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Valmika · 20-02-2019, 17:48

I have a theoretical question about the load a 120 amp (or around there) would put on a 40 something HP diesel engine.

What we are wondering is if you have your motor running and a 2500 watt inverter on and you are running a 1500 Watt heating element in you water heater. Would the drag from the alternator load the engine enough so you could safely idle for 30 to 45 minutes?

pcmm · 20-02-2019, 17:54

That's not a very efficient way to heat water. you'd be better to use a water exchange coil to circulate hot coolant in the water heater. Leave the alternator for charging your batteries

Valmika · 20-02-2019, 18:05

Quote:

Originally Posted by pcmm

That's not a very efficient way to heat water. you'd be better to use a water exchange coil to circulate hot coolant in the water heater. Leave the alternator for charging your batteries

Actually that is what I am talking about, using both. Sorry if I was unclear. The engine doesn't get hot quickly if you are just in idle but would the load of the alternator be enough to maybe get the engine hot faster and keep you from getting in trouble just idling with no load to heat water

pcmm · 20-02-2019, 18:13

Quote:

Originally Posted by Valmika

Actually that is what I am talking about, using both. Sorry if I was unclear. The engine doesn't get hot quickly if you are just in idle but would the load of the alternator be enough to maybe get the engine hot faster and keep you from getting in trouble just idling with no load to heat water

If you could load the engine with an engine driven water maker and/or fridge compressor that would help but running an engine just for the alternator and water heating is going to be inefficient. take a while to get the engine hot when its just idling , you're right.

Stu Jackson · 20-02-2019, 20:54

Quote:

Originally Posted by Valmika

Actually that is what I am talking about, using both. Sorry if I was unclear. The engine doesn't get hot quickly if you are just in idle but would the load of the alternator be enough to maybe get the engine hot faster and keep you from getting in trouble just idling with no load to heat water

1. Just in idle will do squat for charging. Period. You need to run at at least 1500 rpm to get the alternator to actually produce anything worthwhile.

2. Depends on the SC of the house bank, too. Low it'll draw more amps from the alternator. Basic Charging 101, alternator or any other source.

I do this regularly: house bank down to 12.1V after a night on the hook; run engine at 1500-1800 rpm, get a charge and after an hour I get tepid water, without using the inverter.

One of these days I'll remember to use the inverter, but so far after 20 years, this works for me.

**Dockhead** · 21-02-2019, 10:10

Quote:

Originally Posted by Stu Jackson

1. Just in idle will do squat for charging. Period. You need to run at at least 1500 rpm to get the alternator to actually produce anything worthwhile.. . .

That depends entirely on the alternator, and on the gearing.

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School bus alternators have steep output curves, as they are designed to put out power at idle.

Mine, for example, which is geared at 2:3.5, puts out more than 50% of its rated output when the engine is running at 1000 rpm. So at least 1.3kW.

I think the OP's idea was to get hot water from engine cooling loop and a calorifier, and turn on the electric hot water heater just to load the engine for health. I don't think it's a dumb idea at all, but depending on his alternator (and how it's geared), it might not be doing much to help his engine.

But I agree with what others have said -- idling the engine for 20 minutes or so to make a tank of hot water is not going to hurt it, if it's run under load afterwards. Or he could just put it in gear.

**Dockhead** · 21-02-2019, 10:13

Quote:

Originally Posted by Valmika

Actually that is what I am talking about, using both. Sorry if I was unclear. The engine doesn't get hot quickly if you are just in idle but would the load of the alternator be enough to maybe get the engine hot faster and keep you from getting in trouble just idling with no load to heat water

A lot of people put the boat in gear for this purpose. At anchor, just put it in reverse and at idle, with the helm centered. And someone on watch!!

hzcruiser · 21-02-2019, 16:41

Quote:

Originally Posted by Valmika

Actually that is what I am talking about, using both. Sorry if I was unclear. The engine doesn't get hot quickly if you are just in idle but would the load of the alternator be enough to maybe get the engine hot faster and keep you from getting in trouble just idling with no load to heat water

Also keep in mind that, in general, an idling engine is not revving the alternator fast enough to put out max power. Unless you have a smart alternator and replaced the alt pulley with a smaller one. That's what some people do to get the most out of their diesel engine.

BTW, idling for an hour every now and then won't damage the engine (as in glazing the cylinder walls), as mentioned by someone else.

donradcliffe · 20-02-2019, 18:02

It wouldn't be a problem for the engine, but it would take about 50 amp-hours out of your batteries. The heater/inverter will be pulling about 140-150 amps with losses, and the 120 amp rated alternator will only put out about 90 amps if you are lucky.

seabreez · 22-02-2019, 05:12

Quote:

Originally Posted by donradcliffe

It wouldn't be a problem for the engine, but it would take about 50 amp-hours out of your batteries. The heater/inverter will be pulling about 140-150 amps with losses, and the 120 amp rated alternator will only put out about 90 amps if you are lucky.

I second the motion here... to answer your question... it is safe to run the engine 30 minutes, your batteries will not discharge to a point of not starting...

Scott Berg · 20-02-2019, 18:05

Let's see, it's a 120 amp alternator at 12 volts; that's 1440 watts or about 2hp; not much of a load... that said it's not a huge problem so long as you get a heavy load before and afterwards. That said, you are probably spending $10/kw hour this way. I'd find another way...

Valmika · 20-02-2019, 18:29

Quote:

Originally Posted by Scott Berg

Let's see, it's a 120 amp alternator at 12 volts; that's 1440 watts or about 2hp; not much of a load... that said it's not a huge problem so long as you get a heavy load before and afterwards. That said, you are probably spending $10/kw hour this way. I'd find another way...

Thanks Scott, that 2 hp is the number I was trying to come up with. I don't know that formula. I am just trying to figure out a way to get 3 showers and some dishes out of a 6 gallon heater in the cold Channel Islands. I have put the engine in reverse on occasion to put more of a load on it. (Only if the anchor is really set) I figure 45 minutes max on something like this. about 1/3 gallon of fuel.

Paul Kelly · 21-02-2019, 09:23

Quote:

Originally Posted by Valmika

Thanks Scott, that 2 hp is the number I was trying to come up with. I don't know that formula. I am just trying to figure out a way to get 3 showers and some dishes out of a 6 gallon heater in the cold Channel Islands. I have put the engine in reverse on occasion to put more of a load on it. (Only if the anchor is really set) I figure 45 minutes max on something like this. about 1/3 gallon of fuel.

Could you try having shorter showers?