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Old 29-04-2016, 18:42   #46
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Re: Cabin heat from engine

The OP has made up his mind. He is going to do what he sees as a clever way to capture "free" heat. Delusional.
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Old 29-04-2016, 19:06   #47
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Re: Cabin heat from engine

Wondered when we'd see a comment from northern climes. My reaction is - as long as it doesn't go below freezing at night, just get warmer bedding.
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Old 29-04-2016, 23:28   #48
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Re: Cabin heat from engine

Quote:
Originally Posted by Pizzazz View Post
Great input. I believe you may be a little conservative with the heat output.

I think the stored energy system is not that inefficient.
It's horribly inefficient...

I'm an engineer, I'm allowing for efficiency losses all along the way.

So 220 grams per hp/hr at 4 hp is 880 grams or 0.26 gallons per hour. At 138000 btu per gallon that's 36,000 btuh. Subtract 25% for mechanical efficiency and you have 27,000 btuh. About 1/2 the heat goes out the exhaust as hot gas and is cooled by direct water injection at the discharge elbow, Not usable heat without adding an exhaust gas heat exchanger. The other 13500 btu's is cooled via the water to water heat exchanger. If your really lucky and your heat exchanger in the storage tank is designed well, you might get 50% of that into the tank.

That's ~7,000 btu. That's a 21 degree F rise for 40 gallons in the first hour. Assuming 50 degree tank water start temperature, that will take 4.3 hours to heat to 140 degrees. Not one hour.

So say you want to heat the cabin to 70 degrees. That 40 gallons gives you 22,600 btu's usable assuming 140 degree storage temperature and 70 degrees discharge air temp. Actually a tad less, but lets assume 100%.

So that's roughly 5 hours of heat before the water cools to 70 degrees. In reality the efficiency will drop as the delta T drops between water temp and air temp.

The cost of a 40 gallon water heater is $1100 from defender. As others have said, that's a lot of weight in a small boat. Add a pump for $150, bus heater for $250 and piping and wire for another $100 and you have a $1500 heater that is all of maybe 35-40 % efficient on a good day.

From an engineering perceptive, its a non-starter.

Where as a webasto heater on ebay is $750 ish and is 80% efficient. Plus your boat will not be healed over 8 degrees with the 350# of water heater sitting to one side.

But go ahead, spend the money and see how long it takes you to heat that 40 gallons at idle. I'm pretty sure it will take more then 4 hours to bring it up.
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Old 29-04-2016, 23:54   #49
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Re: Cabin heat from engine

Do you know what is the max hot water temperature you are running your boiler at?
The boiler is under pressure, 15psi. It will make up to 250°F without making steam, just hot water. It's oil fired and expensive to run. I have a pellet stove that I put a homemade coil into. That circulates with the boiler and brings the temp to 180°F. The original hydronic system included baseboard radiators thruout the boat. The radiators needed the high heat (250°) to warm the boat in a timely manner. I switched to individual cabin/compartment marine forced air heaters (similar to car heaters). They heated better at 180° than the radiators at 250° and were my 1st step in getting the heating costs down.
Later the pellet stove, and when running new hydronic plumbing, I included the main engines cooling water into the hydronic system. When running I heat with engine heat. A side benefit is in cold weather I can run boiler water thru the engines before starting. All of it controlled by manual and electric valves. Pellets are twice as cheap as oil and 3 times cheaper than electricity. I was burning 5 gallons of oil a day in really cold weather back when diesel was $4/gallon, and only heating a fourth of the boat. Now I spend about $5/day and heat the whole boat.
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Old 30-04-2016, 00:38   #50
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Re: Cabin heat from engine

Quote:
Originally Posted by AnglaisInHull View Post
Wondered when we'd see a comment from northern climes. My reaction is - as long as it doesn't go below freezing at night, just get warmer bedding.
This is also my favourite method. It's nice to sleep in cool air with warm enough bedding. In the morning I may use my Eberspächer diesel heater for maybe ten minutes if it's still too cold inside.

