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Old 09-24-2014, 09:40 AM   #1
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Power budget

In my ongoing search for a quality Trawler/liveaboard/cruiser; There doesn't seem to be much consistency in generator sizes. I've seen everything from 3.5 KW to over 10KW on the same size vessels. It seems to me that AC and immersion type water heaters are the main power criminals. I've researched fuel burn for various size generators.
At approx. 75% load the fuel burn for:
8KW- .67 gph
7KW- .59 gph
6KW- .50 gph
5KW- .42 gph
4KW- .33 gph
3KW- .25 gph

Any real life experience with genny sizes and fuel burn costs would help me build my budget.
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Old 09-24-2014, 09:59 AM   #2
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Where are you planning on boating, and during what seasons? What sort of conveniences and creature comforts do you want? Gas or electric cooking?
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Old 09-24-2014, 10:00 AM   #3
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How much fuel you burn running your generator is much more dependant on how much power your particular boat, and your particular lifestyle than the generator.

That's because within reason the small generators we find on our boats will generally come in fairly close to each other in terms of fuel used per kilowatt hour produced.

Yes, there will be some exceptions to that general rule. Newer design engines are typically more fuel efficient than older design engines. Running a particular engine at a particular loading produce somewhat better results than the norm.

So, in terms of power budget, we burn around 10 gallons per day on the hook in our boat. We have a 47' power hungry boat and we do not try to conserve.

That fuel burn is not an exact number and nobody probably has one, and it wont apply to your boat anyway. The 10 gallon per day number is an estimate based on differences between flowscan readings and actual fuel taken on. It is also skewed by my actual use of the same size generator in home and industrial settings, so it is an educated guess.
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Old 09-24-2014, 10:11 AM   #4
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All good questions and ones I've thoroughly dwelled upon. I plan on cruising for a few years. I'm from Pa. so cold weather is something I'm familiar and comfortable with. I plan on following the seasons; Great Loop style more or less, leaving the Great Lakes later in the season. I prefer open screened windows to AC at home. I sleep with a window cracked in the winter time. It's the southern US that concerns me as I get grumpy when I'm hot. I'll be cruising alone, with an old dog that also gets grumpy when he's hot. Propane stove. I take fast showers, luke-warm showers suffice. I was an active Marine for 6 years, I still consider a shower a luxury.
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Old 09-24-2014, 10:19 AM   #5
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Originally Posted by ksanders View Post
How much fuel you burn running your generator is much more dependant on how much power your particular boat, and your particular lifestyle than the generator.

That's because within reason the small generators we find on our boats will generally come in fairly close to each other in terms of fuel used per kilowatt hour produced.

Yes, there will be some exceptions to that general rule. Newer design engines are typically more fuel efficient than older design engines. Running a particular engine at a particular loading produce somewhat better results than the norm.

So, in terms of power budget, we burn around 10 gallons per day on the hook in our boat. We have a 47' power hungry boat and we do not try to conserve.

That fuel burn is not an exact number and nobody probably has one, and it wont apply to your boat anyway. The 10 gallon per day number is an estimate based on differences between flowscan readings and actual fuel taken on. It is also skewed by my actual use of the same size generator in home and industrial settings, so it is an educated guess.
That's exactly the number I came up with when I averaged all of my consumption calculations (mostly AC) based on 5-6 KW units.
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Old 09-24-2014, 10:25 AM   #6
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Don't forget to add refrigeration as a power consumer. In a 24 period, it's a big energy user, that runs year round.
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Old 09-24-2014, 10:55 AM   #7
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Without giving my reasons or boring you with data I will give you what I feel is correct for a 35-45ft. Powerboat with an owner who wants to be cool when it is hot, take a hot shower, keep food cold, and cook a meal and make coffee. A minimum of 7KW generator, a propane stove, a 2500W Inverter/Charger, a water heater with both electric and engine heat exchanger and a house battery bank that can handle a typical days drain for 3 days before being charged. And if any of this stuff is over 10 years old enough money designated to replace it.
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Old 09-24-2014, 10:55 AM   #8
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Don't forget to add refrigeration as a power consumer. In a 24 period, it's a big energy user, that runs year round.
I've calculated 3.0 to 4.0 amps DC on a 12v system. Half of that on a 24v DC system. Less than the trolling motor on my bass boat. I would think an adequate house battery system should keep my food preseved.
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Old 09-24-2014, 11:04 AM   #9
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Without giving my reasons or boring you with data I will give you what I feel is correct for a 35-45ft. Powerboat with an owner who wants to be cool when it is hot, take a hot shower, keep food cold, and cook a meal and make coffee. A minimum of 7KW generator, a propane stove, a 2500W Inverter/Charger, a water heater with both electric and engine heat exchanger and a house battery bank that can handle a typical days drain for 3 days before being charged. And if any of this stuff is over 10 years old enough money designated to replace it.
I'm a retired airline mechanic, please bore me with data.
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Old 09-24-2014, 11:36 AM   #10
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I have a 38' light and narrow boat with the following: 12kbtu/hr ac/heat, 6.5kbtu/hr ac/heat, 1500w water heater, 140w fridge, and lots of little loads. The 5kW handles all this at around 75% load. To turn on the dive compressor, water heater must be off. That's about the only load management I need to do. And since dive compressor is not regularly used, it is not an issue.

