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Old 11-05-2012, 06:39   #1
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Aluminum Boat: Switch on Negative of Batteries?

Hi all,

I have an aluminum sailing boat, and I've just had some very expensive repairs done due to major electrolysis damage.

The electrician says that the problem is because both the engine batteries and the house batteries are connected directly to the engines at all times through the negative line, which is using the boat as a ground and this has caused the damage. The positive line is disconnected when the engines are not running (manual switch, for which I can choose to be off, start from engine bank, or start from house bank), but the negative stays always connected currently.

So I bought two rather expensive remote magnetic battery switches so that I can isolate the negative before it connects to the engines (two switches for two engines) and have the remote switch installed in the cockpit so that I can isolate the batteries when anchored or sailing, and then connect the batteries again very easily from the cockpit when I want to use the engines. It's important that I don't have to go down to the engine compartments every time I need to turn the engines on in case of an emergency situation.

So my question is: is it a good idea to install these remote battery switches on the negative line so that the negative can be disconnected (and there is no ground)? This means that while anchored the batteries will be entirely isolated (negative & positive) from the engines (and everything else) but while under sail the positive will be connected to the engine but the negative disconnected, then I remote switch the negative on when I want to use the engines. I heard somewhere that it's a bad idea to have the positive connected but the negative disconnected..?

It would be ideal if I could disconnect the positive and the negative always at the same time, but that's impractical because the positive can be switched between engine batteries or house batteries and also it would require 2 more expensive remote batteries switches which I don't have the money to order or the time to wait for them to ship half-way across the world.
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Old 11-05-2012, 08:18   #2
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Re: Aluminum Boat: Switch on Negative of Batteries?

Sorry to hear about your problems
An aluminium boat needs to wired so that the engine is disconnected from ground not only at anchor and sailing, but when the engine is running as well.
This is not difficult o do, you need 2 wire senders an isolated alternator ( some standard alternators are isolated anyway) and a relay ( your remote battery switches will work instead of a relay).
The negative is only connected very briefly while the starter motor is turning. The negative is then disconnected from the engine block and remains disconnected even with the engine running.
Make sure the electrician has checked the shore power, this is a more common cause of problems.
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Old 11-05-2012, 08:29   #3
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Re: Aluminum Boat: Switch on Negative of Batteries?

Thanks noelex for your advice.

I'm not entirely clear. So you are saying that the negative from the engines should be always disconnected, except when I want to start the engines? In which case, I should install the remote battery switches on the negative of each engine (the line coming from the batteries just before it enters the engine) and then to use the engines I flick this on, start the engine, then immediately flick this off? Would it be a problem that the positive is still going to the engine? The engine is also bolted directly onto the aluminum hull, so would the current not flow through the boat in this case, causing even more damage?

What do you mean about using a relay, how would that work? The starter does already have a relay.

As for the alternators... the engines are Yanmar and have the standard Yanmar alternators, plus the port engine has a second 85amp alternator. I know the alternators aren't grounded to anything, and I'm pretty sure they just connect to both engine & house batteries via an automatic voltage switch. What exactly is an isolated alternator?

Thanks!
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Old 11-05-2012, 08:56   #4
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Re: Aluminum Boat: Switch on Negative of Batteries?

Quote:
Originally Posted by chardis View Post
Thanks noelex for your advice.

I'm not entirely clear. So you are saying that the negative from the engines should be always disconnected, except when I want to start the engines? In which case, I should install the remote battery switches on the negative of each engine (the line coming from the batteries just before it enters the engine !
Yes and yes

Quote:
Originally Posted by chardis View Post
Would it be a problem that the positive is still going to the engine? The engine is also bolted directly onto the aluminum hull, so would the current not flow through the boat in this case, causing even more damage?

!
The positive and negative wires are still connected to the alternator, but the alternator is not electrically connected to engine block.(this is an is isolated alternator) The senders which normally have the negative power provided by the engine block need replacing with 2 wire senders that have both positive and negative wires leading o them.
The idea is that the engine block, or anything connected to the hull is not connected to the negative except for the few seconds starting.



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What do you mean about using a relay, how would that work? The starter does already have a relay.

!
The relay is just used to connect the negative for the few seconds of engine start. It is separate to the relay on the starter motor. Your remote battery switches will work just as well. A big relay is normally used because it's cheaper.



Quote:
Originally Posted by chardis View Post

As for the alternators... the engines are Yanmar and have the standard Yanmar alternators, plus the port engine has a second 85amp alternator. I know the alternators aren't grounded to anything, and I'm pretty sure they just connect to both engine & house batteries via an automatic voltage switch. What exactly is an isolated alternator?

Thanks!
The Yanmar alternators are usually isolated, so you are in luck, they just need the ground wire disconnected from the engine block and connected to the battery negative.

I hate to see another aluminium boat owner with problems so if you want to Skype me send me a PM and I will give you my Skype address. I am only a two finger typist which makes explaining this stuff slow.
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Old 11-05-2012, 09:01   #5
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Re: Aluminum Boat: Switch on Negative of Batteries?

Thanks a lot. And I'm sorry for deliberately spelling aluminium wrong, it's just that I expected most people here to be from the US.

Your advise is excellent. I understand now and I'll discuss with the electrician who's coming again tomorrow. Now I have to phone Blue Sea and ask if their remote battery switches actually work on the negative line, since the installation instructions only mention positive.
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Old 11-05-2012, 09:14   #6
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Re: Aluminum Boat: Switch on Negative of Batteries?

