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Old 23-02-2010, 13:55   #1
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Alternator + Inverter vs Generator

I'm going to have a power system of some sort on my boat. My wife demands it, so it shall be.

A 5000W gen set for a boat costs about $6,000-$7000, and weighs 250-300 lb.

A 5000W 24V to 110V power inverter costs around $1200. A pair of 150 amp 24V alternators costs aprox $1000. Batters costs about $100 for a 100 amp-hr 12V unit, and weighs 50 lb. Weight of the whole system? I havn't a clue, but I would figure the alt and the inverter to be less than 300 lb together. The weight of the batteris would depend on how many you use.

Thus for the cost of a 500W gen set a person could buy two 150 amp 24V alternators, a 24V sinewave inverter, and a huge number of batteries for the cost of a new gen set.

Yet I never see this. Why is that?

When the power inverter is putting out juice, and both alts are going, and attempting to charge the battery bank, they should suck up 11 to 15 hp. Is that an insuffent load on a diesel marine engine to keep it fuctioning correctly?

Only other thing I can think of is my internet references costs are a bit off.
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Old 23-02-2010, 15:10   #2
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It's hard to make comparisons with just the data you've given us.

What type and size boat? What engine? What alternator at present? External regulator? What type of boating/cruising? What areas? What do you have now?

What demands will you and your wife expect to make on an electrical system? What estimated power usage will you have at sea? At anchor? Have you done a rudimentary energy budget yet?

All these factors, and more, will go into any reasoned decision about power generation.

That said, it's usually a bad idea to think of using the engine for routine charging. Whether or not it's good for the engine to run at light loads, the efficiency of creating electricity with a main engine is very, very low. It's better with a properly sized generator, but the cost is still high. Much higher than, e.g., solar or wind generators. And, all of these are much higher of course than shorepower.

Nigel Calder did a pretty detailed analysis of the cost of energy generation, and published it in a magazine a few months back. Sorry, I don't remember which...might have been Professional Boat Builder, or Cruising World, or Offshore Navigator, or any of the others he writes for. Sorry. But I'm sure you can find the reference in a search of this Board or SailNet or SSCA's Board.

Bill
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Old 23-02-2010, 16:06   #3
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I agree with Bill.

Also, you should look at a good inverter/charger that will cost $3,500-$4,500 in the 5kW range.

My personal preference is to use the engine for propulsion only, plus a little charging of the starter battery. I use a Northern Lights 6 kW 60 Hz genset for everything else. If you go far from civilization for long periods like we do, you want a backup too. I have two big additional alternators on the engine that charge the house batteries when I enable them. This allows the inverter to work like you describe but I don't need 5 or 6 kW for that. I just missed a bit of umpfhh with a 2.5kW inverter so I switched to 3kW instead and now it can handle the start-up surge from the watermaker pump.

So, basically, I am writing that if you plan to cruise full-time in exotic places, you are best off with a heavy genset (my NL is 3 cylinders for just 6 kW) AND a good inverter/charger.

If all you need is the A/C on weekend outings, you can even research an engine-mounted AC generator. And there are a lot of options in between.

Check out Northern Lights (gensets) and Victron (inverters/chargers/transformers) brands as these are top quality and leading brands in their segments. Others can only try to equal them.

cheers,
Nick.
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Old 23-02-2010, 17:18   #4
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If you go the alternator way then you will end up with the main engine seriously underloaded which is no good for the engine. And you would have no power back up when the engine stops. If your genset stops then you still have the alternator on the main engine to run the basics.

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Old 23-02-2010, 18:24   #5
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Agreed, a genset and an alternator powered off the main engine are electrical backups for each other. Which one you choose to fire up depends on what you want to do. This gives you more flexibility.
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Old 23-02-2010, 18:47   #6
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I beleive your estimates for the invert system are grossly low, that and you increase complexity and lose reliability with this type of system.
Depending on the type and duration of loads you'll expect, an inverter system, far less complicated than you describe may suffice. But if you are planning on larger or even longer duration loads, such as an A/C unit while anchored, a small generator is by far the more efficent and reliable path to take.
If you need more info, just ask.
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Old 23-02-2010, 21:13   #7
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Traditional diesel engines are designed to run at one speed, at their maximum power point or close to it. Run them near idle or without a significant load, and they coke up (clog with carbon deposits) and you wind up decreasing the life and increasing the overhaul expenses. There's some question about newer engines being more tolerant or not.

