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  #191  
Old 04-01-2014
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Re: safe cooking

Quote:
Originally Posted by guitarguy56 View Post
Ok George... I only have one induction stove and no oven.

Here is how I see it at the battery... 1500 watts into the battery is 150 amps per hour... 150 amp/hr... so 150 amps/60 minutes= 2.5 amp/minute so 5 minutes equals 12.5 amps multiplied by 12 volts = 150 watts... 10 minutes gives you 300 watts... this not including inverter efficiency or factor which would alter the above.

The stove of course is 110 vac... loss though the inverter may be .5 amps +/-
The first part of this is basically correct. It should read "...1500 watts out of the inverter needs 150 amp draw from the batteries. 150 amps/60 minutes = 2.5 AH per minute so 5 minutes = 12.5 AH." The change to watts is not needed as DC is measured in amps and amp hours.

Inverter losses are higher - probably around 10%. The correct math is about 84 amps/1000 watts out of an inverter but I (and others) use the higher number - 100 amps - as both an easier number to work with and a way to include the losses.

And one other point. When you use 50 AH from a battery it takes more than 50 AH to replenish it. See Peukert for an explanation.
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  #192  
Old 04-01-2014
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Re: safe cooking

Quote:
Originally Posted by Puddin'_Tain View Post
I think that the 700W figure is the power delivered by the oven, not the current draw. It may not require 1200W, but I would be surprised if it wasn't pretty close to 1kW. While it's true that a few minutes here and there wouldn't tax most house banks too much, it would still be a fairly large percentage of a daily electrical energy usage. At the dock, no biggie. While cruising,....? Of course, there is always the space issue. A second appliance for doing what can be done by the stove isn't usually what a small boat really needs. However, for a live aboard things might be a bit different.
While I have not measured it the actual AC wattage would be higher on full power than the rated 700 watts. But even if it jumps to 80 or so amps it is still not much for a cruiser with a good battery bank and solar. I doubt you will find many boats over 30' in Mexico or the Caribbean cruising without a microwave on board.
And in a warm climate there is no cooler way to cook or heat up food than with a microwave.

I am thinking in terms of a liveaboard.
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  #193  
Old 04-01-2014
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Re: safe cooking

Quote:
Originally Posted by mitiempo View Post
While I have not measured it the actual AC wattage would be higher on full power than the rated 700 watts. But even if it jumps to 80 or so amps it is still not much for a cruiser with a good battery bank and solar. I doubt you will find many boats over 30' in Mexico or the Caribbean cruising without a microwave on board.
And in a warm climate there is no cooler way to cook or heat up food than with a microwave.

I am thinking in terms of a liveaboard.
Well, once you get into a 30' boat there is room for lots of things that the owner of a 27' boat can only dream about; particularly when the 27' boat in question is a pop-top Cal 27.
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  #194  
Old 04-01-2014
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Re: safe cooking

Is this small enough?

https://www.youtube.com/watch?v=psoLkihnVxA
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  #195  
Old 04-01-2014
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Re: safe cooking

I didn't see anyone mention this yet. You are 55 times more likely to have an electrical fire aboard than a stove fire! 24 times greater risk of an engine fire. This concern over galley fuel is not ones primary risk. Get whatever stove you like and mange it properly. Propane or not.

Why Boats Catch Fire - Seaworthy - BoatUS
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  #196  
Old 04-01-2014
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Re: safe cooking

I use a white gas camping stove. depressurize and stow between use gives me what I need. the heat provided heating up early morning coffee is a plus.
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  #197  
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Re: safe cooking

Quote:
Originally Posted by Minnewaska View Post
I didn't see anyone mention this yet. You are 55 times more likely to have an electrical fire aboard than a stove fire! 24 times greater risk of an engine fire. This concern over galley fuel is not ones primary risk. Get whatever stove you like and mange it properly. Propane or not.

Why Boats Catch Fire - Seaworthy - BoatUS
Interesting.

However, this is about fires, not explosions. I suppose the statistics includes galley fires caused by liquid (and solid) fuels. But if there is a propane leak, the result is not a fire but BOOOOM
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  #198  
Old 04-01-2014
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Re: safe cooking

I think BoatUS is combining propane or at least they address it in their article. More telling is they attribute the decrease in stove fires to a decrease in alcohol stoves.

