Does a genset still make sense?

[pwillems]

I have always read that the purpose behind installing a standalone genset rather than just using the main engine to charge batteries is due to the inefficiencies of running a diesel engine with only the light load from the alternator. This makes sense, and I would agree. But, these days, it is quite easy to get hold of some very large alternators and very large inverters, so I am not sure it always makes all that much sense anymore.

Let's say a small catamaran owner has 2 * 20hp and a decent lithium bank that will happily accept high charge rates. A 400amp/12v alternator uses ~16hp (1 hp per 25 amps). At the full 400 amps output, that engine is at over 75% working load. Taking into account heat derating and other inefficiencies, the engine is still getting a pretty good workout for the entire duration of the charge.

Of course, you wouldn't want that alternator putting out its full charge when you actually need that engine to push the boat into some challenging seas, but that is easily managed with a clever regulator, or even a potentiometer if required.

[rslifkin]

It depends on the size of your engines, your power needs, etc. In some cases what you're suggesting can be perfectly viable.

[Joh.Ghurt]

What rslifkin said, it all depends on the boat, where you sail and other factors. I decided to replace the old broken generator because it covers all my energy needs beyond the rather cheap solar installation I could easily fit on my boat. It was cheaper than going with wind, hydro or a really big solar array with the batteries to match.

As to the alternator, my small generator produces 3.5 kW (at 220V) for about 1l/hour. Investing into upgrading the alternator and the pulleys on the main engine wasn't worth it either to me.

In the end a generator made sense in my case. There are also many other good solutions that might fit other people better. You need to think about what works best for you.

[SV Confianza]

I think it depends on how much solar you can reasonably fit. Running the engine all the time under low load, and lower than cruising rpm, will eventually kill it, and most high output alternators will still be low load for the engines. They should be supplemental to another source, like solar.

The Integral solution is about as expensive as a stand-alone generator, but has other advantages like saving space of a dedicated gen. You’re still putting hours on the engine and using up its life span.

There’s really no one size fits all answer.

[mvweebles]

Running hi-output alternators really needs the RPMs to be brought up to cool them down. If you're motoring, should be okay as long as your engine room is adequately ventilated. Running at idle at anchor will not work well.

Much depends on the anticipated load. If you seek A/C, a generator is still the most practical power source.

[Lee Jerry]

Originally Posted by pwillems

Let's say a small catamaran owner has 2 * 20hp and a decent lithium bank that will happily accept high charge rates. A 400amp/12v alternator uses ~16hp (1 hp per 25 amps). At the full 400 amps output, that engine is at over 75% working load. Taking into account heat derating and other inefficiencies, the engine is still getting a pretty good workout for the entire duration of the charge.

Isn't it more like half that power (~7 hp)?

[Pauls]

Yes, it is.

[sailorboy1]

When my inboard generator died I got a Honda (have liked the Honda way more than the inboard). Generators have more uses than just battery charging. But it makes sense to run a small $1200 generator to battery charge than a $20,000 main engine. And a Honda costs less than installing a high output alternator anyway.

[Sabado]

We have a 9kw generator, 2kw solar, about 11kw of lithium and our stock 125a alternators on two engines.

We’ve thought about ditching the generator since 90% of the time, solar takes care of everything including cooking in an electric galley.

There are a few times where we would need to be careful with power. More than 2 very overcast days for example or long passages with the sails shading the solar.

The thing is, we have so much AC charging sources in the boat that with only 2 hours of generator runtime, we go from 35% to 100% with a push of a button and for about 1 gallon of fuel.

The downside (is it a downside?) is that we need to run air conditioning to put an adequate load on the unit. We’re actually thinking about getting a smaller generator. A nice Northern Lights 6kw unit would be sweet.

For us, even with our lithium and solar, the generator is an important part of our system.

[s/v Jedi]

When I had two Lestec 225A alternators for charging the house bank, the load from these was about 7hp so 400A is about 7hp.

What is overlooked by the OP :

- massive bracket, weighing so much with alternator(s) that additional mounts or heavier mounts are needed

- belt system requires additional pulleys, adding to complexity and weight

- wear and tear on propulsion engine just for charging batteries. Replacing a worn out genset is much more cost effective

I took everything out and went back to oem setup.

Now that we made solar our primary charge source, this makes the diesel genset redundant. I may sell it.

[NorthCoastJoe]

I think about stress on other bearings in the belt loop, like the engine water pump. When you pull that much power out of a belt everything else feels it.

I got a Honda generator that weighs less than 50 pounds, puts out 2200 watts, for $1200. Simple easy to take care of and no stress on my diesel.

Our primary power is 660 watts of solar.

I thought about the big alternators but I am concerned about heat in our enclosed engine bay. We have forced ventilation for the engine bay and would have to increase in order not to cook the alternator.

[Greatest Lakes]

Running a 20hp generator is going to be much more efficient than running the 160hp turbo diesel just for the purpose of charging up the batteries, probably a bit faster too. Also, keeping hours and extra wear off the auxiliary is a good idea since a small genset diesel is significantly easier and cheaper to replace.

[sailingharry]

Originally Posted by Lee Jerry

Isn't it more like half that power (~7 hp)?

I wrote a fairly long response with that same conclusion. But before I hit send, I checked online. Several hits on my Google search all confirmed that alternators run some place in the 50 to 70% efficiency range. At 50%, his numbers are exactly right. At 70%, he is still conversationally right.

[sailingharry]

Originally Posted by s/v Jedi

When I had two Lestec 225A alternators for charging the house bank, the load from these was about 7hp so 400A is about 7hp.



What is overlooked by the OP :



- massive bracket, weighing so much with alternator(s) that additional mounts or heavier mounts are needed



- belt system requires additional pulleys, adding to complexity and weight



- wear and tear on propulsion engine just for charging batteries. Replacing a worn out genset is much more cost effective



I took everything out and went back to oem setup.



Now that we made solar our primary charge source, this makes the diesel genset redundant. I may sell it.

I think it's worse than that Jedi. I have 270A mounted on my 50 horse Yanmar, and I keep trying to ignore the negative vibes from the internet about breaking my crankshaft off. I'm not sure that a 20 horse diesel would be at all happy with a 400a alternator!

[Bowdrie]

Originally Posted by sailingharry

I'm not sure that a 20 horse diesel would be at all happy with a 400a alternator!

No problem for 20Hp.
At 14V that 400 amps is ~5.6KW, discounting losses that's about 7.5Hp.
However, for an alternator that large I would want off-engine mounting and drive it with a "Gilmer" type belt.
Even the much-lauded Poly-V belts are still a friction drive and need lots of tension, and that means lots of heat going into the front bearing.