Is idling an outboard for long periods bad for it? - SailNet Community

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Old 02-16-2009
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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 02-16-2009
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2 stroke and 4 stroke outboards can work at idle without any problem. The alternator will not produce its rated output (18 A in your case) at idle. If the boat is in a barn now why not charge your batteries with 110V source?

If the boat is in water you can run the engine at idle. If you need higher revolutions you can do so with the gear in forward or reverse.
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Old 02-16-2009
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Quote:
Originally Posted by celenoglu View Post
2 stroke and 4 stroke outboards can work at idle without any problem.
Good. I was worried it might cause uneven wear, or overheating, or such.

Quote:
The alternator will not produce its rated output (18 A in your case) at idle.
That's what I suspected. I wonder if anyone knows how much a drop there would be. I.e. are we talking ~15A at idle or something like ~8A at idle?

Quote:
If the boat is in a barn now why not charge your batteries with 110V source?
I'm only thinking about the future, on the hook. Right night the AGM house battery bank is fully charged and sitting out the winter in the boat. The starting battery is at home in a warm place connected to an intelligent trickle/conditioning charger.
Quote:
If the boat is in water you can run the engine at idle. If you need higher revolutions you can do so with the gear in forward or reverse.
Well, forward would make for some exciting fun when at anchor ;-)

But I guess reverse would work OK, and it certainly couldn't hurt the set of the anchor...

Would it then be bad to run it in neutral at a higher speed than idle? I.e. why have it in gear at all? Does it require a load at higher RPMs then at idle?
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