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Old 13-11-2022, 05:13   #1
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12v Heaters?

12v Heater?

Is there an opinion on 12v heaters from the forum? I am planning a trip from Annapolis to Kill Devil Hills, NC the first week of December and would like to hear about my options.

My boat: 2005 Catalina 350. Yes, I have a Honda Gen that I can run the onboard heater, but would rather use a 12v, IF available.

Thanks for your thoughts.
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Old 13-11-2022, 05:48   #2
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Re: 12v Heaters?

People have died on boats from CO poisoning running portable generators and 120v heaters, and you are wise to avoid this. There is no safe way to do it because any installation or setup will produce unsafe CO levels below decks under certain wind conditions. A CO alarm is not sufficient to make an unsafe installation, into a safe installation.

It is rare for the 12v system on the boat to provide enough capacity to produce a meaningful amount of heat.

Your options are either to add a fan coil that uses engine coolant, which is inexpensive and will work but is only of use when the engine is running, or to add a diesel heater.

Fan coils:

Diesel heaters:
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Old 13-11-2022, 06:11   #3
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Re: 12v Heaters?

In December 2020, I brought my boat down the Bay from Deale, MD, to Urbanna, VA. I used a Mr. Heater Portable Buddy propane heater rated for indoor use. It has a oxygen shutoff valve. As a precaution, you should have a CO detector on board.

It goes through a 16 oz. propane canister per night, which you should store in the cockpit (assuming you have bridgedeck), or propane safe locker.
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Old 13-11-2022, 06:34   #4

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Re: 12v Heaters?

You will need at least 1,500W of power to heat the boat to what I think is your expectations. !.5KW at 12V is 125A. Your boat's wiring won't support that at all, even if your generator can deliver it. If your generator has 120V output and delivers it to a conventional duplex outlet like the ones ashore, you could use a conventional shoreside electric "plug in" heater. That would bring your temperature below decks to something like 50 or 55 degrees Fahrenheit, PROVIDED you keep the ship "buttoned up". Open a port 'ole, let alone a hatch, for a minute or two and you are back to ambient temperature.

The only SAFE and reliable way to get adequate heat in the boat for a longer term is to install a diesel fueled "truck heater" like a Chinese "knock-off" of the Webasto or Espar, unless you wish to pay the money of a "good one". A knock-off you should be able to get at an auto supply store like NAPA for somewhat under a thousand dollars.

If you have the knowledge and the tools to install one, good. Else you must obviously pay what it costs to have it done professionally.

It's unlikely that you can either do the job yourself or have it done professionally before your scheduled departure date but going to sleep below with any kind of a liquid fueled heater going might well result in an early arrival at Fiddler's Field!

Best just to dress appropriately. Or to wait for spring :-)!

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Old 13-11-2022, 06:45   #5
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Re: 12v Heaters?

There are 12v heated blankets that use acceptable levels of power but any 12v air heater will be a battery killer. My ex purchased a 12v hairdryer (which is just a small air heater) and it could eat a battery bank and was promptly banned.
I have solved the generator gassing yourself with your generator problem stated above with lithium portable drop in powerbanks which can accept charge at 2000w from my 2.2 eu generator whilst motoring or sailing ( and don’t use it when sailing or anchored as hate the noise) and then use these to power a fan heater for a quick heatup which is generally enough when we are inside and the boat is closed up.
They also supply our induction cooktop and electric hotwater meaning no flames anywhere inside our yacht. Our 12volt dehumidifier also performs a slight warming function whilst keeping things dry.
Regarding using generators however we are also lucky that our 28 foot outboard auxiliary yacht came with an integrated generator box designed specifically for the Honda generator that is outside the cockpit accessed from the swim platform and venting over the stern below the level of the cockpit floor. The inboard version of our yacht doesn’t have this facility probably due to the much higher expected alternator outputs available from diesel inboards.
We never use the generator stationary anyway ( for ours and others tranquility) but it would be pretty hard to gas yourself with this set up I feel.
I would worry about anything requiring a flame and exhaust system inside like a diesel heater easily as much as a well positioned external generator.
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Old 13-11-2022, 07:08   #6
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Re: 12v Heaters?

All, I am very aware of the dangers of using the Honda Gen and would NEVER use it inside the boat. Currently it is beside the swim platform and nicely tethered. As I am doing this trip in December I do not anticipate being near any other boats while on anchor. I will use it ONLY to heat the boat while relaxing after a day of solo sailing if needed.