“6) Stove - 1%
Stove fires appear to be less common (1%) than in the past, probably due to fewer alcohol stoves being installed on new boats. Still, alcohol can be a dangerous fuel; though it can’t explode, an alcohol flame is hard to see. One fire was started when a member tried to light the stove and gave up because he couldn’t see the flame. Unfortunately, he had succeeded, but didn’t realize it until he got a call from the fire department. Only one fire was started by propane; a portable stove fell off a counter and ignited a cushion.”

True story. Anyone of you familiar with the Sierra Nevada Mountains and Priest Grade? Priest is an original road (trail?) that went up to Groveland in the gold country. Very, very steep grade that lasts for five miles or more. Big signs on both ends prohibiting large trucks, vehicles with trailers and RVs. It is about 15-20 miles shorter than the “modern” road and cuts travel time considerably. On this particular day an RV driver decided to take the shortcut down and as forewarned, burnt out his brakes about halfway. Fortunately, he was able to use the hillside as a brake to stop his runaway vehicle instead of going over the cliff edge and certain death. The family evacuated safely but the burnt out brakes caught the surrounding grasses on fire which caught the RV on fire. Roaring fire, with the roof burnt off, the vehicle walls acted as a chimney. Gas from the fuel tank added to the conflagration. With fire all around, the propane tank started to cook off. When it did, The poppet valve released and a hundred foot flame shot out through the roof with a whoosh. The biggest explosion were the tires cooking off and expanding then bursting. Those went off with a bang that I would have thought what would happen to the propane tank. Not to say propane can’t explode, but this was one exciting afternoon in any case.

Can I make one last plea for bringing back coal for cooking? It’s not volatile, won’t explode, compact, and has about twice the BTUs as wood or charcoal.
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  #199  
Old 04-01-2014
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Re: safe cooking

Actually the Cookmate two-burner is cheap because it is A TOY.

Two 6800 BTU burners is equal to ONE real stovetop burner. It is that simple. If you want to boil a small kettle of tea, or one cup of soup, that's fine. If you want to get a decent weight 4-quart pot to a boil...This is why so many boaters just buy a $20 (now $25) "Korean BBQ special" butane canister one-burner stove, and if it rusts in five years, toss it. With due care for handling the butane and storing it, it is still real damn hard to beat the value and power of the $25 burners.

A 1500-watt device will use 1500 watts times one hour, or 1500 WATT HOURS in one hour of use. At 12.6 volts (from battery voltage, not alternator or genset) that's 1500/12.6 => 119 Amps being drawn.

In ten minutes of use, it will use 1500/ (60/10) => 1500/6 => 250 "WATT HOURS" of power consumed in those ten minutes.

250 WATT HOURS on a nominal battery system, with nearly fully charged batteries at 12.6 volts, would be about 20 AMP HOURS out of the bank, drawn during those TEN MINUTES, if I've done my math right.

And since inverters all have efficiency losses, and "volts time amps" actually equal "volt-amperes" which in turn are NOT the same as Watts (there's another conversion loss to be factored in on AC systems)...ten minutes on the 1500-Watt induction stove, or any other AC device, will probably use closer to 25 AH from the "12" volt battery bank. More like 30AH if the battery is at a slightly lower voltage, and there are higher inverter losses, and so on.

If you use that induction burner for a bacon and egg breakfast, and then again to skillet-sear a steak or boil diner...I'd hate to use 50? 70? amp hours per day for cooking. I think for most of us, that's a huge dent in the energy budget.
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  #200  
Old 04-01-2014
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Re: safe cooking

Units are important. There are a couple of people on this thread with near saintly patience. Kudos to Puddin' Tain and GeorgeB who have been getting it right.

Units are important. If you are using units like amps/hr (or /min or /sec) you don't understand. If you use amps to measure power (W, kW) or energy (Ws, Wh, kWh, or (unfortunately since it adds voltage dependance) Ah) you don't understand. Carry the unit terms through your calculations and you'll see you the error of your ways. This is Jr High School algebra and you should understand it.

I don't intend to offend anyone directly. I hope to help you understand just how important this material is, and that if you get the units straight you will be an electrical rock star.