The biggest problem with the OP's method is weight. It would be nice to be able to take some extra seawater in and heat it, when I'm about to anchor. That would be free energy. But this system could be a bit too complex, and it would take some space. In a marina running the engine would be quite noisy.

Running the Eberspächer all night has the problem that the fan consumes electricity. I might need to run the motor just to recharge the batteries, and I would lose some of the the better fuel efficiency of the Eberspächer.

I might consider also the option of having a radiator and fan in the engine cooling loop. That would give me some heat e.g. in the morning. No heat storage though, but maybe I would not need it.

While in a marina with electricity, running the Eberspächer is no problem. In this case a small electric heater can be even better. Maybe I should check if there are some small heat pumps available on the market. That would save energy when compared to direct electric heating. But should the heat source be water or air, and where should I install that heat exchanger?

If you use the engine to heat the water, you could take part of the energy out from the alternator to an electric water heater. Maybe too much added complexity, but lower RPM is the bonus if you use the motor just to generate heat.

Lots of options, and none quite perfect.

A warm down duvet is low tech, very light, guaranteed to work, very comfortable. I recommend that, in addition to any other solutions you might have.

The Story - Joutsen
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Old 30-04-2016, 04:53   #51
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Re: Cabin heat from engine

Check Webasto for their diesel fueled heaters, hot air with a wall thermostat would be a very easy small installation. Probable for a similar price as a big Raritan tank and all those parts. They sip fuel and draw 3-4 amps when running.
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Old 30-04-2016, 05:33   #52
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Re: Cabin heat from engine

To be serious for a change, it's an interesting thought excercise, but Sailorchic has provided a pretty detailed response that clearly expresses the challenges. (My one thermodynamics course is way too many years ago to try to verify the calculations).

I'm renting a canal boat later this year that uses the engine for hot water (not cabin heat). The boat information warns that it takes "several" hours to bring the water up to shower temperature. Not a problem when you plan to motor all day, but annoying in a sailboat.

And, yes, the weight. I once sailed a boat that had (for good reasons) an extra-large holding tank. I don't remember how big, but probably close to 200 liters. A fairly heavy 36 footer took on a distinct heel by the time that sucker was full.

Finally - and take this with a grain of salt because it's based on an old fart's memories of old systems - radiant hot water heating systems tend to use large, heavy components to get the heat out of the water and into the air.

So, maybe it can be done, but you have to wonder if it's worth the trouble.
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Old 30-04-2016, 09:14   #53
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Re: Cabin heat from engine

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Originally Posted by AnglaisInHull View Post
I'm renting a canal boat later this year that uses the engine for hot water (not cabin heat). The boat information warns that it takes "several" hours to bring the water up to shower temperature. Not a problem when you plan to motor all day, but annoying in a sailboat.
That is a long time to get shower temperature water. In my boat it takes something in the category of 10 minutes of motoring to heat up the hot water tank. The water will be very hot, but it will be mixed with cold water before it is led to the hot water pipes. Therefore the hot water tank need not be big (saves weight), and you can get as hot water as you ever need (50°C) for a long time.

I have not done the calculations for heating the boat, but at least I can get a days need of hot water by just motoring to the anchorage. Showering woks too, but depending on one's preferences, maybe not for very many showers.

(One more random thought. If you have a grey water tank and you take a shower, maybe that "wasted" heat could keep the boat warm for a while.)
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Old 30-04-2016, 10:41   #54
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Re: Cabin heat from engine

Would a small kerosene or electric heater make sense?
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Old 30-04-2016, 12:53   #55
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Re: Cabin heat from engine

