the engine block from the DC negative, and that the relay was just to temporarily reconnect the engine block to turn the starter.

I'm just writing to add the kill solenoid to your list of things to think about.

[foolishsailor]

@tashtego, great stuff thanks, just what I was looking for.

Did you use the same solenoid as your starter? Seems like a good idea to be able to have it as a potential spare?

[foolishsailor]

Originally Posted by Tashtego

It will connect the engine block to the starting battery's negative post when the starting circuit is energized and will disconnect once you let go of the starter switch/button just like the starter solenoid. Naturally you won't run any other ground wire to the block. Right - - - RIGHT!

Nothing like being home and sick for a week, gives one plenty of time to cruise the forum....

Was gnawing on your info and I am confused by the above.

My understanding is that the engine won't start without a proper ground. So when a boat is running a floating ground the solenoid/relay sits between the hull and engine in the ground wire and is only closed during the starting process effectively creating a normal ground during the starting process and then opens after the engine is started removing the ground and bringing the engine back into a floating potential?

If my whole electrical system is floating, which it is, then using the battery as a ground will not achieve this?

Also curious if anyone out there has made the conversion and can comment on any change in efficacy in the alternator between normally grounded systems and floating ground systems.

[goboatingnow]

Originally Posted by foolishsailor

Nothing like being home and sick for a week, gives one plenty of time to cruise the forum....

Was gnawing on your info and I am confused by the above.

My understanding is that the engine won't start without a proper ground. So when a boat is running a floating ground the solenoid/relay sits between the hull and engine in the ground wire and is only closed during the starting process effectively creating a normal ground during the starting process and then opens after the engine is started removing the ground and bringing the engine back into a floating potential?

If my whole electrical system is floating, which it is, then using the battery as a ground will not achieve this?

Also curious if anyone out there has made the conversion and can comment on any change in efficacy in the alternator between normally grounded systems and floating ground systems.

I would not isolate the engine from the boat merely isolate continous dc negative from the engine block

To do this use Isolated senders all the sender manufacturers do them.

Then use an isolated alternator.

For starting use two approaches. ( a) leave the engine block connected to the dc negative and just start the engine as normal. One the starter is disconnected the ground strap carries no current

For complete isolation use an isolated starter, it requires a search.

In my view switching the dc ground strap via a relay will not achieve much , there are too many stray current paths to the engine to be comfortable that it is isolated.

Dave

[Tashtego]

Leaving the engine block connected to "ground" will cause a problem with current from the house battery flowing through to the block. If the block is not disconnected then as current returns to the house battery it will see two paths, one through the house battery monitor shunt to the house battery negative terminal and another through the starting battery negative (or "ground") to the block and on to the hull and/or the sea. The purpose of adding another solenoid to the starting negative ("ground") is to isolate the starting battery charging system from the engine block and its connection to the hull and/or the sea. There should be NO stray ground connections to the block. With two conductor, isolated, gage sending units and some attention to the other connections, like the stop solenoid, no current should flow through the block except the starting current. Remember the question is how to wire a metal boat to avoid electrolytic corrosion.

Any good high current starting solenoid will do the job. But keep an eye out for the ground connection of the solenoid. An automotive starting solenoid may be meant to be grounded through its case mounting point to the car's frame or body and may need some isolation as well. Better yet, connect it to the starting battery positive and negative terminals and wire the starting switch (2 wire) there as well.

PS: Foolish Sailor - a gas engine won't run without a ground because the ignition current runs into the plugs, across the gap (causing the spark) and on into the block which must be connected back to the current source. A diesel will run all day with no electrical connections (although it may be hard to stop and won't have any gages).

[bobcat22]

Originally Posted by foolishsailor

Nothing like being home and sick for a week, gives one plenty of time to cruise the forum....

Was gnawing on your info and I am confused by the above.

My understanding is that the engine won't start without a proper ground. So when a boat is running a floating ground the solenoid/relay sits between the hull and engine in the ground wire and is only closed during the starting process effectively creating a normal ground during the starting process and then opens after the engine is started removing the ground and bringing the engine back into a floating potential?

If my whole electrical system is floating, which it is, then using the battery as a ground will not achieve this?

Also curious if anyone out there has made the conversion and can comment on any change in efficacy in the alternator between normally grounded systems and floating ground systems.

After nearly 10 years and this thread is still the first in the Google search list. I can add some thoughts to this to clarify and explain my plan.

The problem we face is that aluminum specifically is subject to galvanic corrosion due to being lower on the noble scale that some other common marine metals like SS and bronze. The best way to approach this is to isolate both your AC and DC grounds so there is no contact with the hull. For AC, that means forgoing shorepower or using an isolation transformer. In my case, my shore power feeds only a battery charger that is isolated from ground. All AC is delivered via inverters. Note, you can’t do this with a combination inverter charger because ABYC requires them to provide a hull to shore ground connection.

