Why electric AC?

[EngineerRetired]

I suppose the reason the AC units on most boats are electrical is so they can be ran from shore power. For real cruisers and more active boaters, wouldn't a system ran by a small diesel be better?


Diesel runs an electric generator, electric power runs a motor, motor drives a pump/compressor. vs, Diesel drives a pump/compressor. Just sayin...


Tech wise its a no brainer, multi-evaporator systems have been around a long time. Think large Greyhound passenger buses. Single condenser and compressor, multiple evaporators. Running the lines should not be a huge issue and a pressure transducer on the "high" side would throttle the diesel up or down to meet demand.


Just asking if anyone has done this?

[RaymondR]

I have not done it but have pondered on how it might be done a number of times.

One of my ideas when spending a lot of time motoring in the tropics was the use of a lay shaft driven by the engine with a clutch on one end and an AC motor on the other. This way one could engage the clutch for aircon underway and then disengage it to run the aircon with the AC motor when in the marina connected to shore power.

What came to mind next was whether one could set the AC motor up as an induction generator also to provide AC when underway.

[rwidman]

I think most boaters want their air conditioning to run when the boat is not running. That is, when anchored or docked.

Some boats do have a genset capable of running the air conditioner when anchored.

[EngineerRetired]

Originally Posted by RaymondR

I have not done it but have pondered on how it might be done a number of times.

One of my ideas when spending a lot of time motoring in the tropics was the use of a lay shaft driven by the engine with a clutch on one end and an AC motor on the other. This way one could engage the clutch for aircon underway and then disengage it to run the aircon with the AC motor when in the marina connected to shore power.

What came to mind next was whether one could set the AC motor up as an induction generator also to provide AC when underway.

I was thinking more along the line of a stand alone small diesel, 5HP or so with a serpentine belt that drives an automotive AC compressor say 2.5 - 3 ton (30K to 36K btu), Water maker high pressure pump and a 200ish Amp alternator to charge house batteries. (Obviously not all at the same time. Common sense required)



I've used a couple of small Diesels like this on the farm as "pony motors" for pumps, generators, winch to lift hay bails, etc. and they are real work horses and very economical. Run them at 1800 to 2000 RPM and they last forever.

[tkeithlu]

Here's the choice: A stand alone diesel or a diesel genset. The stand alone only pushes the pump, but is efficient. The genset does a bunch of tasks, anything that runs on electricity, but is less efficient at doing it, for the reasons you list.

Personally, I use air conditioning only occasionally while away from the dock, so installing a third diesel (propulsion, genset, AC) really isn't worthwhile.

Ahh, but then, maybe that's not where I really am. I recently added a small gasoline genset which sits on the deck (yes, I dealt with the CO) and will power a 5,000 BTU airconditioner. I'm thinking of running it at night. It's not alll that efficient, but it will be quieter than the big genset.

[Sailmonkey]

I think most “real cruisers” don’t need ac on the hook but occasionally. A boat at the dock needs ac far more often.

So, to really overcomplicate this sytem, we have a dual compressor system, one belt driven and one hermetically sealed electric.

This is a violation of the KISS principle for extremely small gains in efficiency.

[Lake-Effect]

Originally Posted by EngineerRetired

I suppose the reason the AC units on most boats are electrical is so they can be ran from shore power. For real cruisers and more active boaters, wouldn't a system ran by a small diesel be better?

I believe most people who want A/C away from shorepower have met this goal with a generator and electric A/C. Less efficient for the specific case of air-conditioning on the hook, but using more "conventional" and available units, and more versatility from each device.

Also good for ensuring you'll be alone in your anchorage (gen + A/C running, and crew shouting over them )

[wingssail]

Originally Posted by tkeithlu

Here's the choice: A stand alone diesel or a diesel genset. The stand alone only pushes the pump, but is efficient. The genset does a bunch of tasks, anything that runs on electricity, but is less efficient at doing it, for the reasons you list.

Personally, I use air conditioning only occasionally while away from the dock, so installing a third diesel (propulsion, genset, AC) really isn't worthwhile.

Ahh, but then, maybe that's not where I really am. I recently added a small gasoline genset which sits on the deck (yes, I dealt with the CO) and will power a 5,000 BTU airconditioner. I'm thinking of running it at night. It's not alll that efficient, but it will be quieter than the big genset.

Most gensets, even the small gasoline ones, while seeming "silent" to the operator, are annoying to others in the anchorage.

But, if plumbing in a second compressor is easy, then why not install an automotive compressor with its self-contained clutch. At least it would be handy while motoring.

HOWEVER, I have thought that the engine (or gen set) adds more heat to the interior of the boat than the aircond takes out. Can't see as it helps much or is even a net gain.

Living aboard in the tropics for over 25 years we've never felt like we needed aircond except at the dock. So our mains powered AC AC works well for us.

[rslifkin]

Even the "quiet" portable gensets aren't at all quiet. No air cooled engine ever will be. A built in genset may be a bit louder from inside the boat (depending on how good the sound insulation is), but if the installation is good, it'll be very quiet from outside. Noise should range from minimal with some water splashing down to a faint hum that's inaudible from 200 feet away, depending on how the exhaust is plumbed.

[Cheechako]

If you are going to run an engine anyway, just run a generator, why reinvent the wheel?

[TheOffice]

There are small DC a/c units on the market - for a price.

[EngineerRetired]

Originally Posted by rwidman

I think most boaters want their air conditioning to run when the boat is not running. That is, when anchored or docked.

Some boats do have a genset capable of running the air conditioner when anchored.

I understand and this would do just that. Run the AC, and charge the house batteries (provide power to the inverter) and when not doing one of those running the water maker. It just wouldn't require such a big (thirsty) diesel.


The only downside is it you would not run the AC off shore power. Myself, and I speak only for myself, I don't like being tied up. I have a home tied to land for that

[EngineerRetired]

Originally Posted by TheOffice

There are small DC a/c units on the market - for a price.

No, don't want to use a diesel to run a generator, to run a motor, to drive a compressor. Just drive a compressor directly.

[EngineerRetired]

Originally Posted by Cheechako

If you are going to run an engine anyway, just run a generator, why reinvent the wheel?

With a genset the diesel runs at at a constant speed to turn a generator that produces electricity that is used to spin an electric motor, that turns a compressor. I am proposing driving the compressor directly with the diesel. It's RPM can can easily be altered to meet demand. Get the boat cooled off or only cool one cabin and the RPM can drop to a fraction of full RPM.


Less fuel, longer life, not nearly as expensive. When not being used used for the AC, the same small diesel can charge house batteries or run the water maker directly.


Again, this is all off the shelf tech and should be fairly low cost to build.

[Na Mara]

I think there was a company that did this with small Kubota engines. You could add take offs to them for everything from water makers to fridge compressors.

A good reason for not doing this is redundancy. All the systems powered off that engine are lost if that engine fails . Contrast that with electrical systems powered off a generator. These can also be powered off the battery bank through an inverter, off a backup petrol generator, or even off shore power. This redundancy is worth the slight loss in efficiency in my view.