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Go Back   SailNet Community > On Board > Gear & Maintenance > Electrical Systems
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  #1  
Old 06-22-2009
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Inverters for Dummies

Ahoy

OK - what I need is an "Inverters for Dummies" reply here. I understand AC and DC. I can do wiring. I have two deep cycle batteries and a Yanmar 2QM20H with "probably" a 50 amp alternator. (Manual is on board so can't look it up)

After reading some posts on hot water heaters, I would like to wire in a 1500 watt inverter and be able to turn it on for about the last hour of motoring so that the water is hot (rather than just warm). The Yanmar does not run hot - but it does run really good - so I do not want to mess with the engine.

The raw water cooling of the engine warms the water in the tank but a bit hotter would be nice.

I am also not in the market for a $2k pure sine wave inverter. I will not be running hospital equipment nor an oscilloscope with it.

Thanks in advance, and yes - I am ready for the 20 different opinions I will probably see.

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Old 06-22-2009
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If your water heater takes 1500 watts—at 12 VDC it will require 125 Amps to run it. If you have a 50 amp alternator, you'd still be shy of power needed by the inverter by about 75 amps. That means you'd draw down your batteries 75 amp-hours in the last hour of running the engine, just to heat the hot water. This is a significant portion of your battery bank capacity, since you only have two deep cycle batteries, and I'm guessing they're not 8Ds.

IMHO, you'd be far better off to turn on the inverter as soon as you start motoring, since, as the water heats up, the hot water heater will require less power, and more power will go to charging the batteries after a certain point. This would probably leave your batteries in a better, higher state of charge than doing what you initially planned, especially if you shut down the electrical side of the hot water heater for the last hour of motoring or so...

That said, I am quite surprised that the hot water heater isn't getting hot enough off the engine cooling system, via the heat exchanger coil inside the hot water heater. The engine should be running at at least 135-140˚ F at a minimum. If it isn't, then it is likely not running hot enough to be running efficiently. Domestic hot water temps are usually about 100-105˚ F, and the engine, even with conversion losses, should be able to get the contents of the hot water tank up that high. Since it can't, you probably should investigate why it can't. It could be as simple as scale build up in the heat exchanger coils in the hot water heater. Fixing this would make far more sense than killing your batteries, which is what running the electrical side will most likely do.
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Old 06-22-2009
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WRT to the assessment that the engine not providing hot water...I too am surprised. After even just an hour of engine runtime, the waters hot enough to scald in my hot water tank.
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On our last boat we had a Yanmar 2GM20F (fresh water cooled) and it ran the temperature I thought it should have.

On this one the Yanmar is much older, is raw water cooled and does not seem to run very hot. Going swimming last week - the water was not overly hot coming out of the exhaust hole.

The hot water tank is brand new.
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I had an electrician install mine, as I didn't have time to do it. He recommended wiring the inverter so it would not feed the water heater. He was about the third electrician that gave me that advice. I guess the prevailing opinion is you will toast your batteries really fast if you turn it on by mistake, or forget to shut it off.

The water heater should get hot enough on the engine. The water flowing out of your exhaust is constant flow and does not warm up as high as the coolant in the system.
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1500 Watts will kill your battery in a short period of time; the battery voltage will go down faster than a prom dress. If the battery bank is 6' from the water heater, you would need 1/0 power cables, which is 0.640" in outside diameter (bigger than the battery cables to the starter).

Inverters are spectacularly inefficient, and one that big will cost about $100, if you don't care what the sine wave looks like (you don't).

A much better plan would be to buy a 2000 W gen set for $200 and run it to heat the water. It means you have to have gasoline on board, but it is much more reasonable than the inverter idea. The bonus is you can charge your batteries in a pinch, and you are all set to add refrigeration and air-conditioning!

Best Regards,

e


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Do you have a temp gauge on the engine. What is the temp the engine ruining at? Is it running cold? It could be a engine thermostat stuck open and not allowing the engine to come up to temp. Your hot water is only going to be as hot as the engine water temp. If this is case, $20 fix.
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Last edited by bubb2; 06-22-2009 at 04:35 PM.
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Old 06-22-2009
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Quote:
Originally Posted by bubb2 View Post
Do you have a temp gauge on the engine.
Just the little red lights.

I have purchased an infrared thermometer but it is a few miles away from here. I will pick it up in two weeks, then I can do some serious checking.

So - I guess I will not waste my money on the inverter (the little $30.00 one charges the commodore's Mac).

Thanks all of you - I will explore the engine temperature some more.
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Quote:
Originally Posted by eolon View Post

A much better plan would be to buy a 2000 W gen set for $200 and run it to heat the water.
.::.
Where on earth are you seeing 2000 W generators from a reputable brand for $200?!!

Cheapest I've seen was the Honeywell 2000W Honda lookalike on clearance at Home Depot for $299.
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Quote:
Originally Posted by rikhall View Post
Just the little red lights.

I have purchased an infrared thermometer but it is a few miles away from here. I will pick it up in two weeks, then I can do some serious checking.

So - I guess I will not waste my money on the inverter (the little $30.00 one charges the commodore's Mac).

Thanks all of you - I will explore the engine temperature some more.
If it were me, you have 2 good indicators the engine is running cold. (temp of hot water and temp of exhaust water.) I would pick up a thermostat and just go ahead and change it out. You did say it was a older engine. By the time you get done messing around checking the running temp of the engine you could have the thermostat changed out. Just consider it normal maintenance.
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