But that's the basic issue. Using an engine as a genset is like swatting flies with a shotgun. Not the right tool for the job, so folks who can afford the space and expense use a real genset that is (again) sized to the job they expect of it.
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Old 24-02-2010, 00:19   #8
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I'm going to buck the trend here and not rush to judgement on this question. What do you plan to use A/C power for? If you don't have air conditioning or a washing machine or other heavy power consumers, and you don't intend to spend days at a time on the hook, and you don't need redundancy in the system, your idea might work fine.

Out boat had a generator on board when we bought it; it had only 180 hours of use in 10 years. It was hardly used because the boat was used coastally and was either on shore power at night or with batteries full from motoring into the anchorage.

So the first question is where do you intend to use your boat, and how, and what do you plan to use power for?

If you need A/C power just for a microwave and TV, say, then an inverter should be all you need.

There is, however, a big hole in your cost calculation -- you forgot the charger. A powerful charger (or probably better charger/inverter) is thousands of dollars. If the charger isn't powerful enough (my own problem at the moment) you can't charge a big battery bank from the generator in a reasonable run time.

But since you also forgot to include cost of installation of the generator (also thousands and thousands of dollars), the relative cost benefit of no generator is still correct.

Everyone will warn you not to use your main engine for charging. I have heard this lecture over and over again; it must be true in theory. But in practice I think the dangers may be exaggerated. Our old boat was used for more hours of charging than for propulsion I think, over 12 years, and at probably 8,000 hours still runs like a champ, doesn't smoke or use oil, starts instantly etc. Diesel trucks are idled all night long at truck stops and for years on end.

And again, if you're moving every day, this does not apply -- you get the power for free.

Add some solar and/or wind into the equation, and you may be just fine. I don't think that the majority of people on this board even have generators; most of us make do just fine without them.
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Old 24-02-2010, 01:11   #9
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I agree with dockhead.
First you need to figure out what your going to use the power for...
If its Air conditioning at anchor, then a generator is your only option. Same goes for electric resistance cooktops and heat. Just what kind of generator is a different question. But you can run smaller units off the honda gas generator.
If its microwave, blow drier, hand tools, electric lights, that sort of stuff, then you do not NEED a generator. In truth a large battery bank, efficent chargers, alternate engergy in the form of wind and solar, and larger alternators with good battery management and regulation can do that for you.
So then, depending on your boat size and space available, you can have 600 amp hours of agm batteries for 1500 usd, a 3000 watt victron inverter charger for 2300 usd, say additional 500 to install it, a few solar panels for 1200, a solar charger controller, and a small honda 2000 for the same price as a 6KW genset installed (roughly 10000 USD).
I for one want both, and have the space for both, but not the money or real need yet, so will wait for retirement and the final cruise, then install a North Lights 5Kw genset.
Also agree that using a engine for charging is not going to damage it much, if its a good engine and is used for voyaging as well as charging. Charging does put some load on it, and if you up the RPM's some, it should do fine. Granted, its not idea, but what is.
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Old 24-02-2010, 03:16   #10
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I am equipped the traditional way, i.e. diesel genset and smallish engine alternators but am reconsidering the other paths to electrical heaven.

One idea I am mulling over is switchable 300A Alternators driven by the main engines and a portable petrol generator (e.g. Honda EU3000i or maybe even the 2000? ) for the odd days that you need airconditioning and as a backup charging device.

This seems like a cost, space and weight efficient solution.

So far, the main drawback I see is the need to store 30 gallons of petrol.

Anyone else see other drawbacks?
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Old 24-02-2010, 03:56   #11
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Originally Posted by neelie View Post
I am equipped the traditional way, i.e. diesel genset and smallish engine alternators but am reconsidering the other paths to electrical heaven.

One idea I am mulling over is switchable 300A Alternators driven by the main engines and a portable petrol generator (e.g. Honda EU3000i or maybe even the 2000? ) for the odd days that you need airconditioning and as a backup charging device.

This seems like a cost, space and weight efficient solution.

So far, the main drawback I see is the need to store 30 gallons of petrol.

Anyone else see other drawbacks?
Gasoline storage is a big drawback, but there are huge advantages to the little Honda generators. No installation cost (!), quiet, reliable, dirt cheap. Sounds like a great backup/supplement to an inverter-based AC system.