Quote:
Originally Posted by anonymous
If a microwave is used sparingly, as they nearly always are, the DC amp usage is pretty low. An inverter is needed. But your 1200 watt example need not be that high. A small microwave at 700 watts AC will use about 70 amps DC through the inverter. 5 minutes of use to heat something up or make popcorn or whatever will use 70 divided by 12 or a bit less than 6 amps DC.
This is a great example. Lets walk through it.

First remember that watts are a measure of power that is NOT voltage dependent. That makes them great for dealing with voltage conversions that happen at inverters or battery chargers. Let's use my little Tappan microwave instead of the undefined one above. It is sold as a 600W microwave because that is the output of the microwave transmitter in the oven. If you read the label on the back you'll see it draws a maximum of 850 watts; the difference is due to the efficiency of the transmitter, losses in the power supply, and the control board, clock, etc. So we use 850 watts.

Actual RMS AC voltage (what you read on a volt meter) for US spec appliances should be between 117 and 120 VAC at 60 Hz (Hertz is just the frequency of the alternating cycle (AC) power). Let's use 120 to keep things simple.

850 watts (volt∑amperes neglecting power factor which isn't relevant until we talk about refrigerators, air conditioners, or other device with significant rotating loads) / 120 volts = 7 amps ON THE AC CIRCUIT supplied by the inverter. We'll go back to watts to work our way through the voltage conversion in the inverter since it is voltage independent. My Mastervolt 2kW inverter has an efficiency of 95% - actually pretty decent. 850 watts on the AC side turns into 850 watts/0.95 = 895 watts on the DC side. If my batteries are fully charged and providing 13.6 VDC the microwave will draw 895 watts / 13.6 V = 66 amps. That's a lot of amps. If I heat up a casserole for 8 minutes I'll use 66 amps * 8 minutes / 60 minutes/hr = 8.8 Ah from my 675 Ah battery bank, about 1.3% of capacity. Not bad at all.

Lets look at a spaghetti dinner on an induction cooktop. Assume 3 minutes at 1500 watts to boil the pasta water and 11 minutes to cook the pasta at a lower setting - say 600 watts. We also heat canned spaghetti sauce (I'm making this simple so we don't have to saute meat and mirepoix, and sauce and paste, yadayadayada) at about 300 watts for the same total time.

3 min * 1500 W + 11 min * 600 W + 14 min * 300 W = 15300 W∑min (aren't watts wonderful?)
Through the inverter 15300 W∑min/.95 efficiency = 16105 W∑min
Again with fully charged batteries that means 16105 W∑min / 13.6 V / 60 min/hr = 20 Ah
Not the end of the world, but that does mean that if spaghetti is representative of a cooked meal and you make two a day aboard and that if refrigeration draws about the same 20 Ah you could cook with propane and have two big refrigerators and a separate freezer and come out ahead. If that's a good trade-off for you, fine - just make the decision with your eyes open and don't kid yourself that there is some magic efficiency changing the rules of physics.

Now one watt is roughly 3.4 BTUs, so a 1500 W induction hob is equivalent to a 5100 BTU propane stove burner. It can't be different. Force 10 and Eno propane cookers (for example) have large burners that run between 8000 and 8500 BTUs. The smaller burners hover around 3400 BTU. So in our spaghetti example above, the propane stove will boil the water faster than the induction hob, but that only reduces the initial 3 minutes. It isn't relevant to heating the sauce or cooking the pasta at reduced heat.

In fairness, most propane cookers have an electrical solenoid in the propane locker that draws a surprising amount of power. Many draw about 1 A the entire time they are switched on. So our spaghetti dinner on a propane cooker consumes 14 min * 1 A / 60 min/hr = 0.23 Ah. Not much, but it should be counted.

All the efficiency claims of induction are relative to electrical resistance heating, not gas.

Exercise: look up and understand current (amps), power (W), and energy (Wh, Ah, BTU).

In my opinion (all the above is fact - this is opinion) propane is the best choice for cooking aboard although it does carry some small risk. If you use a gas powered generator like the Honda to top up your batteries to support electric cooking the risk is similar. If you use a diesel generator, a diesel main, or alternative energy the risk is slightly higher for propane.

If you are so concerned with the explosive potential of propane as to rule it out you can make inductive cooking work but your battery bank better be big (off hand, 1200 Ah or so) and you should have the charging capacity to match unless you eat out a lot.
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