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Old 15-07-2015, 12:02   #1
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Why do marine generators (and A/C) require seawater?

Generators (and air conditioners) work just fine on land. Indeed the land units seem smaller and cheaper.

Why do they need seawater?

It adds the problematic thru-hull (leak risk + noise transmission to the marina), and the problematic impeller, problems from junk being sucked in, etc.

Is it to provide extra cooling? Is that really it?

I wonder why i don't just grab a regular gennie from amazon and drop it into the locker? Much cheaper, easier install. Many have remote start buttons. Harder to find one that runs on diesel, of course.
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Old 15-07-2015, 12:17   #2
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Re: Why do marine generators (and A/C) require seawater?

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Originally Posted by Whomda View Post
Generators (and air conditioners) work just fine on land. Indeed the land units seem smaller and cheaper.

Why do they need seawater?

It adds the problematic thru-hull (leak risk + noise transmission to the marina), and the problematic impeller, problems from junk being sucked in, etc.

Is it to provide extra cooling? Is that really it?

I wonder why i don't just grab a regular gennie from amazon and drop it into the locker? Much cheaper, easier install. Many have remote start buttons. Harder to find one that runs on diesel, of course.
Cooling with sea water is far more efficient, because of the far greater specific heat of water compared to air.

Sea water cooling does bring a number of complications, however, so some people do use air cooled fridges. An air cooled generator would be hard to provide with enough cooling air, and would be noisier.
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Old 15-07-2015, 12:26   #3
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Re: Why do marine generators (and A/C) require seawater?

The heat generated by the generator has to go somewhere. An air cooled gen set cant safely live in your locker etc. So Marine units are water cooled. if you have a big deck and can live with the corrosion then a land unit could be used up there.
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Old 15-07-2015, 12:35   #4
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Re: Why do marine generators (and A/C) require seawater?

How about AirCon?

Is that also simply about cooling the compressor?
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Old 15-07-2015, 12:47   #5
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Re: Why do marine generators (and A/C) require seawater?

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Originally Posted by Whomda View Post
How about AirCon?

Is that also simply about cooling the compressor?
It's about cooling the freon. Serves the same purpose as the big shiny coils that live outside your house on the AC unit.

As mentioned, water is much more efficient at cooling than air is, that's why the water/water and water/freon heat exchangers on marine equipment can be so small compared to water/air (radiator) heat exchangers on cars.
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Old 15-07-2015, 12:47   #6
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Re: Why do marine generators (and A/C) require seawater?

In order to cool the air in your boat, the AC unit must remove the heat and moisture from the boat air and transfer it to someplace else. If you used an air cooled unit you need a means to get rid of the waste heat. If you feel the air blowing out of you central home AC (not the cooled interior return air), you can feel the heat.

Water cooled units get rid of the waste heat by transferring it to the seawater.

Yes you could design an air cooled system, but it would be bulky, energy inefficient, and likely noisy.

Safe sailing
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Old 15-07-2015, 13:12   #7
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Re: Why do marine generators (and A/C) require seawater?

I have an 8k btu Dometic Turbo marine unit and a March 3CP pump (oversized vs the A/C unit). It cools one hull of the cat - the one we sleep in. This can be powered either by shore power, or a Honda 2000 does the trick as well. (Run's 10 hrs on less than a gallon of gas.) It is a very quiet generator. I have a friend that runs the 16k btu Dometic Turbo with the same generator. It burns fuel faster (spends less time on eco-mode).

I have friends that have gone the route of buying the window unit A/C's, and mounted them in companionway hatch boards. They spend a couple of hundred dollars for the window unit (or less), and can toss them when they get rusty and no longer work. My built in A/C, the pump, the wiring, ductwork, registers, etc., was a few thousand dollars. I don't have to deal with climbing around a window unit, storing it, etc., but have to maintain the built-in.

So cheap alternative is there.
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Old 16-07-2015, 08:45   #8
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Re: Why do marine generators (and A/C) require seawater?

Seawater cooled Air Con. I converted a 12,000 Btu split type Panasonic air conditioner by removing the condenser and installing the compressor in the bottom of a hanging locker along with a titanium coaxial coil condenser and a March seawater pump with strainer in front of it and it works magic.
It has a remote control just like a house unit that I can program it to come on when I am away and keep the boat fresh.
It is quieter than most marine air conditioners, has more control and can be purchased and installed for less than $2,000. The only thing is the attractive evaporator unit is normally exposed and quite visible in our salon.
This unit can be doubled up with a double split unit. Two evaporators in two areas, hulls/staterooms or salon areas and run off the single compressor if sized properly.
I have the boat on the hard and it is set to run for two hours everyday to keep the moisture down and boat fresh.
I have a write up on the installation if anyone is interested. Send me a PM and I will email it off to you. I also have pictures of my on the hard cooling set up that also keeps my water maker in fresh water flushes.
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Old 16-07-2015, 08:52   #9
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Re: Why do marine generators (and A/C) require seawater?