For the DC side, you really want the engine isolated from the hull with rubber mounts and a drivesaver type isolated coupling to the shaft. The only path for current from block to sea is then via raw saltwater running through the heat exchanger. Thats a tough one to remedy.

The starter grounded to the block then connected to battery common is no big deal as it isn’t on for long. I like the idea of a solenoid in the engine block connection to ground wired up like the starter, but it doesn’t sound easy. Mine has a mag switch which operates the solenoid which then connects the starter. I can imagine low power leads off the actual starter solenoid wired in parallel to a duplicate solenoid leading to the battery negative would do the trick. Accessing those leads may be nontrivial, but I’m going to try.

The engine controls and senders may also be grounded to the block like the starter, so it’s best to make sure thats not the case, especially on a power boat or one running the engine for long periods. Their grounds need to lead directly to battery negative. These include but are not limited to water temp, oil pressure, tach, mag switch, panel power, alarms, shutdown solenoid and other sensors like transmission pressure, etc. Yuck.

The alternator is a particular beast. It should be of the isolated kind otherwise current might stray from the bronze heat exchanger (or iron block) via the salt water cooling system and start eating away the hull. Time to check those zincs - again.

[Bowdrie]

Although my boat is not of metal construction, I went with an electrical scheme that has received the "blessings" of an ABYC inspector, who also was a teacher for ABYC electrical classes.
I use 2 alternators; the "stock" alternator, (engine mounted,) charges only the start batt bank, and neither that alternator nor the start batts are in any way sharing any connections with the house system or its batts.
The engine lives in its own little world, isolated by a Drivesaver from the prop shaft.
The house batt bank is charged from a second alternator which is "off engine" mounted, and it only charges the house batts.
Their are zero connections between the engine and house, no ground paths, and as such the engine does not serve as a "Grand Central Station" for grounding connections.
Their are also 2 batt chargers, one for engine batts, one for house,, no interaction or connections between them.
Note; the two chargers allow diff charging regimens and float voltage for the "deep cycle" house batts and the "automotive" type starter batts.
The system works very well, and has been trouble free,, no under or overcharged batts, no ground loops, no corrosion, no currant leaks, no stray voltages.

[skipperpete]

Originally Posted by foolishsailor

Nothing like being home and sick for a week, gives one plenty of time to cruise the forum....

Was gnawing on your info and I am confused by the above.

My understanding is that the engine won't start without a proper ground. So when a boat is running a floating ground the solenoid/relay sits between the hull and engine in the ground wire and is only closed during the starting process effectively creating a normal ground during the starting process and then opens after the engine is started removing the ground and bringing the engine back into a floating potential?

If my whole electrical system is floating, which it is, then using the battery as a ground will not achieve this?

Also curious if anyone out there has made the conversion and can comment on any change in efficacy in the alternator between normally grounded systems and floating ground systems.

Yes I’ve converted a few engines to insulated earth, new Yanmar engine installations replacing insulated earth Volvo MD22’s in French built alloy yachts.
I was fortunate in that the temp senders on the volvo are already insulated so the rest was fairly easy apart from the alternator. Here’s what needs to be done.
1. The main earth from the battery goes directly to a Cole Hersee or Brillman solenoid and the other cable off the opposite terminal goes to the block, preferably a starter motor mounting bolt.
2. A 20 amp Bosch relay goes into the start circuit to operate the start solenoid AND the earth solenoid
3. The oil pressure sender and alarm switch are removed and mounted on the engine room wall, connected by plastic high pressure hose.
4. The temp senders can be bought from VDO as insulated ground. Measure the resistance of your old temp gauge sender to order new.
5. I had to physically insulate the alternator by using plastic shim material for washers and plastic pipe for the bolt shanks.
6. A negative bus bar ( insulated) is installed either on the engine or on the bearer near the starter and earth wires are individually run to each insulated component bearing in mind that while the senders are low power draw ,the alternator needs an earth wire at least as big as its positive cable. None of this is any use at all if other stuff is grounded to the hull.... radios and bilge systems are 2 offenders.
Ovni and some Meta builds have an alarm/indicator panel that monitors the insulated status of both positive and negative for both the engine and the entire hull/windlass etc.
hope this helps.
Pete.

[noelex 77]

Pete has outlined how this is done on most aluminium (and some steel) boats, although many engines have insulated oil pressure senders, alarms and alternators available which greatly simplifies the process.

With most marine engines it is a easy conversion.