Aluminum Boat: Switch on Negative of Batteries?

[atoll]

it sounds like a good solution for the negative side of things,how ever the solenoid on the starter motor on the pos side is all ready a battery switch,so effectively you will now have 2 switches/breaks in the pos cabeling to the engine! with the remote switch fitted.

also remember you will also need to replace any temp/oil pressure senders for isolated twin terminal units.

also on most wiring looms the alternator and instruments take their the pos feed from the +battery cable on the starter solenoid,this will need rerouting and able to be turned off when the motor is not running.

Originally Posted by chardis

I'm a bit confused. Actually I wasn't talking about the alternator, but just the way the battery connects to the engine.

So my idea was:

I use the remote battery switch I have now on the positive line, since it is only for the positive. This means I put it in series with the positive line just before it goes into the engine.

I connect the output of the remote battery switch to both the engine AND the engagement for the relay which is on the main negative line, again just before it connects to the engine. This should mean that the relay will engage whenever the remote battery switch is engaged, and it should disengage whenever the remote battery switch is disengaged. So effectively it would be one remote switch that controls both the positive and the negative just before they enter the engine.

So to start the engine, I switch my remote battery switch at the cockpit, turn the key, then switch the remote battery switch off again. Both positive & negative are engaged, then entirely isolated again.

As for the alternators, I rewire these so that they connect directly (both positive & negative) to the battery and don't get any of their power through the engine. Totally separate wiring system. I assume in this case that the alternators are thereby isolated and are not grounding themselves through the engine and causing electrolysis this way, if this is the case I'll need a different solution.

Does that sound right?

[chardis]

Hmmm... sorry I'm really inexperienced when it comes to this type of thing. I'm confused by the last 3 posts.

It's easy to disconnect the battery from the engine... but then of course the alternator still needs to be connected to the battery in order to charge. But the alternator is grounded to the engine you are saying? So this then makes it an impossible situation as far as I ca tell, because as long as the battery is connected to the alternator then there is current flowing through the engine and into the hull and causing electrolysis?

I don't understand... why can't use the remote battery switch on the positive line to engage/disengage a relay on the negative line, thereby isolating the engine from the battery that way?

Then if the alternator is grounded to the engine by design then I don't see what can be done to stop the current flowing through the boat..?

Right now I have a manual switch going from the battery to the engine on the positive line which can be set to 0 (off), 1 (engine battery) or ALL (house bank), and I have 2 remote battery switches which are net yet installed, and I plan on installing the relays on the negative, except I don't understand whether you're saying this is a good idea or not?

Sorry if I'm being stupid, but I'm still learning all of this stuff, and I really appreciate all your help!

[noelex 77]

Originally Posted by chardis

Hmmm... sorry I'm really inexperienced when it comes to this type of thing. I'm confused by the last 3 posts.

It's easy to disconnect the battery from the engine... but then of course the alternator still needs to be connected to the battery in order to charge. But the alternator is grounded to the engine you are saying? So this then makes it an impossible situation as far as I ca tell, because as long as the battery is connected to the alternator then there is current flowing through the engine and into the hull and causing electrolysis?

I don't understand... why can't use the remote battery switch on the positive line to engage/disengage a relay on the negative line, thereby isolating the engine from the battery that way?

Then if the alternator is grounded to the engine by design then I don't see what can be done to stop the current flowing through the boat..?

Right now I have a manual switch going from the battery to the engine on the positive line which can be set to 0 (off), 1 (engine battery) or ALL (house bank), and I have 2 remote battery switches which are net yet installed, and I plan on installing the relays on the negative, except I don't understand whether you're saying this is a good idea or not?

Sorry if I'm being stupid, but I'm still learning all of this stuff, and I really appreciate all your help!

You cannot use a ground alternator. It has to be isolated. Generally the Yanmar engine do have isolated alternators. They just have a wire from the negative terminal on the alternator to the engine ground. Remove this wire and the alternator is isolated.
On some alternators you cannot do this. The alternator case is permantly grounded, so where it bolts to the engine there is a ground conection. If you have this sort of alternator it needs replacing ( there are sometimes conversion kits that will change ground to isolated)

Yes a relay on the negative is needed

Yes you can use a remote battery switch on the positive line to engage/disengage a relay on the negative line.
But the relay only need a small switch carring a couple of amps to activate it so a large battery switch is not needed, just to activate the relay. You cannot disconect the positive to the alternator while the engine is running and there is little point disconecting the + to the starter motor because it's not connected to engine block or hull.

So a positive and negative connection is needed to the alternator when the engine is running. With an isolated alternator this is not connected to the engine ground. The positive lead also goes to the starter motor, but this is never electrically connected to the engine block or the hull. The negative to the starter motor is only connected by the relay during starting.

There is also a need to supply positive and negative to the senders and the engine panel when the engine is running. So the senders, like the alternator need to be isolated. This usually means purchasing new senders for an aluminium boat.

The usual wiring is
+ wiring
start battery - battery switch-start motor- alternator- engine panel and senders.

- wiring
start battery - battery switch-alternator- battery side of the relay-engine panel and senders
From the engine side of the relay starter motor

With this sort of wiring, an isolated alternator and senders there is no electrical contact to the engine block or hull with either + or - wiring other than when starting.

[chardis]

Got it!

Thanks a lot for your help. I'll discuss this all with the electrician tomorrow.

[mitiempo]

The simple solution, advocated by Steve Dashew who knows a thing or two about building aluminum boats, is to just switch the negative off when the engine is not running and don't worry about the hours it is running.