Engines and AC units generate heat that needs to be dissapated. On land this is done with large amounts of air flow generated by fans that blow fresh air over their radiators / cooling coils. Boat simply cannot support air flow based cooling.
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Old 18-08-2015, 12:12   #10
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Except power?

So a common conclusion here is that the efficiency of seawater cooling is significantly better than air/fan cooling.

Except looking at the statistics, this doesn't seem to be true. Window-mounted A/C units have similar or lower power requirements than marine units. Example: Frigidare 8k BTU unit: 6.5A (115V), Frigidare 10k BTU unit: 8.0A versus Webasto FCF 9K BTU 8.3A. Split units (outside fan/condenser, inside scroll fan) seem to be about the same amperage.

Am I missing something? Shouldn't the marine units be much more efficient given the seawater advantage?

The marine units do seem to be marginally smaller -- i guess the condenser can be smaller for the same BTU given the cooling efficiency. They weigh about the same, but perhaps the marine unit is sturdier.

Is the extra money (6 to 8 times the cost) plus seawater problems worth the smaller size?
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Old 18-08-2015, 12:27   #11
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Re: Why do marine generators (and A/C) require seawater?

The conclusion is that the hot air/or heat has to go outside! Seawater allows a permanent installation that isn't sticking out of the boat somewhere, so the boat can be used properly.
Window units work fine on boats, just mount it on top of your overhead hatch with wood work allowing blowing into the cabin... a $250 unit will freeze you out. Trouble is, you really cant go to sea with that. Bottom line is though, you don't need it at sea. Stick it in a locker and mount it if you anchor in the summer... of course it's a messy, storage problem issue so not many do it, unless they are just living aboard at the dock.
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Old 18-08-2015, 13:52   #12
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Re: Why do marine generators (and A/C) require seawater?

It's about space not efficency. A well designed air cooled unit can be just as efficent.

The main advantage of a marine unit is the cooling system is smaller.

That said, we trashed the marine Air/con a couple years back for a portable unit. It doesn't look as nice but no more clogged intakes and the condensation never overflows into the bilge.
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Old 18-08-2015, 13:59   #13
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Re: Except power?

Quote:
Originally Posted by Whomda View Post
So a common conclusion here is that the efficiency of seawater cooling is significantly better than air/fan cooling.

Except looking at the statistics, this doesn't seem to be true. Window-mounted A/C units have similar or lower power requirements than marine units. Example: Frigidare 8k BTU unit: 6.5A (115V), Frigidare 10k BTU unit: 8.0A versus Webasto FCF 9K BTU 8.3A. Split units (outside fan/condenser, inside scroll fan) seem to be about the same amperage.

Am I missing something? Shouldn't the marine units be much more efficient given the seawater advantage?

I don't think you can directly compare seawater/air cooling efficiency and electricity usage efficiency. Some of the power requirements may simply come from design choices: a bigger water pump here, a larger compressor there, here a fan, there a blower, etc etc etc

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Old 18-08-2015, 14:32   #14
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Re: Why do marine generators (and A/C) require seawater?

Quote:
Originally Posted by Canuck Sailor View Post
Seawater cooled Air Con. I converted a 12,000 Btu split type Panasonic air conditioner by removing the condenser and installing the compressor in the bottom of a hanging locker along with a titanium coaxial coil condenser and a March seawater pump with strainer in front of it and it works magic.
It has a remote control just like a house unit that I can program it to come on when I am away and keep the boat fresh.
It is quieter than most marine air conditioners, has more control and can be purchased and installed for less than $2,000. The only thing is the attractive evaporator unit is normally exposed and quite visible in our salon.
This unit can be doubled up with a double split unit. Two evaporators in two areas, hulls/staterooms or salon areas and run off the single compressor if sized properly.
I have the boat on the hard and it is set to run for two hours everyday to keep the moisture down and boat fresh.
I have a write up on the installation if anyone is interested. Send me a PM and I will email it off to you. I also have pictures of my on the hard cooling set up that also keeps my water maker in fresh water flushes.
For some reason I can't send you a pm from my tablet. I would love to have you email me the specs on your install. Rnewby1 @ hotmail .com thanks in advance
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Old 18-08-2015, 14:39   #15
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Re: Why do marine generators (and A/C) require seawater?

Here is another option to butchering wood to install a house window unit this goes over the bow hatch
My Boats Gear
I found a coupe listed on craigslist here for 2"Benjamin's so not to bad of a price
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