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Old 04-02-2015, 06:40 PM   #1
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AC power requirements

Hello all,
I'm a sailor, considering buying a 1973 Defever 40 for weekend cruising. The boat has a non-working 10kw genset (I believe Onan or Kohler). I'm curious to know what could possibly have required that much power on the boat in the past? The generator is almost as big as the Ford Lehman! It's a west coast boat, so no a/c. Fridge runs on propane but I suppose there might have been an AC fridge in there at one time.

More generally, do casual/weekend cruisers even need gensets any more? Are they simply left over from pre-inverter days? Am I missing something important that runs on AC? In my camper, I run the microwave from a single group-24 lead acid battery--even that works for 3 or 4 days. This boat has four group-31 batteries. I have a hard time imagining that you'd need that much power on the hook, either DC or AC.

Thanks.
Bill
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Old 04-02-2015, 06:58 PM   #2
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If you don't need it, don't worry about it. Many boats have ac / heat units for extending the season. Also at anchor you may want to recharge the batteries, run the water heater, and then there is the blender.

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Old 04-02-2015, 07:11 PM   #3
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Refrigeration is usually a boat`s greatest consumer of electrical power. Propane relieves your batteries of that demand. Without that, or a large solar array and more battery, you`d likely need a working genset. For most boaters a genset is far from being an anachronism.
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Old 04-02-2015, 09:27 PM   #4
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42 year old boat, could have been any number of things in there at one point or even today. Reverse cycle heat (and A/C in southern climes... who knows where the boat's been?) electric range and stove, as noted perhaps an AC fridge, washer/dryer, maybe some lighting and so on. Advise you accompany your surveyor and do a thorough inventory of what all electrical demands are; find out what might be involved in getting the genny working again, and if not, what size you really need for the way the boat is configured now.
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Old 04-03-2015, 07:04 AM   #5
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Generator sizing is always interesting. Typically with a 40 ft boat you find 6, 8, 10, or 12 KW generators. The larger sizes are selected to be able to run everything at one time including AC, electric stove and 110v refrigeration.

There are differences of opinion as to how to select the proper size, but a 10 kw is not unusual.
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Old 04-03-2015, 07:04 AM   #6
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Generator sizing is always interesting. Typically with a 40 ft boat you find 6, 8, 10, or 12 KW generators. The larger sizes are selected to be able to run everything at one time including AC, electric stove and 110v refrigeration.

There are differences of opinion as to how to select the proper size, but a 10 kw is not unusual.
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Old 04-03-2015, 07:10 AM   #7
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Gensets are OPTIONS on new boats , so hugely oversized is common as the percentage of profit is greater with an outsized more costly unit.

Sadly with diesels the oversized unit is costly as it eats more fuel with minor loads that rightsized , and suffers mechanically from lack of loading , shortening its service life.

Bit outsized is really common.
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Old 04-03-2015, 11:44 AM   #8
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Genset was standard on my boat. It came with a 9KW Cummins/Onan. It was sized to drive 3 air conditioners, stove, and water heater at the same time. The next size down is a 7KW version. Would have probably worked except in the rare instance that everything is on at once. Fuel consumption between the two models at 50% load is .4gph vs .5gph. I don't see any real advantage to go with the smaller version.
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Old 04-03-2015, 01:04 PM   #9
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At the same kw output modern gennies in the same manufacturer's series consume the same fuel regardless of their total capacity. IMO the unde-rloading issue is way overplayed. No load at all is the real enemy and modern engines are even more tolerant of that.
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Old 04-03-2015, 03:58 PM   #10
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Barnecleb

Your 40 foot Defever is a lot like my 40 foot Marine Trader.

The way I see it is: A generator was provided as a alternate source of power to be used when shore power is not available. It follows that the generator then should be equal in size to the shore power. If your Defever was provided with 2- 30 amp shore cords then the generator to be equal would be 7.5Kw. If the shore power were two 50 amp cords or a split phase 50/250 then the generator size would be 12.5 Kw
This makes the boats systems all operate off the shore power or ship power just the same. Now you can re-visit all this ancient logic and decide you want to operate differently and make up many reasons for a smaller generator. However it is not easy to make up reasons for a generator that is larger than the shore power capability.
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Old 04-03-2015, 04:22 PM   #11
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Quote:
Originally Posted by obthomas View Post
Barnecleb

Your 40 foot Defever is a lot like my 40 foot Marine Trader.

The way I see it is: A generator was provided as a alternate source of power to be used when shore power is not available. It follows that the generator then should be equal in size to the shore power. If your Defever was provided with 2- 30 amp shore cords then the generator to be equal would be 7.5Kw. If the shore power were two 50 amp cords or a split phase 50/250 then the generator size would be 12.5 Kw
This makes the boats systems all operate off the shore power or ship power just the same. Now you can re-visit all this ancient logic and decide you want to operate differently and make up many reasons for a smaller generator. However it is not easy to make up reasons for a generator that is larger than the shore power capability.
This all being said here is what I did: I live aboard and added a washer dryer and a humidifier and other stuff so that my 60 amps wasn't enough and I was quite often tripping breakers. I changed my 60 amps shore to 100 amp shore and left my 8 Kw generator alone. What this says is on my boat I do not have as much generator as shore power. When away from shore I am careful to not overwhelm the generator. So far all is well.
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Old 04-03-2015, 04:42 PM   #12
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Quote:
Originally Posted by obthomas View Post
Barnecleb

Your 40 foot Defever is a lot like my 40 foot Marine Trader.

The way I see it is: A generator was provided as a alternate source of power to be used when shore power is not available. It follows that the generator then should be equal in size to the shore power. If your Defever was provided with 2- 30 amp shore cords then the generator to be equal would be 7.5Kw. If the shore power were two 50 amp cords or a split phase 50/250 then the generator size would be 12.5 Kw
This makes the boats systems all operate off the shore power or ship power just the same. Now you can re-visit all this ancient logic and decide you want to operate differently and make up many reasons for a smaller generator. However it is not easy to make up reasons for a generator that is larger than the shore power capability.
Great post and an excellent explanation of how generators are sized by boat manufacturers. The issue on old boats, as obthomas gives an example of, is what gets added or subtracted to the boat by successive owners.
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Old 04-06-2015, 10:31 AM   #13
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That's the most logical explanation I've heard. When we lost power in Ohio for a week, I ran most of the house on a 2000-watt Honda generator, but I had to be careful about what was running at the same time.

Thanks everyone for the informative comments.

Bill

Quote:
Originally Posted by obthomas View Post
Barnecleb

Your 40 foot Defever is a lot like my 40 foot Marine Trader.

The way I see it is: A generator was provided as a alternate source of power to be used when shore power is not available. It follows that the generator then should be equal in size to the shore power. If your Defever was provided with 2- 30 amp shore cords then the generator to be equal would be 7.5Kw. If the shore power were two 50 amp cords or a split phase 50/250 then the generator size would be 12.5 Kw
This makes the boats systems all operate off the shore power or ship power just the same. Now you can re-visit all this ancient logic and decide you want to operate differently and make up many reasons for a smaller generator. However it is not easy to make up reasons for a generator that is larger than the shore power capability.
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