[David M]

I have an aluminum hull and have the batteries ground connected to the engines all the time and have no electrolysis problems. I never ground anything directly to the hull (like you sometimes see in cars). This is called a floating system My guess is that your engine is not the problem provided you have cables in good condition where the ground current, the electrons, prefer to take the path of least resistance....the copper cable and not the metal with slightly more resistance, the aluminum hull.

There are many sources of electrolysis on aluminum boats. You may want to investigate other potential problems.

Another potential problem is your shore power ground which needs to be open, except for when it is needed. This is taken care of with a galvanic isolator. This device is a must for aluminum boats. Otherwise your boats hull could become the sacrificial anode for all the boats in the marina.

http://www.newmarpower.com/pdf/Manual-GI-30&50.pdf

[noelex 77]

Shore power problems are certainly a much more common cause of problems with aluminium boats.
A galvanic isolater is the very minimum standard and an isolating transformer is much better. An alternative is the European system where protection is provided by more sensitive RCD.
If you design and maintain an aluminium boat with the best electrical system you are very unlikely to ever have any problems. Some aluminium boats suffer no problems even with poor electrical design, but they expose themselves to risk particularly for example if faults develop in neighbouring boats, or the barrer coating under the antifouling becomes damaged.

[Florida Mike]

What you are trying to do is simple from my perspective.

Bring all batteries and electrical circuits to one isolated ground point.
Bring your engines ground to one point.
Get a relay similar to but much heavier duty; Amazon.com: Stinger 80 Amp Battery Relay Isolator: Car Electronics
This really has an isolated windings and will not care if it is in the negative or positive lead. I would recommend going to a heavy-duty truck parts supplier.

It gets a little bit more complicated if you want to start your engines with just one pushbutton per engine. The reason I say this is that I’m sure you want the negative connection made before the positive connection. Just my two cents, Mike.

[Stu Jackson]

The detailed answers you have been given are remarkable, given the hunt and peck typist.

If you talk to your electrician, it could well be the blind leading the blind.

There have been detailed books written about electrical systems in non-GRP (fiberglass) hulls.

With your admittedly limited electrical knowledge, it might be helpful for you to obtain one of these books and study it.

Good luck.

[jrambo316]

Originally Posted by Stu Jackson

The detailed answers you have been given are remarkable, given the hunt and peck typist.

If you talk to your electrician, it could well be the blind leading the blind.

There have been detailed books written about electrical systems in non-GRP (fiberglass) hulls.

With your admittedly limited electrical knowledge, it might be helpful for you to obtain one of these books and study it.

Good luck.

Recommended book titles?

[RaymondR]

If your engine mounts use elastomers to vibration isolate and are therefor insulators and you have flexible couplings between the gearbox outputs and the prop shaft it might prove easier to just isolate the entire DC systems from the hull. On my metal boat this only required insulating the Morse controls from the metal hull with nylon sleaves and washers on the mounting bolts.

[espresso]

Actually trying to isolate the negative of the battery system is futile. It only takes the smallest bit of grime, dried salt or carbon to connect the negative to the hull. Additionally, the negative connected to the hull isn't really the problem. You need the positive to cause corrosion.
If you had to take a voltmeter and measure the resistance between the hull and the negative pole on an "isolated" system, you would be surprised how little resistance there is in practice.
My information is that on the aluminium mega yachts, they don't bother to isolate at all.
What they do is have a huge strap going from the negative terminal onto the hull at a non structural point. They then monitor any current flow on this strap which would indicate corrosion going on somewhere.

[Triumphant]

I have a steel boat with a Perkins 4236M fitted. The boat, including the engine, is floating earth (by this I mean the negatives of Alternator and Starter and everything else on the boat only ever meet back at the battery negative terminal post or bus bar)

So another solution could be to fit a starter motor that is wired for floating earth which is the Perkins solution avoiding relays and the like

I also have a simple test circuit so I can see if there is any current running though the hull which will identify whether is it is a positive or negative fault. This is how I found out that the ICOM 802 SSB grounds to the hull and the Wirie WiFi needs to be insulated from the metal work

I also found this old thread as well
http://www.cruisersforum.com/forums/...tion-2043.html

Good luck

[noelex 77]

Originally Posted by espresso

Actually trying to isolate the negative of the battery system is futile. It only takes the smallest bit of grime, dried salt or carbon to connect the negative to the hull.

Electrically isolating the engine effectively and permanently from the hull is difficult to do, as you note. However to isolate the engine from the negative supply is very easy. A solonoid is simply used to disconnect the negative after the engine is started. Just a battery switch could be used if you want to keep things ultra simple. Most small diesels will happily keep running without any electricity connection. Senders, such as the temperature gauge need to be "2 wire" (ie not getting their negative supply from the engine block), but these are simple screw in replacements. The alternator needs to be an isolated design, but many marine alternators come standard with this configuration.

With this arrangement the engine, propshaft etc do not need to be isolated from the hull, as they are not connected to the DC system (except for a few seconds when starting the engine) so there is no concern about salt grime etc breaching connections like the engine mounts.

Originally Posted by espresso

What they do is have a huge strap going from the negative terminal onto the hull at a non structural point.

An alternative to an isolated ground system is to ground all negative wires to one point in the hull as you describe. This approach is easier and used by some boat builders. It is vital that there is only one negative connection for everything. Double pole circuit breakers are still best with this system, but this step is sometimes skipped to reduce costs.

One difficulty with this system is to determine if a fault has occurred and another negative path has developed.

If the grounding point is not at the engine, the engine still needs to be isolated to avoid violating the rule of a single hull negative attachment point.

It is important with an aluminium boat that the electrically supply is wired correctly. Unfortunately, even people experienced with marine wiring can make mistakes if they follow standard wiring practices that would be perfectly acceptable for a fibreglass boat.