I think the big question is air conditioning. If you have air conditioning, then you really need a permanently installed, diesel generator. If you don't have air conditioning (washing machine, watermaker), you probably don't really need it, although combined with a high powered charger it's nice to have, for those who are away from civilization a lot and hang out on the hook for days at a time.
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Old 24-02-2010, 04:36   #12
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I also agree with Dockhead about charging with the main engine. We move frequently enough that the engine gets its full throttle time. We don't want a seperate generator, due to cost, weight, complexity, and space. We will install a solar panel on the bimini, and since we have modest AC loads (no air conditioning, etc), we will probably get by with one charging session per day, which also cools the fridge, heats the water, and cools the beer! Miraculous!!
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Old 24-02-2010, 08:34   #13
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Guys, it really depends on how the boat is used. Even when you spend "days" at anchor, you can still do without genset. But we have spent years at anchor with the only dock touched being the fuel-dock once a year for 30 minutes (taking 400 gallons diesel). Solar & wind power go a long way and you don't need A/C all the time, but making all your water at anchor, using the computer for entertainment, loud rock music now and then, and cooking inside with all the hatches closed because it's raining and firing up the A/C for relief are all reasons to get this right.

Engine: all the talk about using the engine for charging or not concentrate on carbon build-up. But just think about the run-hours with the maintenance that comes with that: oil changes, impellers, belts, pump-rebuilds, the little oil leaks that appear with so many hours, the extra diesel consumption from a (too) big engine etc. make a genset the more cost effective solution.

Also, running an A/C in hot climates or a heater in cold climates is not just a comfort item, it's also a maintenance item because you remove huge quantities of moisture from the boat. When you're in the tropics and you run an A/C a couple of hours every week, you will not have mildew or worse inside the boat.
When you are at anchor in an area with some wind (trade winds perhaps) you don't really need an A/C if you use awnings and good ventilation. But when it starts to rain or you move to a dock, things change.

We're out here 7 years now and just replaced the genset and A/C. You will spend about $100 per month spreading the cost of these items over the time used and that includes maintenance (not the fuel).

cheers,
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Old 24-02-2010, 08:57   #14
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Originally Posted by s/v Jedi View Post
Guys, it really depends on how the boat is used. Even when you spend "days" at anchor, you can still do without genset. But we have spent years at anchor with the only dock touched being the fuel-dock once a year for 30 minutes (taking 400 gallons diesel). Solar & wind power go a long way and you don't need A/C all the time, but making all your water at anchor, using the computer for entertainment, loud rock music now and then, and cooking inside with all the hatches closed because it's raining and firing up the A/C for relief are all reasons to get this right.

Engine: all the talk about using the engine for charging or not concentrate on carbon build-up. But just think about the run-hours with the maintenance that comes with that: oil changes, impellers, belts, pump-rebuilds, the little oil leaks that appear with so many hours, the extra diesel consumption from a (too) big engine etc. make a genset the more cost effective solution.

Also, running an A/C in hot climates or a heater in cold climates is not just a comfort item, it's also a maintenance item because you remove huge quantities of moisture from the boat. When you're in the tropics and you run an A/C a couple of hours every week, you will not have mildew or worse inside the boat.
When you are at anchor in an area with some wind (trade winds perhaps) you don't really need an A/C if you use awnings and good ventilation. But when it starts to rain or you move to a dock, things change.

We're out here 7 years now and just replaced the genset and A/C. You will spend about $100 per month spreading the cost of these items over the time used and that includes maintenance (not the fuel).

cheers,
Nick.
Well, I think that's what everybody said. Generators and air conditioning go together like cookies and cream. If you've got A/C, you will want a gennie. If not, then it's optional. Nobody questioned the value of A/C in hot climates, certainly not me, but not everybody needs, or has it.

FWIW, I will definitely install A/C in our boat before we move her down to the sultry Mediterranean next year. Then the generator (a really nice 6.5kW 1500rpm Kohler) will really come in handy. At the moment, on the U.K. south coast, it's not really needed and it's just another complex system with maintenance expenses and headaches. The added expense and complexity of a whole nother system certainly outweighs the efficiency gain of using a smaller engine for non-propulsion uses. But when you have heavy alternating current consumers on board, like A/C, you have no choice.

We don't have A/C (yet), but we do have a washer/dryer, which is a terrific thing to have on board. The generator means we can use it any time we want (except at sea). This is nice, but not worth it, by itself, to have that big beast of heavy, expensive equipment, which costs more than a boat buck a year just in routine maintenance.
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Old 24-02-2010, 11:02   #15
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Dockhead: what I meant is that a genset even makes sense without A/C in some situations, like spending years at anchor without shore-power and water-hose conveniences. The costs of the genset are less than the extra cost on the main engine in that case.

In England you need heating, not A/C ;-)
Also note that US washing machines use very little power and you can run them off an inverter easily. The reason is that they don't heat water... you connect them to a hot water supply.

cheers,
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