A 5kW suits my needs very well. I think most trawlers could run on a 5 unless you start getting into bigger boats with the larger airs. I could probably run a 16 in the salon and still use the same gennie.

The 5kW with fridge and AC's cycling on and off all night averages about one gal in three hours. Probably averages about 1/3 to 1/2 load.

When not using ac's, a 1000w inverter runs fridge, coffeepot, laptop, etc, and gennie is off.

Very pleased with the system design.
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Old 09-24-2014, 12:28 PM   #11
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I have a 38' light and narrow boat with the following: 12kbtu/hr ac/heat, 6.5kbtu/hr ac/heat, 1500w water heater, 140w fridge, and lots of little loads. The 5kW handles all this at around 75% load. To turn on the dive compressor, water heater must be off. That's about the only load management I need to do. And since dive compressor is not regularly used, it is not an issue.

A 5kW suits my needs very well. I think most trawlers could run on a 5 unless you start getting into bigger boats with the larger airs. I could probably run a 16 in the salon and still use the same gennie.

The 5kW with fridge and AC's cycling on and off all night averages about one gal in three hours. Probably averages about 1/3 to 1/2 load.


When not using ac's, a 1000w inverter runs fridge, coffeepot, laptop, etc, and gennie is off.

Very pleased with the system design.
Worst case scenario; 100ē and 90% humidity somewhere on the TennTom. 50 bucks a day in fuel to be comfortable.
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Old 09-24-2014, 12:46 PM   #12
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I find when actually traveling, even a hot muggy day the boat is comfortable enough with windows, doors and hatches open. The killer is trying to sleep on such a night, that is when the gennie is used. But even then, run gennie and ac's for a couple hours, cool and dry out boat and usually can turn it off and sleep fine. Easily can stay comfortable on summer days for a gallon a day for gennie.

Most of my gennie hours actually come from winter running. When cruising, the "bus heater" keeps cabin warm using coolant from the main engine. Sleeping, though, the boat gets cold fast and gennie stays on all night running the ac's in reverse mode for heat.
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Old 09-24-2014, 12:53 PM   #13
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I find when actually traveling, even a hot muggy day the boat is comfortable enough with windows, doors and hatches open. The killer is trying to sleep on such a night, that is when the gennie is used. But even then, run gennie and ac's for a couple hours, cool and dry out boat and usually can turn it off and sleep fine. Easily can stay comfortable on summer days for a gallon a day for gennie.

Most of my gennie hours actually come from winter running. When cruising, the "bus heater" keeps cabin warm using coolant from the main engine. Sleeping, though, the boat gets cold fast and gennie stays on all night running the ac's in reverse mode for heat.
Ahh. Yes. Nights. Like a camper when it's cold.
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Old 09-24-2014, 01:03 PM   #14
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My worst case scenario includes having guests aboard. Most likely my 80 year old parents (fit and healthy) for a week or so at a time. Many of my friends are retired airline and can cruise between one airport and another. Does my $50.00 per day "worst case" budget need adjusted downwards? I have a fairly reliable and effective method of predicting the "use cost" of something: I call it ECAD. Estimate Closely And Double.
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Old 09-24-2014, 01:13 PM   #15
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Gen fuel really inconsequential. Main engines eat the most. Figure a 80mile day at 2nmpg and that is 40gal per day. Most trawlers do better, mine is close to 4nmpg at 7kts. Depends on what you end up buying. Most 40 to 44' call it 2nmpg for estimating purposes. Fits your ECAD model, which I like!!