OK, so... Blue Sea says that the remote battery switches are only for the positive. Pity that this was not mentioned anywhere before purchase. So I have $300 of battery switches that will not do what I bought them for. I suppose I can use these to entirely isolate the positive, since I have them. Unnecessary, since there is already a switch on the positive, but it will save me a few seconds every day at least due to the remote switch. Perhaps in 100 years I'd have made my money back in time.

Meanwhile... how does this relay thing work? I expect that I can pick those up locally. An isolating relay, right? Where and how should it be installed? And this is automatic (not manual switching)?
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Old 11-05-2012, 09:29   #7
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Re: Aluminum Boat: Switch on Negative of Batteries?

Can I use something like this:
Amazon.com: Stinger 80 Amp Battery Relay Isolator: Car Electronics

I just install it in series along the negative line? Do I need more than 80amps?

Thank you!
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Old 11-05-2012, 09:42   #8
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Re: Aluminum Boat: Switch on Negative of Batteries?

The relay is simply a heavy duty switch that connects the negative wires
The relay needs to be rated at about 100a no name large *automotive or boat anchor winch relays *are cheap and can used. *The higher the rating the better and silver contacts are a bonus.
this would be ideal. An isolated case ( separate negative wire) so the relay has 4 connections ( 2 heavy duty to connect the negative wires ) and two thin connections that supply + 12v and ground to close the relay.



relays continuous duty 12 volt & 24 volt DC power relays,starter relays
Sbj-4201


The relay just connects the two negative wires ( from the battery and engine) together when starting.
Many boats wire this so when the start position of the ignition switch is engaged it automatically closes the relay. I prefer a separate switch. So to start the engine I throw a switch which closes the relay and connects the negative, then I start the engine normally. Once the engine is started the relay switch is released and the negative is disconnected from the engine and hull.
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Old 11-05-2012, 09:53   #9
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Re: Aluminum Boat: Switch on Negative of Batteries?

Quote:
Originally Posted by chardis View Post
Can I use something like this:
Amazon.com: Stinger 80 Amp Battery Relay Isolator: Car Electronics

I just install it in series along the negative line? Do I need more than 80amps?

Thank you!
That relay would probably be fine, on an engine 30 HP or less, but larger current rating is a better, and would give a longer life.
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Old 11-05-2012, 11:32   #10
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I have an aluminium boat and had the same questions. There is some good info on this thread.

Converting Diesel to Floating Ground

Edit: this is a really normative thread as well

Alternator and Starter Isolation
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Old 11-05-2012, 18:12   #11
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Re: Aluminum Boat: Switch on Negative of Batteries?

OK. So my plan for the battery is like this:

Install the remote battery switch on the positive, have this then feed into the engine and the control for the relay which is on the negative line. Then when the positive & negative will be tied together, when I flick the switch both turn on, and when I flick it off both turn off.
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Old 12-05-2012, 00:10   #12
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Re: Aluminum Boat: Switch on Negative of Batteries?

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Originally Posted by chardis View Post
OK. So my plan for the battery is like this:

Install the remote battery switch on the positive, have this then feed into the engine and the control for the relay which is on the negative line. Then when the positive & negative will be tied together, when I flick the switch both turn on, and when I flick it off both turn off.
No you want the positive to the alternator to stay on when the engine is running. You also want the negative to the alternator to stay on when the engine is running.
The negative to the engine block ( and therefore hull) is only connected by the relay for the few seconds when starting the engine.

It is usually wired like this.

The engine battery switch is turned on. Unlike on a non metal boat this battery switch connects and disconnects both the positive and negative, so it acts like two switches. The negative is connected to the alternator, but not the engine block.
The engine will run, but not start like this, because the starter motor gets its negative from the engine block ( you can as an alternative get an isolated starter, but these are difficult and expensive to get for most engines).
To start the engine turn the ignition on giving power to the gauges. Then activate the relay to connect the negative to the engine block ( i use a small momentry switch next to the start button) hit the start button, when the engine starts release the relay switch and the negative is no longer connected to the motor block.
( A common alternative is to automatically connect the relay when the start button is pressed)
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Old 12-05-2012, 03:12   #13
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Re: Aluminum Boat: Switch on Negative of Batteries?

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?
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Old 12-05-2012, 03:35   #14
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Re: Aluminum Boat: Switch on Negative of Batteries?

You will need extra battery switches on both the positive and negative wires to alternator. These are needed because a means of disconecting large wires is needed for safety and also because on an aluminium boat circuits not being used should be disconnected.
This will mean you will end up with a lot of battery switches, particularly if you are using separate switches for positive and negative.
You will also need to install 2 more large thick battery cables, from the start battery to the engine bay via battery switches somewhere.
Finally you will need to supply positive and negative power to the engine panel for gauges etc from the "alternator" wires.
It seems like a complicated solution, for no real gain, but i think it would work.
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Old 12-05-2012, 03:49   #15
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Re: Aluminum Boat: Switch on Negative of Batteries?

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Originally Posted by chardis View Post



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.
A non isolated alternator grounds through the case of the alternator, so will not work with any isolated ground system.
An isolated alternator has a separate negative stud.
In an isosolted system this stud normally connects to battery ( via a battery switch) and the isolating relay. The other end of the isolating relay is connected to the engine block itself, so the negative wire only connects to the block when the relay is activated during starting.
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