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Giulietta 01-07-2008 01:43 PM

WARNING..shore power ON, engine ON is a NO-NO

Just got back from some sailing, as we landed a few friends were discussing in a friend's boat.

The boat almost caught is the deal.

He had several problems with his Alternator, and had to change them twice before.

This time it almost set fire to the is why..

He has his boat with the shore power allways connected, every single day.

3 times a week, a care taker starts his engine (and mine) and lets it run in neutral at higher everyone here does.

Turns out that his alternators are being damaged because the shore power is connected while the engine's alternator is also charging the batteries...

I always have disconnected (for more than 20 years) the shore power when we start the engine, but here its not a comon practice, annd most run the engine whilst still on shore power.

Normally the alternator's are protected with a bridge or something, but...never knwing..

So be warned, if you run your engine, make sure you disconnect the shore power, just in case.

sailingdog 01-07-2008 01:50 PM

Hmm.... interesting...

I'd be curious to see how he had things wired on his boat. Of course, if you don't have an AC-based charger, this isn't going to be an issue.

TwentySeven 01-07-2008 01:54 PM

That's a coincidence. Last weekend I disconnected the shore power to start the engine for the first time ever (I suddenly recalled having heard that is was better, without knowing why).

Knowing why and the possible consequences will definately help to remember to do it in the future. So thanks a lot for the warning.

NOLAsailing 01-07-2008 01:57 PM

As an aside, why are you running your motor without load that frequently? Small diesels don't really respond well to that over time.

Giulietta 01-07-2008 04:11 PM


Originally Posted by NOLAsailing (Post 246773)
As an aside, why are you running your motor without load that frequently? Small diesels don't really respond well to that over time.

Over here is comon practice to start the engine every week for 10 to 15 minutes, leave it idle for 2 minutes, then rev it up to 1500 or more, in neutral for a few minutes.

We all do it, and we never had any problems. Mind you most boats get used a lot so this is not needed. Only for boats that are long time stopped.

We know about cylinder glassing, but its a minimal price to pay for not runing the engines. Our diesel is also very good and has algae deterrents from the refinery, and is sold as marine diesel.

Idiens 01-07-2008 04:17 PM

I guess there must be an odd way to wire a boat so that the shore powered battery charger fights the alternator charger.

Not a problem on my boat though, DC is DC and diodes normally are diodes. Sounds like he had a diode failure somewhere.

teshannon 01-07-2008 04:19 PM

I always disconnect first but I've been doing it just because starting the engine is usually the last thing I do. Did not realize it could be a problem if you don't. Is that a normal problem or is something wired wrong on that boat?

JohnRPollard 01-07-2008 05:11 PM

I have started our boat many times while still attached to shore power without any incidents. But I have always shut down the shore power/charger almost immediately thereafter. So I can't say whether I would have a problem or not if I left it connected during prolonged periods of engine operation. Edit: We are running a Xantrex TrueCharge20 charger.

It would be interesting to know more about the wiring details.

swadiver 01-07-2008 05:38 PM

You are the man. I often start my engine if I will not be sailing for more than a week. I try to remember to disconnect the shore power but sometimes I forget. I have not experienced a problem, but this make sense to me. I will be sure to incorporate shore power disconnect prior to engine start in the furture. Thanks so much for your post.

artbyjody 01-07-2008 06:03 PM

Shore power
Older chargers that do not have blocking diodes (such as some of the cheap automobile variety) - yes, probably not a good idea to have shore power attached. However, if you have a newer system in place - you will never have a problem.

Here is why:

When the internal engine is running and the alternator is running correctly - the alternator has Voltage sensor just as the AC chargers do. There will be an initial switch on and switch off between the units but the Alternator will usually win the battle for charging the missing cranking amps, as the AC Charger is more sensitive and cuts off more quickly when sensing the voltage is higher than the float / max charge level (which the alternator puts on the line). The alternator will be a bit slower as it is sensing the pull it using to crank the engine but the AC charger will immediately sense it and shut off for protection. However, if the battery is severely drained - it is possible for both to charge at the same time - but the AC charger will go into trickle mode.

However, older systems - older Charge Units - are not necessarily as sensitive / smart with the charging but they too usually should have no problem being on when the engine is running as they are all designed to shut off when max Voltage is detected.

Causes of fires etc, are usually improper grounding or not using the appropriate gauge of wiring on either of the systems causing overheating and eventually fire. Or placement of fuel tanks too close to the engine without the space being properly ventilated before being engine is started. Most accelerated corrosions or quickly dying alternators are usually caused by fuel vapor or oil coming in contact with with the windings in the Alternator. These deposits in the end cause the alternator to arc and spark internally (and if not fire - will kill the alternator rather quickly).

Considering that both RV and Marine utilize similar techniques (actually the same - except the chargers for boats are 'marinized', for example the same Xantrax charger I have on my boat is the same as the on on my Airstream) . I would of blown my truck's alternators a long time ago by having AC power hook-up to the RV at the same time the truck is running and connected to the RV (as the RV's battery banks can be charged from the trucks trailer connection).

While it probably is best to disconnect - as it puts one in the habit of not pulling out without dragging the shore power cable from the dock... it is very unlikely with the proper system in place - that electrical issues would result with having shore power and the engine running less a failed component, poor ventilation if fuel vapor is present in the area of the engine (alternator), or improper wiring.

Just my 2 pesos...

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