So main engine eats 40gal/day, that's $160.
Gennie eats 2gal/day in minimize mode, that's $8.
Run gennie all day, figure 10gal, that's $40.

Budget $200/day worst case in fuel. Could be better, but prepare for that.

Most older folks can handle the heat as long as there is a breeze, especially outside of cabin in the shade.
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Old 09-24-2014, 01:36 PM   #16
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Gen fuel really inconsequential. Main engines eat the most. Figure a 80mile day at 2nmpg and that is 40gal per day. Most trawlers do better, mine is close to 4nmpg at 7kts. Depends on what you end up buying. Most 40 to 44' call it 2nmpg for estimating purposes. Fits your ECAD model, which I like!!

So main engine eats 40gal/day, that's $160.
Gennie eats 2gal/day in minimize mode, that's $8.
Run gennie all day, figure 10gal, that's $40.

Budget $200/day worst case in fuel. Could be better, but prepare for that.

Most older folks can handle the heat as long as there is a breeze, especially outside of cabin in the shade.
Thanks. That's very helpful. I've guesstimated various fuel burn rates for the size and style Trawler I want and their various propulsion systems. Because 10,000 is a nice round number and 24 months is another nice round number I know my avg. daily miles is 13.8. That's the magic number (and the cost of Diesel) when it comes to estimating use cost for engine fuel.
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Old 09-24-2014, 01:51 PM   #17
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When cruising in my 40, in December where I use a diesel heater (about 1 gal/day), and go the distance from NJ to Ft Pierce, FL and back....only anchoring occasionally...I spend about $1000/week for EVERYTHING from toothpicks to eating out to fuel. That's for 2 and a large dog.

When I find free dockage, anchor out more and stay at a place that's inexpensive for a monthly rate....I knock that back to around $750/week for the 4 month trip average (16-18 weeks). There are weeks that we only spend a couple hundred...but then we tend to splurge a little more some place else.

That's not a lot of dining out or other entertainment...just plain old living aboard a boat on the go.

I have an 8kW Genset...that I use when it's needed...but I also have a 1000W Honda that I use just to top off batteries 2X a day in some situations where I really don't need the 8kW.
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Old 09-24-2014, 02:00 PM   #18
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Ski is right on the mark as usual. We have anchored out in June/July for the last few summers. Where you drop your hook makes a world of difference. The outer banks has a lot of great locations. The closer you are to the ocean, the better comfort. Cape look out, Ocracoke to name a few are great spots to open all hatches and let the breeze blow though the boat. BTW, I hate sleeping hot !!!! and I slept great. BUT if you drop your hook near the marsh or creeks inland you will not have a breeze and the bugs will eat you alive. You will need to run your genny all night. So why not just go into a small town and plug into electricity and do some sight seeing. Stay the whole month and get a better deal. If I'm anchoring out for extended periods of time I usually run my genny 1.5 hrs in the am and 1.5 in the pm to keep the batteries charged. Of course you need to use power management to run systems because they all wont run at the same time. It never has been a problem. Lets see here, bulk charging the batteries, running both A/C's, while you are making coffee and baking cookies also washing and drying clothes. Nope, not gonna pull the load. It took a while to convince the wife this was normal.
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Old 09-24-2014, 04:07 PM   #19
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>I've calculated 3.0 to 4.0 amps DC on a 12v system. Half of that on a 24v DC system. Less than the trolling motor on my bass boat. I would think an adequate house battery system should keep my food preseved.<

4 amps DC for 12 hours or more probably 16 hours a day is 48 to 64 AH , so one 120AH deep cycle battery will be down to 50% each day.

The house bank size will decide how many days you can anchor with no noisemaker.

The quality of the charging system will decide if 5 hours or 10 hours is required to get mostly recharged.

I usually figure a noisemaker all up cost is about $10.00 per hour.

But that includes fuel, maint , repairs and a kitty for the new one when the installed one dies.

Therefore the first question should always be , how can I have a great lifestyle with NO noisemaker.

IF Air cond is required you can only go to the power pole if cost is a concer.

If AC is not required , there are many ways to live just fine , quietly , but not for Free!
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Old 09-24-2014, 04:29 PM   #20
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We have a 12KW NL genny we run everywhere we go. We average ~1GPH
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