As I thought, there really isn't a good option for a 12v unless one is fitted with a good solar array and a powerful inverter, which I do not have either.

Only looking for opinions and suggestions, I do not need lectures.

Thanks to all who have done the former and not the last!

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Old 13-11-2022, 07:38   #7
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Re: 12v Heaters?

You are getting good advice above.

Over the course of thirteen January trips down the ICW from NC to FL we have evolved an ICW winter travel system of long underwear, bus heater, 12V bunk heater, and cooked breakfasts. We dress for the weather and pile blankets on the vee berth. It's not summer, so we don't expect to be in shorts. Long underwear and wooly caps are the uniform. We have a Heatercraft 'bus heater' that provides about 40,000 BTU/hr when the engine is running which is enough to keep the interior warm when we are motoring during the day with the companionway open and the interior drying out. Late in the day we turn on our Electrowarmth 12V bunk heater that lays across the vee berth sideways between our pillows and knees. It warms and dries out the vee berth keeping it warm to crawl into as soon as supper is finished. In the morning I fix a cooked breakfast firing up the stove and oven to knock the morning chill off the boat before my wife emerges. Then, we are off again with the bus heater blasting its heated air into the boat as we wash up and shave underway. During our rare marina stops, two 125v ceramic heaters with external thremostats keep the boat warm. and
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Old 13-11-2022, 08:19   #8
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Re: 12v Heaters?

Ed, the only heater I have ever had on board is a simple Oriogo-style alcohol burner type. It produces more than enough heat for our needs, and we cruise fairly cool areas (northern L. Superior and now Newfoundland).

They are relatively cheap, and easy to use. I wouldnít recommend using it underway, although we do at times. And you have to be careful about venting to avoid CO issues, but all I can say is, it has worked well for us.

On the 12v heating idea, Iíve not tried this, but electric blankets or bed warmers should work. And there is an array of 12v heating items in the motorcycle world (gloves, jackets, shirts, pants, etc) which I suspect might work on a boatís 12v system.
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Old 13-11-2022, 08:22   #9
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Re: 12v Heaters?

Diesel forced air heater. Going on year 3 with my Chinese knockoff, about $300 when you add the thru hull skin fitting and upgrade the muffler and exhaust. Lots of heat 5kw, ecomical with fuel and 12v.
Just install correctly, add co alarm.
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Old 13-11-2022, 08:22   #10
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Re: 12v Heaters?

WOW, a bunch of really good opinions and suggestions, thanks to all.

As it will be just me sailing south, I think I will go with my best hunting long johns (merino wool of course) and nice wool cap and a heavy coat. If my wife were aboard it would be very different, but as we will be living in OBX, I do not think installing a perm heater system would be warranted.

Thanks to all, never a lack of experience and knowledge on this forum, it is appreciated.

Safe sailing to all.

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Old 13-11-2022, 12:41   #11
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Re: 12v Heaters?

Originally Posted by wsmurdoch View Post
We have a Heatercraft 'bus heater' that provides about 40,000 BTU/hr when the engine is running
My memory was bad. The real figure is 28,000 BTU/hr.
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Old 13-11-2022, 15:31   #12
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Re: 12v Heaters?

You've had good advice. The two main options, which are not mutually exclusive, are bus heater, and Webasto etc. diesel heater. The bus heater will give you abundant free heat while you're motoring, and the diesel heater will keep you warm on nights at anchor.

The Webasto uses some 12v power so you will need a reasonable battery bank.
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Old 18-11-2022, 16:17   #13
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Re: 12v Heaters?

It would take a phenomenal amount of 12v power to have an appreciable effect. Use your oven. Bake a lot.

The 12v could warm an electric blanket or throw. They make such that run on 12v. Or use an AC one on inverter. But for big heat donít count on creating it from 12v.
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Old 19-11-2022, 06:51   #14
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Re: 12v Heaters?

Even boiling water for a pot of tea will be bloody slow off 12V no matter how big a bank or how many panels.

For space heating electricity in general is a non-starter. Burn fuel directly, the electrics contribution is for control systems and fans / pumps only
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Old 19-11-2022, 11:00   #15
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Re: 12v Heaters?

Singlehanded podcast by math Rutherford. This guy has lived on boats everywhere but mainly Annapolis for years. He does a podcast on the best way to heat a boat and talks of pros and cons of each. He says he’s basically tried everything. Worth a listen
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