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Old 16-02-2009, 01:49   #1
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Is idling an outboard for long periods bad for it?

I have a new Mercury 50hp 2stroke on my boat, with an alternator output (per the manual) of 18A and have two questions:

1. Is that 18A output "peak", i.e. only at high RPM? What might I expect at idle?

(I'd just check myself on the boat, but it's -7C (19F) outside and the boat is tucked away in a barn for the winter)

2. Is it bad to run an outboard at idle for long periods of time?

(the outboard has just over 20 hours on it so far, so not sure if it is fully broken in and how that might affect the situation)

I'm planning on doing some extended cruising this coming summer where I will be fairly long periods on the hook in one place (e.g. 5-7 days) and am wondering whether running the Mercury outboard a certain amount each day would be sufficient to top up my batteries (250AH total in house bank, 90AH deep cycle starting batter). I know that this is fairly standard practice with folks with diesel engines, but am unsure if a gas outboard would be as tolerant to e.g. idling for 2 hours, or whether the alternator output would be enough for it to be worthwhile. It's also possible to increase the speed of the engine when in neutral, so it would be technically possible to e.g. run the engine at pretty much anywhere in its normal operating range, but again, not sure if that's good to do with the engine not being under load, etc. (noise at higher RPM would of course also be an annoyance, but perhaps tolerable as a single-season solution).

(And yes, I know lots of folks recommend a portable generator such the Honda, and yes, that would probably be a good solution, but no, I don't really have a boat buck to throw at one...)

Thanks.
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Old 16-02-2009, 03:46   #2
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Your 18 amps is peak output- and thats not going to happen anywhere near idil speed- no where near enough to keep up with charging a house bank-of 250amh- its only designed to charge a starting battery - nothing more- you need at least 25% of bank size to be able to do it and thats Min.
IF you discharged your batterys say 100 amps a day you would need to run the outboard at about 6 hours a day- and likly at more than half speed to charge the batterys
buy a solar pannel of 200 watts you can get one for $8-900 about the same price as a honda genset and in the long run cheaper and a whole lot quiter- or buy the Honda- they work well,
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Old 16-02-2009, 04:46   #3
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Some outboard engines have an un-regulated (& even un-rectified AC) “lighting or power” supply, not a rectified & regulated charging output. Such engines should NEVER be used for battery charging, unless the optional regulator/rectifier has been added.

Alternator output is almost always specified “cold” at full (alt’) RPM. See the curves below.
At a “fast” idle, a typical outboard Alternator (see above) might put out up to 10% of it’s rated capacity (in your case about 1.5 - 2.0 A/Hr). As the Alternator heats up over time, this output will drop.
At a “slow” idle, the Alternator will have no appreciable output.

Long-time idling tends to foul plugs, use oil, and overheat most “older” outboard engines. I wouldn’t recommend operating engines for long times at idle. Perhaps newer engines are less susceptible to the problems.

An outboard engine is generally considered fully broken-in at about 10 hours run time. See the “Break-In” chart for your specific engine.

See also:
Alternator Specification Basics

See the sample output curves for alternators at various RPM (below). Note that the RPM we are talking about is alternator RPM, typically 2.5 times engine RPM. Typically alternators have their full output rated at 6000 RPM (cold) but can continue to spin up to 12,000 RPM or more without any additional increase in output.
Note how elevated Alternator temperature reduces power output.
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Old 16-02-2009, 07:06   #4
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unless the optional regulator/rectifier has been added.

Gord,
I have the PDQ with twin Yamaha 9.9's When the batteries are close to charged up I have to turn on 12v loads to keep from overcharging the batteries. If I don't, I easily see 17 - 18 volts. How would I rig an external regulator to protect the batteries and also keep a load on the stator coils to protect them?

Tom

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Old 16-02-2009, 07:58   #5
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Goto page 57 etc. of the Yamaha Outboard Shop manual:

Yamaha Outboard Shop Manual: 2-90 HP ... - Google Book Search

Yamha will have a Rectifier-Regulator kit as pictured in Figure 48 on page 62.
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