I only read half the responces so forgive me if I am recovering ground. The wallas stove might be an ideal solution. Its a deisel cookstove with a smoth top close the lid over the cooktop and it blows air over the cook surface to heat the boat. Yes they cost a couple grand but take up little space think old shcool laptop need only a small throughhull like fitting in side of boat. Exhaust is clean and cool enough it wont melt a fender hanging in front of the outlet.
Oh and they work well for cooking food too.
Personally I have an old cast iron heater with a heat exchanger which i have thought of hooking up to my water heater heat exchanger and engine. Much more staightforward than your plan but even that hardly seems worth the effort. Except when I am driving the boat in freezing conditions from the unheated rear helm station.
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Old 30-04-2016, 12:56   #56
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Re: Cabin heat from engine

A quick note on the space pen mentioned much earlier. Yes it exists i believe parker still sells them. The problem with pencils in o gravity is that the lead or grafite is a conductor. Floating in zero grav it can mess up sensitive electronics.
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Old 30-04-2016, 14:22   #57
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Re: Cabin heat from engine

If the large water tank solution was efficient then surely the truck market would be using it rather than the diesel heaters. Astonishingly a Wesaco ST2000 is cheaper in the UK. £500 plus shipping against $2200 in Defender US.

Webasto heater Air Top 2000 STC Diesel Single Outlet 12v | 4111385C
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Old 02-05-2016, 11:37   #58
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Re: Cabin heat from engine

[QUOTE=sailorchic34;2109558]It's horribly inefficient...

I'm an engineer, I'm allowing for efficiency losses all along the way.

So 220 grams per hp/hr at 4 hp is 880 grams or 0.26 gallons per hour. At 138000 btu per gallon that's 36,000 btuh. Subtract 25% for mechanical efficiency and you have 27,000 btuh. About 1/2 the heat goes out the exhaust as hot gas and is cooled by direct water injection at the discharge elbow, Not usable heat without adding an exhaust gas heat exchanger. The other 13500 btu's is cooled via the water to water heat exchanger. If your really lucky and your heat exchanger in the storage tank is designed well, you might get 50% of that into the tank.

That's ~7,000 btu. That's a 21 degree F rise for 40 gallons in the first hour. Assuming 50 degree tank water start temperature, that will take 4.3 hours to heat to 140 degrees. Not one hour.

So say you want to heat the cabin to 70 degrees. That 40 gallons gives you 22,600 btu's usable assuming 140 degree storage temperature and 70 degrees discharge air temp. Actually a tad less, but lets assume 100%.

So that's roughly 5 hours of heat before the water cools to 70 degrees. In reality the efficiency will drop as the delta T drops between water temp and air temp.

The cost of a 40 gallon water heater is $1100 from defender. As others have said, that's a lot of weight in a small boat. Add a pump for $150, bus heater for $250 and piping and wire for another $100 and you have a $1500 heater that is all of maybe 35-40 % efficient on a good day.

From an engineering perceptive, its a non-starter.

Where as a webasto heater on ebay is $750 ish and is 80% efficient. Plus your boat will not be healed over 8 degrees with the 350# of water heater sitting to one side.

But go ahead, spend the money and see how long it takes you to heat that 40 gallons at idle. I'm pretty sure it will take more then 4 hours to bring it up.[/QUOTE

During the Age of Enlightenment, Science trumped Myth (Geez, I'm beginning to like that verb!). I don't think it is necessary to look further than the above, well-stated and articulate explanation by SC when considering your heating needs. As I stated earlier, how complicated do you want to make a 31-foot sailboat? Good luck and safe sailing.
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Old 02-05-2016, 12:14   #59
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Re: Cabin heat from engine

Off topic, but

Fact or Fiction?: NASA Spent Millions to Develop a Pen that Would Write in Space, whereas the Soviet Cosmonauts Used a Pencil - Scientific American
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Old 03-05-2016, 07:29   #60
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Re: Cabin heat from engine



Great article, Pilot and another example of waste in government spending. And, I do believe it relates to this discussion in the sense that simplicity and cost effectiveness are as important in our Space Program as they are in boat ownership. KISS or you'll be a boat slave and dock sitter with an unending list of "to do's" that never ends. A worthwhile read. Good luck and safe sailing.
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