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).